BEAM MODIFICATION

DEVICES

Dr. Ayush Garg

Definition• Beam Modification Defined as desirable modification in the spatial distribution of radiation - within the patient - by insertion of any material in the beam path.

Why the need for beam modification?• Irregularity in patient exterior contour .

• Tissue deficit .

• Dose inhomogeneity .

• Acute normal tissue reactions.

• Detraction from overall effective treatment.

Types of Beam modification• Shielding: To eliminate radiation dose to some special parts of zone at which the

beam is directed.

• Compensation: To allow normal dose distribution data to be applied to treated zone, when the beam enters obliquely through the body or where different types of tissues are present.

• Wedge filtration: Where a special tilt in isodose curves is obtained.

• Flattening: Where the spatial distribution of natural beam is altered by reducing central exposure rate relative to peripheral.

Field blocking and shaping device

SHEILDING BLOCKSAims of Shielding:• Protect critical organs• Avoid unnecessary radiation to surrounding normal tissue• Matching adjacent fields

An ideal shielding material should have the following characteristics:• High atomic number.• High-density.• Easily available.• Inexpensive.

• Most commonly used shielding material for photons is Lead (Pb).

Choice of shielding also depends on type of beam used

SHEILDING BLOCKS• Thickness of shielding block used depends upon energy of radiation.

• Shielding material which reduces beam transmission to 5% of its original is considered acceptable.

• Half value-layer is an expression for the attenuation produced by any material.

• Half-value layer is defined as the thickness of material, which will reduce the intensity of the primary beam by 50%.

• Practically thickness of lead between 4.5 - 5 half-value layers results in 5% or less of primary beam transmission.

Custom Blocks

Material used for custom blocking is known as the Lipowitz metal or Cerrobend.

Melting point 70°C.

Density 9.4 g /cm3 at 20°C (83% of lead).

1.21 times thicker blocks necessary to produce the same attenuation.

Most commonly thickness of 7.5 cms used.

Lead, 26.70%

Bismuth, 50.00%

Cadmium, 10.00%Tin,

13.30%

Bismuth LeadTin Cadmium

Styrofoam cutter

Disadvantages• Production is time consuming• Cumbersome to use• Increase in treatment time• Production of toxic fumes while fabricating (exhaust require)• Increase setup time• Space requirement• Inaccurate daily variation (manual placement using template)

Independent Jaws/Asymmetrical jaws

• Used when we want to block a part of the field without changing the position of isocenter.

• Independently movable jaws, allows to shield a part of the field, and this can be used for “beam splitting”.

• Here beam is blocked off at the central axis to remove the divergence.

• Use of independent jaws and other beam blocking devices results in the shift of the isodose curves.

• This is due to elimination of photon and electrons scatter from the blocked part of field.

Multileaf collimator• Multileaf collimators are a bank of large number of collimating

blocks or leaves• Can be moved automatically independent of each other to

generate a field of any shape.• 40 pairs of leaves or more having a width of 1 cm or less

(projected at the isocenter).• Thickness = 6 – 7.5 cm• Made of a tungsten alloy. • Density of 17 - 18.5 g/cm3.Primary x-ray transmission:• Through the leaves < 2%.• Interleaf transmission < 3%.• For jaws 1% • Cerrobend blocks 3.5% .

Multileaf Collimators• MLC systems may have double focus or single focus leaves.• The latter are shaped to match the radiation beam along

the Y axis only as the upper end is narrower than the lower.

• Single focus leaves are also rounded at the ends.• This can lead to significant beam transmission (20%) when

the leaves abut each other.• Both designs are to ensure a sharp beam cut off at the

edge.• In order to allow fast interleaf movement, while reducing

radiation transmission a tongue and groove design is often used.

• This design in turn leads to some under dosing in the region of the tongue (17 – 25%).

Multileaf CollimatorsThe degree of conformity between the planned

field boundary and the boundary created by the MLC depends upon:

• Projected leaf width.• Shape of target volume.• Angle of collimator rotation.• The direction of motion of leaves should be

parallel with direction in which the target volume has smallest cross-section.

Multileaf CollimatorsADVANTAGES• Time for shaping and inserting of custom blocks is not required.• The hardening of beam, scattered radiation, and increase in skin doses and doses outside

the field, as seen with physical compensators is avoided.• Automation of reshaping and modulation of beam intensity in IMRT.• MLCs can also be used to as dynamic wedges and electronic compensators (2D).

DISADVANTAGES• Island blocking is not possible.• As the physical penumbra is larger than that produced by Cerrobend blocks, treatment of

smaller fields is difficult, as is the shielding of critical structures, near the field.• The jagged boundary of the field makes matching difficult.

Micro-MLC• 52 leaves(26 pairs): • 14 pairs of 3 mm up to 42 mm FS at isocenter. • pairs of 4.5 mm leaves • pairs of 5.5 mm leaves • Max. Field size: 98 mm x 98 mm • Max. distance between adjacent leaves

= 95 mm (nearly full field)

• Also available as anAdd-on Device

• MLCs with leaf widths between about 2 mm and 5 mm as mini MLCs (mMLC) and below about 2 mm as micro MLCs (μMLC).

• The mMLC allows to achieve an excellent target coverage without any hotspots, and at the same time it yields a reduced dose load for normal tissues.

In a study done by Shiu et al

Comparison of dose-volume histograms (DVHs) for the treatment of an 8.5 cm3

brain metastasis with circular fields using one or two isocenters, and an 18 field mMLC treatment. Left: target, right: normal tissue.

IMRT with an mMLC or μMLC• Studies done by Cardinale et al., Bues et al., found-

• A significant reduction of hotspots in the target volume and superior normal tissue sparing for some of the targets in comparison with conventional uniform beam techniques.

• The use of intensity modulation in SRT with a MLC seems to be a promising approach.

Principles of Intensity Modulation with a MLC

Compensators• A beam modifying device which evens out the skin surface

contours, while retaining the skin-sparing advantage.

• It allows normal depth dose data to be used for such irregular surfaces.

Compensators can also be used-• To compensate for tissue heterogeneity. This was first used

by Ellis, and is primarily used in TBI.

• To compensate for dose irregularities arising due to reduced scatter near the field edges (example mantle fields), and horns in the beam profile

CompensatorsThe dimension and shape of a compensator must be adjusted to account for :• Beam divergence.

• Linear attenuation coefficients of the filter material and soft tissue.

• Reduction in scatter at various depths due to the compensating filters, when it is placed at the distance away from the skin.

• To compensate for these factors a tissue compensator always has an attenuation less than that required for primary radiation.

• As the distance between the skin and compensator increases the thickness ratio (h’/h) decreases.

Compensators• The thickness ratio depends on:

• Compensator to surface distance.• Thickness of the missing tissue.• Field size.• Depth.• Beam quality.

• Of these, the distance is the most important factor.

• The formula used for calculation of compensator thickness is given by: TD x (τ/ρc), where TD is the tissue deficit and ρc is the density of the compensator.

• The term τ/ρc can be directly measured by using phantoms.

• The term compensator ratio is the inