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Gamma irradiation and virus inactivation

Table of contentsGamma irradiation basics 3Radiation sterilization for health care products Written by Fairand, B. P., reprinted with permission of CRC Press

Steris-Isomedix Services 31Introduction to the Sandy, Utah Facility

New findings and old theories 33Gamma irradiation and virus inactivation Hanson, G. (GE Healthcare), Wilkinson, R. (HyClone Laboratories, Inc.), and John Black (American BioResearch Laboratories, Seymour, TN)

Viral safety in serum for cell culture use 37Reprint from Art to Science vol. 16 no. 2 Written by Hanson, G (GE Healthcare) and Foster, L (HyClone Laboratories, Inc.).

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Gamma irradiation basicsThe following excerpt is a reprint of Fairand, B.P. Radiation sterilization for health care products: X-ray, gamma, and electron beam, ISBN: 1587160749 (2002) with the permission of CRC Press LLC, Boca Raton, FL, USA.

Chapter 1. Historical perspective

ProcessesRadiation sterilization is exposure of a product to high-energy electrons or high-energy electromagnetic radiation in the form of X-rays or gamma rays in a controlled and safe manner. Particle accelerators are used as the source of high-energy electrons, and these same accelerators can be used to generate X-rays in the form of bremsstrahlung radiation by placing a high-atomic number conversion target in the electron beam. Radioisotopes of cobalt and cesium are the principle sources of gamma rays for this efficacious method of sterilization. High-energy electrons, bremsstrahlung radiation, and gamma rays penetrate deeply into most products, even when they are in their final packaged state, thus ensuring that all elements of the product have been effectively sterilized. Radiation sterilization is a mature industry that traces the existence of the first commercially viable irradiator facilities to the late 1950s (1). Today, high-power electron beam machines and gamma irradiators containing millions of curies of isotope are capable of sterilizing millions of cubic feet of product per year. It is perhaps the efficacy of the process and the relative simplicity of treatment that attracts many to select this method of sterilization. Use of particle accelerators to generate bremsstrahlung radiation is a more recent development in the field of radiation sterilization (2,3). With the availability of robust and very high-power electron beam accelerators, bremsstrahlung irradiators began to be brought on line in the late 1980s to service the radiation sterilization industry (4).

For electrons to be an effective bulk sterilant, they must have energies of at least several million electron volts (MeV). X-rays and gamma rays, also called photons, are pure electromagnetic energy that makes up the high frequency part of the electromagnetic spectrum, which includes other sources of radiation such as radio waves, microwaves and visible light. It is the high frequency attribute of X-rays and gamma rays that allows them to penetrate through large volumes of product and function as an effective bulk sterilant. The high-energy electrons, X-rays, and gamma rays that are used for sterilization applications are capable of removing electrons from atoms in the materials that are irradiated. This process, called ionization, is a basic characteristic of these forms of radiation that separates them from other forms of radiation.

The lethal effect of radiation on microbes is well understood and extensively documented in the literature (5-8). High-energy electrons, X-rays, and gamma rays interact with microorganisms through ionizing events that lead to scissions of

bonds in the biologically active macromolecules of the microbe. The nucleic acid, DNA, which contains in its exact structure the blueprint for future generations, is particularly sensitive to these effects. Depolymerization of the DNA and subsequent changes in its chemistry effectively destroy its reproductive capacity. High-energy electrons, X-rays, and gamma rays do not destroy the microorganism; they simply preclude its ability to reproduce. This method of sterilization has proven to be highly effective. This text will focus only on gamma rays.

Gamma raysCobalt-60 and cesium-137, which are radioactive isotopes of cobalt and cesium, are the principal sources of gamma rays that are used for sterilization of health care products. Of these two isotopes, cobalt-60 has been widely used for sterilization of medical device products, while cesium-137 has been used on a much more limited basis. The principal reasons for this selection process relate to the fact that cobalt-60 gamma rays penetrate much more deeply into products than gamma rays from cesium-137, and cobalt-60 can be produced to high specific activities, which permits manufacture of very compact energy-efficient sources. Cobalt-60 is also manufactured in a metallic state that is inherently more stable than salts and other material compositions.

During the late 1970s and early 1980s, when the gamma processing industry was undergoing rapid growth, there was a shortfall in supply of cobalt-60 sources; however, this limitation was soon overcome once suppliers ramped up their production to meet industry needs. Shortfall existed for a few years, principally due to the long time to complete the manufacturing cycle. Today, major suppliers in North America, Europe, and Asia offer an ample supply of cobalt-60 sources for use by the radiation processing industry. Safe shipment of these sources to irradiators around the world is now possible using shielded shipping containers that have been approved by national and international regulatory bodies.

Use of colbalt-60 for radiation processing began in a rather modest way, circa the 1950s. In this time frame, underwater facilities were being used for material modifications (i.e., curing and cross-linking) and small, self-contained dry source irradiators were being used principally for research applications. It soon became evident that radiation offered a good alternative for sterilization, particularly for those devices that were heat sensitive. With techniques available for producing large quantities of cobalt sources, radiation processing became commercially viable, and production-size irradiators were being commissioned by the mid-1960s.

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The cost and size of these high-throughput facilities deterred their use at individual hospital sites. However, large manufacturers of medical device equipment were able to accommodate these factors while capitalizing on the high throughputs, and thus, they were the first to realize the true advantages of using gamma radiation to sterilize medical device products on a commercial scale. The burgeoning industry also gave rise in the 1970s to contract sterilization plants, which were able to service larger producers of medical equipment as well as smaller manufacturers. By 1991, more than 150 large-scale gamma sterilization plants were operational worldwide, of which more than half were contract facilities (9).

RegulationsThe critical nature of health care products and inherent dangers of intense sources of radiation when improperly employed necessitated development of appropriate standards and close control of irradiators by regulatory bodies. In 1965, the first draft of an international code of practice was developed to ensure that the radiation sterilization process was properly carried out (10). Since then, other codes and regulations on control and maintenance of the process have appeared at both the national and international levels. Agencies now exist to confirm that radiation sterilization plants are in conformance with these regulations. For example, in the United States, the agency is the Food and Drug Administration (FDA), and in the United Kingdom, the comparable agency is the Department of Health and Social Services (DHSS). Licensing of electron accelerators and X-ray bremsstrahlung facilities are regulated only at the state level. Other regulatory agencies may impose requirements to accelerator design and operation of employee safety (e.g., Occupational Safety and Health Administration [OSHA] in the United States) and environmental protection (e.g., U.S. Environmental Protection Agency [EPA]). In the case of gamma facilities, the use of large quantities of gamma-emitting radioisotopes by irradiators also dictates that nuclear agencies have regulatory jurisdiction to ensure safe operation of the gamma facility. The International Atomic Energy Agency (IAEA) is the operative group at the international level, and in the United States, the comparable agency is the Nuclear Regulatory Commission (NRC). The proceeding agencies and regulatory bodies, with their oversight of the radiation sterilization industry as well as the high standards set by the industry, provide assurance that irradiators operate in a safe mode and consistently deliver the appropriate dose to sterilize products that are used in the health care industry.

Absorbed doseDefinition

The amount of radiation imparted to a material is defined in terms of absorbed dose. It is defined as the mean energy imparted to an incremental unit of matter, divided by the mass of that matter. The international unit of absorbed dose in use today is the gray (Gy), which is 1 joule per kilogram. The previous unit of absorbed dose was the rad, which equals 100 ergs per gram. Based on these definitions, it is seen that one gray is equal to one hundred rads.

Methods for setting

Methodologies for setting minimum absorbed dose to achieve acceptable sterility assurance levels in the medical devices and other critical items are well established (11,12). By the early 1960s, radiation was being used to effectively sterilize sutures by exposure to a minimum absorbed dose of 25 kGy (13). The criteria for establishing sterility of medical devices were the same as those used by other methods of sterilization, e.g. steam and ethylene oxide. The maximum tolerable risk of finding a viable microorganism on a device was set at one per million units or less. Early work on radiosensitivity of a large number of different types of microorganisms supported the use of 25 kGy as the minimum absorbed dose as long as good manufacturing practices (GMPs) were used in the manufacture of the devices (14-17). The concept of GMPs was in place at this time, thus ensuring that device bioburdens were kept to low values. However, use of a minimum absorbed dose of 26 kGy to achieve the desired sterility assurance level was not universally accepted. For example, work by Christensen based on Streptococcus faecium as the biological indicator suggested the need for a minimum absorbed dose of at least 35 kGy (18).

As commercial irradiators came on line in the late 1950s and early 1960s, it became clear that guidance was needed not only for setting the dose but also on process controls to ensure reliability of the process. In 1966, IAEA called together a panel to consider a code of practice for radiosterilization of medical devices. Results of this panel’s work were presented in 1967 at an IAEA symposium on radiosterilization of medical products that was held in Budapest, Hungary (19).

As the industry grew and became more widespread and more demands were placed on the use of radiation as a preferred method of sterilization, a North American Working Group under the auspices of the Association for Advancement of Medical Instrumentation (AAMI) was created to develop guidelines for sterilization of medical devices by irradiation. The outgrowth of this work by this group led to methodologies that allowed absorbed doses less than 25 kGy dependent of microbiology and application of the device (11,20). This group, which is now under the auspices of the International Standards Organization (ISO) with AAMI as secretariat, continues to refine and develop guidelines for sterilization in the health care industry.

In addition to a specification of a minimum dose, a maximum dose must be set at a value to ensure that no detrimental effects are introduced in the medical device. The setting of the maximum dose usually takes into account biocompatible as well as physical and mechanical properties of the device. Considerable work, principally by manufacturers of polymers and medical device materials during the years after radiation processing first became a viable method of sterilization, led to the development of a wide variety of radiation-compatible materials. Standards and technical information reports are available to guide a user in selection of radiation-compatible materials and of methods in determining a maximum acceptable dose.

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Fig 2.1. Electromagnetic spectrum.

Process controlsMedical device manufacturers must follow GMPs in the design, manufacture, and sterilization of medical devices. These GMP requirements were first codified in the United States in 1976 and were later revised and updated in 1996 (21). With the 1996 revision of the regulations, the term GMP was replaced with the term quality system regulations (QSRs). The device manufacturer is ultimately responsible for sterility of the device, whereas the irradiator certifies the absorbed doses that are delivered to the device. However, even in those cases where radiation sterilization is performed by a contract sterilizer, the contract sterilizer is considered to be an extension of the device manufacturer and, consequently, must adhere to the appropriate QSRs. Existing standards provide complete guidance on radiation sterilization of health care products (13,14). This includes requirements for validation as well as routine process control. In addition to conforming to QSRs, the irradiator facility must be in conformance with rules and regulations that are delineated in its license with the nuclear regulatory agencies or appropriate state agencies [22].

Chapter 2. Basic principlesGamma raysGamma rays are pure electromagnetic energy capable of propagating through space and interacting with materials. Gamma rays make up the very high-frequency end of the electromagnetic spectrum that consists of many other forms of radiation including X-rays, visible light and very low-frequency

radio waved. Figure 2.1 shows how gamma rays fit into the overall electromagnetic spectrum. All of the different sources of radiation shown in Figure 2.1 belong to the same electromagnetic spectrum and differ only in the frequency of the radiation.

Sources of gamma raysGamma rays that are used in sterilization applications come from radioisotopes. The atoms in a radioisotope element contain unstable nuclei that decay at a predictable rate into other nuclei (23). This decay process is often accompanied by emission of gamma rays. Cobalt-60 is an example of a radioisotope that has found widespread use as a source of gamma rays for sterilization of materials. Cesium-137 also is used in research applications and sterilization of low-volume products. However, cesium has not been widely used for sterilization of health care products and, for this reason, is not discussed in this book.

Cobalt is found in its natural state as an ore that is refined to produce a metal. The stable isotope of cobalt contains 27 protons and 32 neutrons in its nucleus, which totals 59 particles in each nucleus of cobalt. The total number of particles in the nucleus is referred to as the mass number (23). Therefore, the stable isotope of cobalt has a mass number of 59. Cobalt-60 is formed when cobalt-59, the stable isotope, is exposed to neutrons in a nuclear reactor. The cobalt-59 nucleus absorbs a neutron to create the radioactive isotope cobalt-60, which has a half-life of 5.26 years and decays to form stable nickel-60. Two gamma rays with energies of 1.17 MeV and 1.33 MeV are emitted every time a cobalt-60 nucleus decays to form nickel-60. These high-energy photons can penetrate deeply into most medical device products, thus providing an effective source for bulk sterilization applications. Even though cobalt-60 photons are sufficiently energetic to effectively sterilize large volumes of product at a given time, the photon energies are still below the threshold energies that are necessary to create other radioisotopes, that is, it is energetically impossible for cobalt-60 photons to make other materials radioactive.

Fabrication of sourcesIn preparation for irradiation in a nuclear reactor, the refined cobalt-59 metal is metallurgically formed into slugs or cylindrical rods that are typically less than 0.5 inch in diameter. These slugs or rods of cobalt-59 are usually plated with a metal such as nickel and then encapsulated in a stainless steel or Zircaloy™ tube. This configuration forms the target that is irradiated in the nuclear reactor. After exposure in the nuclear reactor that typically can take up to 18 months or more, the radioactive rods are removed from the nuclear reactor and transferred in special containers called casks to hot cells where final encapsulation in stainless steel occurs. The final configuration is usually in the form of a thin rod about 18 inches long by 0.5 inch in diameter. An example of a commercially available source element is shown in Figure 2.2. Each of these sources can contain 10 000 curies or more of cobalt-60, and many of these sources are grouped together inside an irradiator cell to form the source or radiation that is used to sterilize medical device products. One curie of a radioactive isotope decays at the rate of 3.7 × 1010

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disintegrations per second, and because cobalt-60 emits two gamma rays per disintegration, each curie of cobalt-60 emits 7.4 × 1010 photons per second. Because an industrial irradiator facility typically contains more than a million curies of isotope, products that are processed in an irradiator are literally bathed by an intense flux of photons. This allows large volumes of product to be sterilized in relatively short periods of time, that is, in a few hours.

Fig 2.2. Colbalt-60 source element (courtesy of MDS Nordion).

Interaction of gamma rays with matterCompton scattering

At intermediate photon energies that are characteristic of photons emitted by cobalt-60, the dominant channel for interaction of radiation with matter occurs via a process called Compton scattering. In this process, photons that are emitted by cobalt-60 transfer their energy to matter principally through scattering events with orbital electrons of atoms that make up the target material. The scattering event is shown in Figure 2.3. Classical electromagnetic theory is unable to describe the inelastic component of scattering of electromagnetic radiation from charged particles, and it was not until 1923 that publications by Compton and Debye quantified the relationships governing the energetics and scattering angles of photons off of atomic electrons (24,25). The work of Compton and Debye was followed in 1929 by the work of Klein and Nishina who applied relativistic quantum theory to scattering of photons from electrons to derive an expression for the cross section of Compton scattering (75). Today, this process is fully understood, and transfer energy to materials can be accurately calculated.

Fig 2.3. Compton scattering.

Partition of energy

A photon undergoing Compton scattering with an orbital electron transfers sufficient energy to the electron to strip it from the atom. This process is called ionization. Each cobalt-60 photon typically undergoes several Compton scattering events within the target material before its energy is dissipated. Therefore, a single photon can eject several high-energy electrons, which are referred to as primary electrons, from their bound states around atoms. The mean free path between Compton scattering events, even in a unit density such as water, is on the order of several centimeters or more, whereas the range of the primary electrons is several millimeters or less. The interaction of cobalt-60 gamma rays with matter is shown in Figure 2.4. The large mean free path of photons in low-density medical device products allows a large volume of product to be treated to approximately the same absorbed dose.

Fig 2.4. Interaction of gamma rays with material.

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The primary electrons rapidly dissipate their energy through numerous low-energy scattering events with other atomic electrons. These secondary electrons can cause further ionization and excitation of atoms and dissociation of molecules. It is these low-energy events that result in physical and chemical changes in materials, including destruction of pathogenic microorganisms. Gamma rays function as an initiator of this cascade of interactions, and thus, they only act as a precursor to the ultimate absorptive events that lead to sterilization of products and alteration of material properties. Changes induced by Compton scattering or photons are concentrated around the track of the primary electron, which as we have already seen, has a range of several millimeters in unit density material. At the atomic and molecular levels, these changes occur almost instantaneously, that is, on the order of 10-10 seconds, in the form of ionized species, excited and dissociated molecules, and thermal processes. Recombination of some excited species, for example, free radicals, might be chemically active for hours or even days.

Chapter 3. DosimetryDosimetry, which is the method of measuring absorbed dose, is an important part of the radiation sterilization process. Knowledge of the minimum and maximum absorbed doses delivered to a medical device is necessary to confirm that the part received sufficient dose to make it sterile yet did not receive a dose that could compromise its intended function. In the Chapter 7 on processing with irradiations, we will see that medical devices can be released for final distribution to the healthcare provided solely on the basis of knowledge of dose delivered to the product. This process is called dosimetric release. To satisfy this release process, knowledge of dose delivered to the product is critical. Another process, termed parametric release, also may be used as a method for post-irradiation release of the product. A detailed understanding of an irradiator’s dose delivery characteristics as well as proper monitoring techniques and controls during the processing cycle are essential for implementation of a parametric release program. Dosimetry might not be required during routine processing when parametric release is invoked; however, dosimetry is expected to continue to play an important role in validation of the system and process and in periodic audits. General guidance on use of dosimetry in the radiation sterilization of healthcare products is given in an ASNI/ISO standard on radiation processing (11). Standards on calibration and use of several dosimetry systems also have been published by the American Society for Testing and Materials (ASTM) and approved ISO Standards (26-32). These standards guide the user in use of dosimetry systems to obtain quantitative measurements of absorbed dose.

Types of dosimetry systemsA dosimeter is defined as a device that, when irradiated, exhibits a quantifiable and reproducible change in some property of the device, which can be related to absorbed dose in a given material using appropriate analytical techniques. The key words in the definition of a dosimeter that separate it

from most other materials are quantifiable and reproducible, because many materials undergo qualitative changes when exposed to radiation. Dosimeters are well-characterized tools that are intended for quantitative measurement of radiation environments. A dosimetry system is a system for measuring absorbed dose that includes dosimeters as well as measurement instruments, associated reference standards, and procedures for the system’s use.

Four different classes of dosimeters are used in the radiation processing industry. These dosimeters, which have different applications and levels of quality, include the following:

• Primary standard dosimeters: these dosimeters are of the highest metrological quality and are established and maintained as an absorbed dose standard by a national or international standards laboratory. These dosimeters are normally retained by the standards laboratories and are used to calibrate radiation fields against which other classes of dosimeters are calibrated. Calorimeters and ionization chambers are two examples of primary standard dosimeters (33-35).

• Reference standard dosimeters: these dosimeters are of high metrological quality and are used as a standard to provide measurements traceable to, and consistent with, measurements made using primary standard dosimeters. Reference standard dosimeters are available for use at irradiation facilities and are used to calibrate radiation fields and routine dosimeters. Several reference standard dosimeters are available for use in high-dose applications. Examples of these dosimeters include alanine, ceric-cerous sulfate and aqueous solutions of dichromate (27-29).

• Routine dosimeters: these dosimeters are calibrated against a primary, reference or transfer standard dosimeters and are used for routine absorbed dose measurements that include dose mapping and process monitoring. Examples of routine dosimeters are dyed and clear polymethylmethacrylate, radiochromatic films, and cellulose acetate (30-32).

• Transfer standard dosimeters: these dosimeters, which are often used as reference standard dosimeters, are suitable for transport between different locations and are used to compare absorbed dose measurements. They are specially selected dosimeters that are used for transferring dose information from a calibration facility to an irradiation facility.

Dependent of the method of analysis, dosimeters can be divided into two types: either physical or chemical systems (36). Methods of analyses and examples of dosimetry systems that use a particular method of analysis are shown in Figure 3.1. Physical systems provide basic methods for measuring absorbed dose and are usually associated with primary standard dosimeters such as calorimeters and ionization chambers. In calorimeters, energy is deposited as heat in an adiabatic chamber and recorded as an increase in temperature in a thermally isolated mass. In ionization chambers, energy is deposited in a medium that produces electron-ion pairs with known energetics. Application of a voltage to the medium produces a current that is measured with an electrometer.

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Fig 3.1. Dosimetry method of analysis.

Chemical systems can be either liquids or solids. Dichromate and ceric-cerous are examples of liquid dosimeters where absorbed dose changes the ion concentration in the aqueous solutions that can be measured with spectrophotometers as change in absorbance of the medium. Solid chemical dosimeters, which include radiochromatic films and polymethylmetha-crylate as examples, change color when exposed to radiation. Spectrophotometers are typically used to determine the increase in absorbance that is induced by the change in color. Alanine, which is used as a reference standard dosimeter, uses an electron paramagnetic resonance method for analysis.

Calibration of dosimetry systemsCalibration of a dosimetry system includes the analytical instrumentation that is used for analysis of the dosimeters as well as the dosimeters. They need to be calibrated following documented quality procedures. Details on calibration of dosimetry systems are given in “Guide selection and calibration of dosimetry systems for radiation processing” (ASTM) (26).

InstrumentationInstrumentation used for absorbed dose measurements depends on the type of dosimetry system that is selected to make the measurements. Several types of dosimeters are analyzed using photometric or spectrophotometric techniques. Others, such as alanine, use electron paramagnetic resonance spectroscopy for analysis, or in the case of ceric-cerous, a spectrophotometer or potentiometer may be used. Regardless of the type of equipment, these instruments must be calibrated before use and at periodic intervals thereafter. Other ancillary equipment such as thickness gauges, analytical scales and filters also must be calibrated. The calibrated instrumentation should be traceable to national or international standards. This may not always be possible due to the method of analysis. For example, electron paramagnetic spectrophotometers are calibrated using irradiated alanine dosimeters and other calibrated devices that are discussed in “Practice for use of the alanine-EPR dosimetry system” (ASTM) (27).

DosimetersMeasurements using primary standard dosimeters are based on physical principles, and consequently, these systems do not need to be calibrated against other standards. Reference standard dosimeters and transfer standard dosimeters, if they are reference dosimeters, are calibrated by national, international or accredited laboratories. Calibration of routine dosimeters usually is the responsibility of the user of these systems. Three methods of calibration of routine dosimeters are in “Guide selection and calibration of dosimetry systems for radiation processing” (ASTM) (26). These calibration procedures include the following:

• Irradiation of the routine dosimeters in an irradiation calibration facility, for example, a national calibration laboratory

• Irradiation in an in-house calibration facility that has an absorbed dose rate measured by reference or transfer standard dosimeters

• Irradiation of the routine dosimeters together with the reference or transfer standard dosimeters in the production irradiator, for example, an in situ method of calibration

Each method of calibration has its own potential advantages. The first two methods have the advantage that the dosimeters are irradiated under well-controlled and documented conditions. The third method has the advantage that the dosimeters are calibrated in the production irradiator under conditions similar to those encountered during routine production. Effects on dosimeter response from possible differences between calibration conditions and conditions uses are minimized. In some cases where the dosimetry system is sensitive to changes in environmental conditions, an in situ approach might be the only viable method of calibration (37).

Selection of a specific method of calibration, which is the responsibility of the user, is dependent of the type of dosimetry system being used, the irradiator environment and practical considerations such as cost and logistics. Regardless of the calibration procedure, traceability to national standards must

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be demonstrated. For example, in the United States, this requires traceability to the National Institute of Standards and Technology (NIST), and in the United Kingdom, it is the National Physical Laboratory (NPL). Due to the international use of irradiation sterilization, which includes extensive trade in irradiated products, as well as the limited number of standard laboratories that provide calibration services, the International Atomic Energy Agency (IAEA) also has set up a high dose intercomparison program. This program promotes dosimetric accuracy in products that are processed in irradiator facilities (38). Other recognized laboratories that meet stringent requirements can also be used for the calibration purposes (39).

Selection and use of routine dosimetry systemsSeveral technical factors need to be considered in selecting a dosimetry system for routine measurement of absorbed dose at an irradiator site. Also, there are several practical considerations that need to be taken into account in this selection process. The irradiator environment and method of analysis are key elements in selecting an appropriate routine dosimetry system. Technical factors that need to be considered include the following:

• Stability and reproducibility of the system: these properties are basic to any dosimetry system. Dosimeters are normally stored and analyzed in a controlled environment to stabilize their response. For dosimeters with significant changes in response after irradiation, it may be necessary to set strict time constraints for analysis.

• Dependence of dosimeter response on dose rate and energy spectra: again, it is desirable, when possible, to select a system that has a response that is insensitive to these factors. For example, PMMA dosimeters and radiochromic films are relatively insensitive to the energy spectra typically found in gamma and X-ray irradiators. If the dosimeter response cannot be corrected for these effects, it may be necessary to use an in situ method of calibration.

• Effects of size, location, and orientation of the dosimeter: these properties are usually more important in electron beam dosimetry, where large dose gradients are frequently encountered.

• Variance of the dosimetry system response within established limits about a fixed calibration curve: it is important that the response of the dosimetry system can be easily fitted to an analytical function over the range of use. The calibration curve consists of a discrete number of data points; however, the calibration data must allow calculation of dose over the entire range of use. Goodness of fit to the data is an important factor in this calculation.

Practical considerations that need to be considered in the selection of the routine dosimetry system include the following:

• Ease and simplicity of use: consumption rates of dosimeters typically are high, and irradiators may operate around the clock.

• Ruggedness of the dosimeter: production irradiators typically are not laboratory environments. Dosimeters must survive under conditions that can sometimes be harsh.

• Availability and cost of dosimeters and associated instrumentation: routine dosimeters are typically used in large quantities, and sometimes, multiple irradiator sites can be involved. Multiple pieces of analytical instrumentation might be required.

• Time and cost to complete the analysis and certification of dosimeter results: oftentimes, irradiation is the last step before marketing of the product. Delays in this process can be costly or unacceptable.

Uncertainty in the measurement processSeveral important elements in the sterilization process rely on results of absorbed dose measurements. Among other things, results of dosimetry are used in the setting of sterilization doses, the selection of minimum and maximum dose zones for routine monitoring of the process, the setting of cycle times or conveyor speeds, and as the basis for dosimetric release of product. It is important to demonstrate that dose measurements are repeatable and are not subject to significant bias. A quality system for conducting these measurements must be in place, and the overall uncertainty in the measurement process should be estimated. The estimate of uncertainty is a quantitative process that involves an investigation and estimate of component of uncertainty and elimination of significant sources of bias. This process demonstrates that the measurements are consistent with good use of the system. It should be remembered that no number based on a measurement process has value unless it is assigned an uncertainty.

Sources of uncertaintyPrincipal sources of uncertainty in dose measurements are traceable to the following (40):

• Uncertainty in the absorbed does received by dosimeters during system calibration

• Analysis of dosimeter response

• Fit of dosimetry data to calibration curve

• Routine use of dosimeters in a production irradiation facility

The individual components of uncertainty need to be combined to find a single number for uncertainty in the measurement of absorbed dose. If the components are independent of each other, they can be combined in quadrature. This method involves taking the square root of the sum of the squares of the individual components. For components that are correlated, other methods for combining these components should be used (40). The final number for uncertainty should be given at some confidence

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level. The combined uncertainty is multiplied by a coverage factor that has a value of approximately 2 at a 95% confidence level and a value of approximately 3 at a 99% confidence level. If procedures for good use of a dosimetry system are followed and the dosimetry system is properly calibrated, the combined uncertainty in the measurement of absorbed dose should not exceed + 10% at a 95% confidence level.

Use of uncertaintyThe resultant value of overall uncertainty is a gage for determining if the measurement system is under statistical control and if proper procedures are being followed. The process of estimating uncertainty allows sources of bias to be identified and eliminated. This ensures that systematic errors of significant magnitude are not included in the dose measurements. The estimated value of uncertainty has practical value for setting run parameters to avoid dose measurements outside of the acceptable limits. This approach for setting process parameters is referred to as the target dose concept. Results of the estimate of uncertainty in the dose measurements also act as a guide for steps that may need to be taken to improve the accuracy of the measurements. These steps could involve changing the method of analysis and techniques for handling the dosimeters or, in the extreme, precipitate a decision to select a different routine dosimetry system that is better suited for monitoring dose in the chosen environment. Without knowledge of the uncertainty in the measurement process, it also would be difficult to understand the contributing factors to variability in dose measurements, whether the variability in dose was traceable to the dosimetry system per se or attributable to changes in process parameters.

Chapter 4. Effects of radiationMaterialsThe effect of radiation on materials is strongly dependent of the type of material being irradiated. The geometry of the part and properties of the radiation environment can also play a role in response of a material to absorption of energy from high-energy electrons, X-rays and gamma rays. In assessing the ability of a medical component or device to survive the effects of radiation, additional factors such as biocompatibility, discoloration and functionality of the part might need to be taken into account. Materials can be broadly divided into metals and nonmetals. As shown in the following sections, metals are impervious to the effects of radiation, whereas nonmetals, which include various types of polymers, paper products and natural products (i.e., cotton, glasses, and ceramics), exhibit a wide range of response to the effects of radiations. Some materials are negatively affected by radiation even at low levels of absorbed dose and, consequently, are not compatible with the radiation sterilization process, while many other materials can survive absorbed doses far in excess of those needed to sterilize the part. Early in the development of the radiation processing industry, major suppliers of polymers and other materials that are commonly used in the medical

device industry began to develop radiation resistant materials so that today, many materials can be classified as radiation compatible and are suitable for rotation sterilization. (11,43,64). The possibility of using radiation to sterilize pharmaceuticals has stimulated interest in the studies of the effects of radiation on drugs. These studies date back to the later 1960’s and early 1970’s (41,42).

MetalsThe inherent stability of metals to effects of radiation becomes evident when the basic theory of metals in juxtaposed with the interaction of high-energy electrons, X-rays and gamma rays in materials. High-energy electrons from accelerators interact with materials through ionizing collisions with atomic electrons in the material. The secondary electrons that are produced in these ionizing events, in turn, dissipate energy through additional collisions with other atomic electrons. Bremsstrahlung radiation and cobalt-60 gamma rays deposit energy in materials principally though Compton scattering events. In this process, energy is transferred to atomic electrons, which dissipate energy though multiple collisions with other electrons. Regardless of the type of incident radiation (i.e., high-energy electrons, X-rays, or gamma rays) the absorbed energy creates a cascade of low-energy electrons that is produced from absorption of the incident radiation transfers energy to this sea of free electrons, which in essence, functions as an energy absorber. There is no change in the properties of the metal other than some heating of the metal may occur upon thermalization of the electrons.

Radiation does not directly affect metals; however, the surface of metal components could discolor or show evidence of corrosion when irradiated with some types of polymers. These synergistic effects are initiated by free radicals and decomposition products that are formed from radiation induced dissociation of the molecular constituents in the polymers. For example, very small amounts of hydrochloric acid (HCl) are formed when polyvinylchloride (PVC) is irradiated. The effect of hydrochloric acid on metals is well known. Localized heating of a metal part also could have possible deleterious effects on the performance of a medical device component, for example, delamination at a metal-polymer interface.

Polymeric materialsPolymers and their derivatives make up the bulk of materials that are found in single-use medical devices. Polymers are normally divided into three major groups that include thermoplastics, thermosets and elastomers. Thermoplastics are perhaps the most widely used type of polymers in medical devices. These polymers are normally processed using technologies such as injection molding, extrusion, or spinning. Thermosets include epoxies, among other types of polymer materials. Elastomers are made up of rubbers, both natural and synthetic. Other fibrous materials that generally are polymeric in nature include paper products and natural products such as cotton.

11

All polymers consist of basic molecular units called monomers, which are arranged in a repeated fashion to form a macromolecule or polymer. The effect of radiation on polymers is highly dependent on the type arrangement of these monomeric building blocks. The surrounding environment during irradiation and other factors such as the presence of additives, which can serve as plasticizers or antioxidants, play roles in the response of a particular polymer to irradiation. For this reason, it is always important in the validation of new product to complete a proper material qualification study.

Polymers respond to radiation in two fundamental ways. They can either undergo chain scissions wherein molecular chains are ruptured, which lower the molecular weight of the polymer, or molecular chains can cross-link to form three-dimensional structures. Both modes of change occur when a polymer is exposed to radiation. Some polymers are changed primarily through chain scissions, while others principally undergo cross-linking to form three-dimensional structures. Polymers that have a propensity for chain scissions usually are more sensitive to the effects of radiation that those that are prone to cross-link. Lowering a polymer’s molecular weight through radiation induced chain scissions usually result in some loss in its strength properties, and the rupturing of molecular bonds also leads to the evolution of small quantities of gas, for example, hydrogen. Depending on the chemical composition of the polymer, other small molecules such as carbon monoxide, carbon dioxide, and methane can be produced. Polymers that tend to cross-link when exposed to radiation rather than undergoing chain scissions generally show an increase in tensile strength; however, this may come at the expense of their elongation properties, that is, the polymer becomes more brittle.

The potential loss of mechanical properties, which include tensile strength, shear strength, elongation, elastic modulus and impact strength, is frequently the most important consideration in selection of a polymer for irradiation, sterilization. Materials that exhibit little loss in these properties when exposed to the levels of radiation that are necessary for sterilization of a medical device are usually preferred to other materials. In this regard, polymers that contain aromatic molecular structures are generally more resistant to radiation than aliphatic polymers, and the polymers with aromatic rings are usually chosen if mechanical properties are an important issue in performance of a part. Other factors such as density, crystallinity, and additives also need to be taken into account in determining the stability of a given polymer to effects of radiation [43].

The environmental conditions of irradiation can play a role in response of some polymers. The presence or absence of oxygen, that is, anoxic conditions, can influence the recombination kinetics associated with radiation-induced free radicals. In the presence of oxygen, free radicals can be converted into peroxidic radicals, which can promote further chain scissions. Polypropylene is an excellent example of a polymer that autoxidizes and, for this reason, normally requires addition of antioxidants to make it compatible with the irradiation process. For polymers that are sensitive to oxidative effects, such as polypropylene, studies have shown

that irradiation by accelerated electrons has fewer detrimental effects on the polymer than irradiation by gamma rays (76). This is usually attributed to the much higher dose rates that are delivered by accelerated electrons compared to gamma radiation. With accelerated electrons, dose is delivered in a very short period of time compared with the rate of diffusion of oxygen into the material, which controls the rate of oxidative degradation. Dose rates in X-ray-bremsstrahlung irradiators are intermediate to those in gamma and electron beam irradiators. The response of polymers to this source of radiation should fall between radiation-induced changes from gamma and electron beam environments. While some polymers may be sensitive to oxidation, many others are not, and oxidative effects are not an important consideration in selection of the material or type of process.

In some cases, color formation in irradiated polymers is the deciding factor in selection of a particular polymer for use in a medical device. Discoloration may discourage use of radiation simply for aesthetic reasons, or the transparency of the medical device may need to be retained for functional reasons. Such may be the case for syringes, vials and tubing. There are two types or radiation-induced color centers in plastics (44). One is permanent, and the other is annealable. Radiation-induced dissociation of molecular bonds can produce double covalent bonding, which results in formation of color centers. These permanent color centers correspond to stable conjugated chromaphores. The annealable color centers are attributable to free radicals that are formed in the polymer during the irradiation process. This source of discoloration is more frequently observed in rigid plastics, where radicals can be trapped in the polymeric matrix rather than in rubbery polymers. Oxygen in the air diffuses into the polymer and reacts with these free radicals to anneal out the color centers. Annealing rates depend on the permeability of the polymer to oxygen and the temperature at which the process occurs.

The resistance of a polymer to radiation-induced discoloration does not appear to be related to its resistance to changes in mechanical properties. For example, epoxies are highly resistant to changes in mechanical properties from exposure to radiation, yet readily discolor when irradiated, and Teflon™, which undergoes major degradation in its mechanical properties from radiation, is highly resistant to radiation-induced discoloration. The additives in commercial plastics can significantly affect the response of a polymer to radiation. Phenolic class additives are more apt to cause discoloration to the polymer than non-phenolic additives.

As noted at the beginning of this section, most polymers can be radiation sterilized without concern for degradation of the plastic. Some may require additives or antioxidants to make them compatible with the irradiation process, but only a few polymers such as Teflon are limited in their application. The relative stability of various types of medical polymers is shown in Figure 4.1. Tables 4.1 and 4.2 provide additional information on the radiation tolerance of several common elastomers and thermoplastics. Thermosets as a class are highly resistant to radiation.

Epoxy (aromatic)Liquid crystal polymer

PhenolicPolyester (thermoset)

PolystyrenePolyurethane (thermoset)

Polyvinylidene fluoride (Kynar™)Polyethylene (low/medium)

PET (polyester, rigid)PETG (polyester, flexible)

PolyimideECTFE

ABSHi-impact styrene (HIPS)

PolycarbonatePolysulfone (UDEL™)

PVC (Flexible)Polyvinylidene chloride (Saran™)

Natural rubber (latex)Styrene-butadiene rubber

Chlorinated polyvinyl chloridePolyethylene (UHMW)

EPDMPBT (polyester)

Polyamide (nylon 10 and 11)Polyvinyl chloride (rigid/semi)

Cellulose acetate butyrateCellulose acetate propionate

Kraton™ (SEBS)Polyethylene (HDPE)Acrylic co-polymers

Neoprene rubberSilicone rubber

Polyamide (nylon 6 and 12)ABS (Hi-impact)

Polymethyl pentenePolypropylene (stabilized)

Butyl rubberCellulose, natural (cotton/paper)

Acrylic (PMMA)Cellulose acetate

Polyvinyl chloride-acetateFEP (teflon)

Polypropylene, naturalAcetal (Delrin™/Celcon™)

TFE, Teflon

Note: This chart represents the best available data as of this date,and is intended as a guidance, specific resin formulations must beevaluated in the intended application for the effects o radiation and:

1. Residual and functional stress2. Section thickness3. Molecular weight and distribution4. Morphology5. Environment (oxygen/temperature)6. Dose rate

0 25 50 500 1000Dose (kGy)

1500

12

Dose (kGy) in ambient air at which elongation decreases by 25%

Fig 4.1. Radiation stability of medical polymers (courtesy of Karl J. Hemmerich, Biosafety Systems, Inc. San Diego, CA).

Table 4.1. Radiation tolerance of elastomers

Elastomer Tolerance (kGy) CommentsNatural rubber (latex) 600 Stable with sulfur or resin cure systemsNitrile 200 Avoid multiple sterilizationUrethanes 100 to 200 Wide variation in chemistry that affects responseNeoprene 50 to 100 Avoid multiple sterilizationSilicones 50 to 100 Avoid multiple sterilizationButyl 50 Sheds particulateFluor elastomer 50 Avoid multiple sterilization

13

The data in Table 4.1 show that all of the elastomers can survive the first-time doses that are needed to sterilize most health care products. However, that is not the entire story. For example, the response of urethanes can vary widely depending on the chemistry of the elastomers. Armed with that information, it is important to look at urethanes as a class and select the one with a chemical structure that makes it highly tolerant to radiation. Butyl rubbers can survive the exposure to radiation, but if the particular type of elastomers that is selected sheds particulates, it may not be compatible with the radiation sterilization process. Similar comments can be made concerning the data in Table 4.2 on thermoplastics. For example, polycarbonates are highly resistant to radiation, but they have a propensity to discolor when exposed to radiation. This might negate the use of this polymer in some types of applications.

In fact, polycarbonates are sometimes treated with a background dye to disguise the effects of discoloration so they can satisfy aesthetic needs. The information in Figure 4.1 and Tables 4.1 and 4.2 only provides guidance on the response of various polymers to the effects of radiation. For this reason, it is incumbent upon the manufacturer of specific health care products to properly validate the specific resin formulations in the intended application to the effects of radiation.

Glass and ceramicsAbsorbed doses to induce physical damage in glasses and ceramics usually far exceed doses that lead to degradation in polymeric materials. The amorphous nature of glasses makes them insensitive to radiation-induced changes in their molecular structure. The vulnerability of glasses to radiation is dictated by discoloration, particularly in those applications where the glass must remain transparent. In some cases, the color centers can be annealed out through post irradiation heating.

PharmaceuticalsMany of the early studies on the effects of radiation to pharmaceuticals are conducted at very high doses. It was thought at the time that doses of at least 25 kGy where needed to achieve a sterility assurance level (SAL) of 10-6. In some cases,

administration of these high doses produced undesirable results such as loss of potency of the drug or changes in the color and viscosity, which tended to discourage widespread use of radiation for sterilization of pharmaceuticals (77-79). Even today, the treatment of drugs using radiation is only done on a limited basis. Ophthalmic ointments and some antibiotics are examples of pharmaceuticals that presently are being sterilized using radiation (80). Raw materials and excipients have fared better. Excipients such as talcs and gums, which are materials that may be added to a drug to enhance its pharmacological action, are being successfully radiation sterilized in bulk quantities.

Today, many types of drugs are prepared under aseptic conditions, which lower bioburdens to very low values. At these very low bioburdens, it is likely that minimum doses to achieve an SAL of 10-6 are much less than 25 kGy. The possibility of using low doses to sterilize drugs increases the prospects of success. Irradiation of the pharmaceuticals in a dry state and inert atmosphere is preferred because these conditions eliminate the presence of reactive free radicals that promote secondary chemical change. For drugs in an aqueous form, possible negative effects of radiation are reduced if the drugs are irradiated in a frozen state (46,47). Irradiation of the product in a frozen state impedes the diffusion of the free radicals that are formed from radiolysis of the water. Under frozen conditions, the radicals have a higher probability of recombining without affecting the drug. Recent studies also have shown that adjusting the dose rates mitigates the effects of radiation (45). The potential advantages of sterilizing pharmaceuticals after the drugs are sealed in their final container and of sterilizing biologically derived products that may be sensitive to heat make radiation sterilization and attractive alternative to the use of conventional methods of sterilizing drugs such as heat or filtration (45). Radiation also inactivates viruses, which has positive implications for treatment of biologically derived pharmaceuticals (67). Many of these types of drugs are under development today and could enjoy the advantages of being treated by radiation. To enhance the probability of success, radiation sterilization of the drug should be taken into account during the development phase of the drug rather than waiting until the drug has been fully synthesized and tested.

Table 4.2. Radiation tolerance of thermoplastics

Thermoplastic Tolerance (kGy) CommentsPolystyrene > 1500 Stabilized by benzene ring structurePolyethylene (low/medium density) 1000 to 1500 Cross-links and gains strengthPET, PETG (aromatic polyesters) 1000 to 1500 Very stablePolysulfone 1000 StablePolycarbonate 1000 DiscolorsPolyvinylchloride (flexible) 1000 Yellows and releases HClAcrylonitrile/butadiene/styrene (ABS) 1000 Protected by benzene ringPolyacrylates 100 YellowsPolypropylene 20 to 50 Needs to be radiation stabilizedPolyacetals (Delrin, Celcon) 15 Avoid use due to embrittlementPolytetrafluorethylene (Teflon) 5 Avoid use, liberates fluorine gas

101

0 1086Dose (kGy)

Frac

tiona

l sur

vivo

rs

Fungal spore

Bacillus pumilus

42

100

10-3

10-1

10-4

10-6

10-5

10-2

100

0 7654Dose (relative units)

Frac

tiona

l sur

vivo

rs

B A C1 C2

321

10-1

10-2

14

Inactivation of microorganismsInactivation kinetics and the D10 value

The effectiveness of radiation for inactivation of microorganisms was recognized soon after the discovery of X-rays (49). The mechanism of inactivation is common to X-rays, gamma rays and electron beams. As shown in Figure 4.2, when the number of survivors from and initial population of different species of microorganisms is plotted against applied dose, it is noted that the surviving fraction continues to decrease with increasing absorbed dose, approaching zero survivors in an asymptotic manner.

Because delivery of sufficient dose to achieve zero survivors is not possible, sterility is defined in terms of the probability of finding a survivor on a very large number of devices. This is the basis of the term sterility assurance level or SAL, which is used today in the definition of device sterility. The probabilistic nature of the inactivation of microorganisms is fundamental to the interaction of radiation with mater. As already shown in the section on Compton scattering (Chapter 2), the interaction of photons in materials is based on the probabilities that are associated with quantum mechanics. Primary and secondary electrons that are generated in a Compton scattering event have a probability of interacting with a microorganism, “a hit.” This hit can cause sufficient distortion in the structure of the microorganism to render it unable to reproduce. Similar processes occur with electrons and X-rays.

Fig 4.2 Inactivation of microorganisms.

Because dose/survivor curves exhibit an exponential type of behavior, they usually are presented as semi logarithmic plots. Gunter and Kohn (50) and Alper (8) showed that dose/survivor curves were characterized by the four basic shapes shown in Figure 4.3, which were designated as A, B, C1, and C2. Curve A, which is a simple exponential curve with a constant slope, was observed as early as 1912 (51). Curve B is characterized by a constantly increasing sloe and curve C2 by a constantly decreasing slope. Curve C1 initially exhibits a shoulder at low absorbed doses and is characterized by a constant slope at

higher doses. It has been found that most microorganisms are governed by curve A or C1. Dose/survivor curves for typical microbial contaminants on hospital products were characterized by Whitby, who tested 673 isolate resistances from some 70 000 microbes (52).

The linear portions of curves A and C1 have constant slopes that permit these curves to be readily extrapolated to absorbed doses that equate to acceptable sterility assurance levels for medical devices. A parameter called the D10 value, which is defined as the incremental dose that is needed to reduce the surviving population by a factor of 10, is a measure of the radiation resistance of a particular species of microorganism. The mathematical expression relating D10 to a population of microorganisms and absorbed dose is given by Equation 1,

D = -D10 log(NIN0) (1)

where D is absorbed dose, N0 is the initial number of

microorganisms and N is the number of survivors at an absorbed dose of D. If D

10 is known and the dose/survivor curve,

also referred to as the inactivation curve, is characterized by exponential behavior, the dose to achieve the desired sterility assurance level is readily determined from Equation 1.

Bacillus pumilus, a spore-forming microorganism that was once used as a biological indicatory in the irradiation process, has a well-established D10 value and serves as a good example of the use of Equation (1) to calculate the dose to achieve a given sterility assurance level. Extensive studies have shown that the D

10 value of Bacillus pumilus when irradiated in air is 1.7 kGy (51).

Based on substitution of this D10

value into Equation 1, it would require a dose of 10.02 kGy to reduce the population of Bacillus pumilus by a factor of one million.

Fig 4.3. Dose/survivor curves. Reprinted with permission of International Atomic Energy Agency from Manual on Radiation Sterilization of Medical and Biomedical Materials, Technical Reports Series 149, IAEA, Vienna (1973).

In practice, the dose needed to achieve an SAL 10-6 is significantly greater than 10.2 kGy, because bioburdens on health care products are typically > 1.

15

Kill mechanisms

Absorption of a sufficient amount of radiation by a particular species of microorganism will destroy its ability to reproduce. Due to the probabilistic nature of the interaction of radiation with matter, all elements of the microorganism’s structure, including the nucleic acids, proteins and enzymes, are equally subject to damage from radiation. However, the nucleic acids, which contain information essential for growth, are found to be more sensitive to the effects of radiation that other components of the microbe (53).

The biologically active polymer DNA contains in its structure the blueprint for future generations. Therefore, a discussion of the disruptive effects of ionizing radiation usually focuses on damage done to this macromolecule. A principal effect to DNA from intensive exposure to gamma rays, high-energy electrons and X-rays is depolymerization (55). A sufficient number of lesions in both strands of the polynucleotides can exceed enzymatic repair processes, and the DNA will no longer be functional. In addition to the lesions in the polynucleotides, disruption of the base units in the DNA can lead to cross-linking of the two strands, thus preventing the unraveling during mitosis. The presence or absence of oxygen and state of hydration of the microorganism also can have a significant effect on the D10 value and, therefore, significantly affect the amount of radiation that is needed to achieve a desired SAL (51). Destruction of the protein moiety of microbial cells can lead to the demise of microorganisms; however, absorbed doses to achieve this type of kill usually are significantly greater than doses that are based on disruption of DNA.

Radiation resistance of microorganisms

The resistance of microorganisms to the effects of radiation is a function of parameters. The type and species of microorganism obviously play a major role in the resistance of an organism to radiation (54). This is tempered by the physiological state of the organism at the time of radiation exposure, which can significantly increase or decrease its resistance to the effects of radiation. Also, environmental conditions, which include the presence or lack of oxygen, state or hydration and temperature, can play significant roles in defining the levels of resistance of microorganisms to radiation (51). Finally, dose rate and in some cases fractionation of the radiation might need to be taken into account.

Microorganisms can be divided into four general groups, which include bacteria, fungi, viruses, and specs of protein called prions (54,55). Within each of these groups, there are many different species with differing resistances to the effects of radiation. Even for a given species of microorganism, different strains of the microorganism may exhibit different tolerances to radiation. Ignoring numerous exceptions and possible effects of the physical as well as radiation environment on radiation resistance, we can arrange the different types of microorganisms according to increasing resistance as follows: vegetative bacteria (gram negative are less resistant that gram positive), fungi and fungal spores, bacterial spores, viruses and prions.

Vegetative bacteria typically have very low D10 values and, in most cases, do not control the absorbed dose levels that are required to achieve the specified level of sterility for the device.

Oftentimes, these types of microorganisms do not even survive for long times in the physical environment that is present on the surface of a dry medical device. Vegetative bacteria are also particularly sensitive to the presence or lack of oxygen. In the presence of oxygen, D10 values can be reduced on average by a factor of 3 from anoxic conditions (51). One type of vegetative microorganism that has captured the attention of many people due to its possible presence as a pathogenic organism in food products is Escherichia coli, Strain 0157:H7. Its very low D10 value of 0.28 kGy makes it particularly susceptible to elimination by radiation (56).

Fungi and fungal spores respond in a similar manner to radiation as bacteria, yet they can present a broader spectrum of D

10 values principally because of the more complex

morphology and cytology of these types of microorganisms. Fungal spores usually exhibit a shoulder in their dose/survivor curve, that is, C1 shape. The width of the shoulder, which plays an important role in the magnitude of absorbed dose to achieve the desired SAL, is usually attributable to one or more of three factors. The first two factors, which are similar, include multicellular and multinuclear characteristics of fungal spores. Individual cells in a multicellular spore can function independently; thus, all cells must be disabled to disable the entire spore. It is easy to see that on the basis of hit theory, the presence of more than nucleus in a cell would also increase its resistance to radiation. The shoulder in the dose/survivor curve would, in effect, disappear if these two factors were taken into account by proper counting of the level of contamination on a medical device. The third factor that influences the shape of the dose/survivor curve is ploidy that involves duplication of the chromosomes. Cells with this property can be highly resistant to the effects of radiation. The morphology of the fungi can have a significant effect on resistance to radiation. For example, Pyronema domesticum, a fungal growth found in cotton products that go into medical products, has received considerable attention because of its high resistance to radiation (57,58). The impervious nature of its sclerotia to effects of radiation is thought to be the reason for the unusually high resistance of this fungal growth to radiation.

Bacterial spores can have very high resistance to the effects of radiation and oftentimes are the determining factor in selection of a dose to achieve a given SAL. For example, as already noted, Bacillus pumilus, a bacterial spore, served for many years as a biological indicator to test for sterility. Its use as a biological indicator has been largely discontinued today and even discouraged in recent standards that rely on testing against the natural bioburdens already present on the product (11).

Viruses are typically more resistant to radiation than bacteria and fungi principally due to their much simpler construction. However, even among viruses, structures are widely variable, ranging from simple systems with a single strand of DNA and capsid to much more complex structures. Viruses appear to respond to single-hit kinetics and thus, conform in all cases to the inactivation relationship given by Equation 910 (48(. Viruses are known to be sensitive to heat. Therefore, synergism from the simultaneous application of heat and radiation may be beneficial in lowering the absorbed dose needed to achieve sterility (59).

16

Prions are a fourth class of contaminants that have recently come to the attention of people interested in sterility. The 1997 Nobel Prize in medicine was awarded to the discoverer of these elemental structures, and infectivity by prions was a focal subject at the 1998 Kilmer Conference (55). We can expect that these specs of protein, which appear to be void of nucleic acid, are highly resistant to the effects of radiation.

Chapter 5. Gamma IrradiatorsClassifications and regulatory aspectsTo fit the numerous applications for radiation processing in research, industry and other fields, irradiators conform to a variety of designs and methods of operation. Because of the variety of designs, four general categories of irradiators have been established to facilitate the preparation of standards (60-63). Category I irradiators are self-contained, dry source storage irradiators. In this type of irradiator, the sealed source is completely contained in a dry container that is constructed of solid materials. The sealed sources are shielded at all times. Human access to the sealed source and the volume undergoing irradiation is not physically possible in its designed configuration. Category II irradiators are panoramic, dry source storage irradiators. In this type of irradiator, human access to the irradiation chamber is allowed whenever the shielded source, which is contained in a dry container that is constructed of solid materials, is not in use. The shielded source is exposed within the irradiation chamber that is rendered inaccessible during the use by an entry control system. Category III irradiators are self-contained, wet source storage irradiators. In this type of irradiator, the sealed source is contained in a storage pool (usually water), and the source is shielded at all times. Human access to the sealed source and the volume undergoing irradiation s physically restricted. Category IV irradiators are panoramic, wet source storage irradiators. In this type of irradiator, human access to the irradiation chamber is allowed whenever the sealed source is fully shielded in the storage pool. The sealed source is exposed within the irradiation chamber that is maintained inaccessible during use by an entry control system.

Nuclear regulatory bodies have structured rules and regulations for design and safe operation of irradiators around these categories of irradiators. In the United States, licenses and radiation safety requirements for Category II, III, and IV irradiators are codified in the Code of Federal Regulations, Title 10, Part 36 (22). Category I irradiators are not included in the rule principally because they are self-contained, and licensees who purchase this type of irradiator usually do not participate in their design and manufacture. For these reasons, this type of irradiator is licensed under the general requirements for Title 10 Part 30.33 of the Code of Federal Regulations rather than Part 36. The International Atomic Energy Agency has also issued a series of publications on safe design and operation of the irradiator. For example, irradiators must be designed to withstand the effects of natural disasters such as earthquakes and tornadoes and have redundant safety systems and

radiation monitoring equipment to eliminate any possibility of radiation exposure to its workers. When operated in conformance with regulatory guidelines, irradiators have shown a consistent track record of safe operation.

Different types of irradiatorsCategory III self-contained, wet source storage irradiators are not designed for routine processing of large volumes of medical device products. Logistics associated with transfer of products to the bottom of a pool of water to be exposed to the gamma sources basically limits the applicability of this type of irradiator to material studies, processing of samples, curing of plastics and alteration of material properties. Large-scale commercial irradiators that are capable of processing truckloads of medical device products on a daily basis are usually Category II or Category IV panoramic systems. These systems operate in various modes and can differ significantly in overall size. However, they all have certain common features that include a biological shield, a shield for the source, an redundant safety system and a conveyance system for moving product into and out of the irradiation room. The biological shield could be constructed form either concrete or metal, and the source shield could be a solid encasement of concrete or metal for Category II irradiators or a pool of water for Category IV irradiators. Both types of irradiators have an air exchange system to remove ozone that is generated when gamma rays interact with air in the irradiation room. Category IV irradiators use deionizers or similar systems to purify the water.

In general, panoramic irradiators can operate in either what is termed a batch mode or a continuous mode. The batch mode entails moving the product manually or with an automated conveyance system into the irradiation room, while the source is still in the shielded position. Once properly positioned, the source is removed from its shielded position, and the product is automatically conveyed around the source until it has been properly dosed. In a continuous system, product is continuously conveyed into and out of the cell through a maze, while the sources are in the exposed position. The product container can take on various forms. In some irradiators, the product is left on its shipping pallet and processed through the system, or it could be placed in a metal box called a tote or loaded into a larger container called a carrier.

Figure 5.1 and 5.2 show typical carrier and tote box irradiators that are manufactured by MDS Nordion (Kanata, Ontario, Canada). Both of these Category IV irradiators employ a conveyor to automatically move the product units into and out of the irradiation room. The tote box irradiator shown in Figure 5.2 provides a good example of how product containers are moved around the source to uniformly dose the entire product in the tote. The product containers experience multiple passes at two different vertical levels past the source plane before exiting the irradiation room. Furthermore, to ensure uniform dosing of the product, the totes are rotated 180° after each pass by the source plane. As shown in Figure 5.2, the sources are loaded into a rack to form a rectangular array. This allows megacuries of isotope to be loaded into a single irradiator. The

Source hoists

Automaticconveyorsystem Unloading

elevator

Loading stationControlconsoleSource in rack

Storage pool

Radiationroom

Source passconveyor

Source hoist mechanism

Access hatch

Roof plug (3 pieces)

Shipping container Equipment room(air filters, compressor, deionizer, and chiller)

Controlconsole

Source rack

Storage pool

Irradiationroom

Cobalt-60 source

Product unloading

Product loading

Control console

Source storage pool

Radiation shield

Shielding door

Microswitch(top shielding plug)

Top shielding plug

Safety column

Microswitch(door interlock)

Back-up microswitches(for drawer up and drawer down)

Two microswitches(drawer down)

Three microswitches

Collar and sample chamber interlocks

Drawer, up(loading position)

Access tubeDiameter 3.5 cm (1.375 in)

Radiation shield

Shielding plug

Drain

Sample chamber

Sample chamber door

Radioactive source

Lead shielding collar(opened for loading)

17

throughput of these irradiators varies according to their design; however, they typically can sterilize medical device products at a rate of about one cubic foot of product per curie of isotope per year. Therefore, an irradiator stocked with megacuries of cobalt can sterilize large quantities of product on an ongoing basis.

As noted earlier, sizes of irradiators can vary significantly. Figures 5.3 and 5.4 show IBA/SteriGenics irradiators that where specifically designed to be small batch irradiators. The Category IV mini-cell irradiator shown in Figure 5.3 can be placed in an existing warehouse yet is capable of sterilizing up to about two million cubic feet of medical device product per year. The micro-cell irradiator shown in Figure 5.4, which is a Category II dry source storage irradiator, is capable of processing smaller volumes of high unit value products such as pharmaceuticals. A unique feature of these two designs is an integral door system what is combined with steel shielding to minimize the overall size of the cell. Irradiators even smaller than the micro-cell design shown in Figure 5.4 are available; however, they are not designed for high-throughput production applications. Two of these types of MDS Nordion irradiators are shown if Figures 5.5 and 5.6. The gamma cell in Figure 5.5 is used principally for microbiological analyses, production validation studies and calibrations, and the 3000 Elan irradiation shown in Figure 5.6 is used for treatment of blood for the prevention of graft versus host disease.

Fig 5.1. Automatic carrier irradiator (courtesy MDS Nordion).

Fig 5.2. Automatic tote box irradiator (courtesy of MDS Nordion).

Fig 5.3. Mini-cell irradtiator (courtesy of IBA).

Fig 5.4. Micro-cell irradiator (courtesy of IBA).

Fig 5.5. Gamma cell (courtesy of MDS Nordion).

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Fig 5.6. 3000 Elan Irradiator (courtesy of MDS Nordion).

In both cases, the radioactive source is loaded into the cell at the supplier’s facility after which the entire unit is shipped to the customer site. Sometimes, these irradiators are loaded with cesium-137 sources rather than cobalt-60 due to the very long half-life of the cesium-137 isotope, which reduces the frequency for replenishment of isotope to compensate for decay. When it comes time to restock the radioactive source, the gamma cell is shipped back to the supplier. This eliminates any handling of isotope by the user. It is clear from the design of both of these systems that they are Category I irradiators.

Engineering and operational aspects of gamma irradiatorsThe three basic containers that convey medical device products through and irradiator as an entity include totes, carriers, and pallets. In automated systems, product is staged at a load site on the unprocessed side of the warehouse, where it is loaded into the applicable container. In pallet systems, the entire pallet of incoming product may be loaded directly onto the system. For some batch irradiators, the product is manually transported into the irradiator cell, while the sources are in their fully shielded location and, thereupon, loaded into the containers. In automated systems, the loaded containers are transported into the cell via various conveyance systems.

Tote systems

Totes are usually constructed from aluminum alloys or stainless steel. The size of totes will vary depending on irradiator design; however, the width of the tote, which is the dimension that is perpendicular the source plane, usually does not exceed about 24 inches. This is a critical dimension that governs the uniformity of dose delivered to the product unit, because gamma rays from

the soured must penetrate through this dimension to dose all product that has been loaded into the tote. The length of the tote can vary between about 2.5 up to about 5 feet, and the height is typically about 3 feet. In automated systems, totes are transported from the load site into the irradiator cell using a floor or overhead conveyor. Floor conveyors move product using belts, pneumatic systems or live rollers. Overhead conveyors may employ a chain or pneumatic system to move product. When the overhead conveyor is chain driven, the totes are usually loaded onto a carrier. Chain conveyors operate on a power-free principle, wherein drive dogs pull the carriers through the system until the carriers are detached from the drive dogs by pneumatically driven stop units.

The irradiator shown in Figure 5.2 uses a floor conveyor to transport product into the cell. As shown in the figure, this irradiator is a two-level system, wherein elevators are used to transfer totes vertically from the bottom level to the top level. Figure 5.7 is a schematic showing a top view and end view of the totes as they appear around the rectangular source place. It is noted in the end view shown in Figure 5.7 that the totes extend vertically above and below the source plane. This source/product configuration is referred to as a product overlap system. It allows product to be uniformly dosed in the vertical direction by exposing the top half of the tote on the bottom shelf and the bottom half of the tote on the top shelf. The product overlap geometry also allows a greater percentage of the gamma rays that are emitted isotropically from the source to be absorbed in the product rather than escaping the system to be absorbed in the floor, ceiling and walls of the cell. As shown in Figure 5.7, the flow of totes around the source plane is designed so that each side of the tote faces the soured plane for an equal period of time. This ensures that dosing across the width of the tote is uniform. The double pass of totes on each side of the source plane also enhances the intrinsic efficiency of the system by allowing more of the gamma rays to be absorbed in the product rather than escape from the system. To further enhance system efficiency, some multi-pass irradiators have been designed with eight or more passes; however, this is done at a sacrifice in turn time of product runs. On the other hand, in those cases where turn time is the critical parameter, irradiators have been designed where totes only make a single pass on each side of the source plane, albeit at a lower intrinsic efficiency. Single-pass designs have an advantage over multi-pass designs in those cases where runs with disparate processing parameters are irradiated on a routine basis. In a single-pass design, a run can be quickly flushed from the cell to allow the processing of parameters to be changed to accommodate a subsequent run.

Some tote irradiators have three rather than two vertical levels. These systems are designed for uniform dosing of product and for achieving high system efficiency. In this type of irradiator, totes are typically moved via a floor conveyor to a transfer sigh that is exterior to the cell. At the transfer site, the totes are loaded onto an overhead conveyer, which transfers them into the irradiator cell. The motion of the carriers past the source planes is depicted in Figure 5.8. As seen from Figure 5.8, the irradiator has two source planes rather than one. This

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Fig 5.7. Product flow-tote box irradiator. Fig 5.8. Product flow-three-level irradiator.

configuration allows product to be transported past the source planes while equally dosing both sides of the totes without the use of product rotation devices within the cell. Three-level systems as well as two-level systems operate in what is termed a shuffle-dwell mode. Totes or carriers shuffle to a location in the cell where they accumulate in rows on each side of the soured plane(s). The product units dwell in these locations for a preset cycle time after which they shuffle to a new location where the process is repeated until the irradiation process is completed.

Carrier systems

Carriers are usually constructed from the same types of materials as totes, that is, aluminum or stainless steel. In large-scale irradiators that typically process millions of cubic feet of product per year, carriers are typically designed to be about three times the volume of a tote. The footprint of the carrier is about the same as that of a tote, but its vertical dimension can be up to nine or 10 feet in height, which is about three times the height of a tote. Insofar as motion of carriers past the source, they follow similar patterns to that of totes. Carrier irradiators are also designed as single or multiple-pass systems and operate in a shuffle-dwell mode. The carrier system shown in Figure 5.1 is a multi-pass irradiator. Unlike tote irradiators, the source plaque is designed to extend above and below the product. This source/product geometry which is shown schematically in Figure 5.9, is called a source overlap design. Because the source overlaps the product, to get uniform dosing of product in the vertical direction of the carrier, the source needs to be graded. This is done by placing more isotope at the top and bottom of the source plaque than new the vertical

centerline. To maintain reasonably high system efficiencies, the standoff distance of the carriers from the source is usually much less the standoff distance of carriers from the source is usually much less than the standoff distance for product overlapping irradiators.

Fig 5.9 Source overlap irradiator design.

Carrier systems in which the size of the product unit is smaller than the size of a standard tote have been designed. This type of irradiator is designed to process smaller volume, high unit value products such as pharmaceuticals. The micro-cell irradiator that is shown in Figure 5.4 is an example of this type of design. The source is configured in a cylindrical geometry, and the carriers revolve around the source so that in the completion of a full revolution, all four sides of the carrier have been exposed to the source for equal periods of time. The efficiency of this type of irradiator is much less than that of a large-scale system, but this is done for the benefit of achieving uniform dosing of the product, which is important for some types of products that are sensitive to the effects of irradiation.

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Pallet systems

A major advantage of pallet irradiators resides in the fact that many of the material handling aspects of the process are eliminated, because the product can be loaded in its entirety onto the system. This not only simplifies product count and tracking aspects of the process, it is particularly advantageous in those regions of the world where labor costs are high. However, due to the large size of a pallet compared to a tote or carrier, it ma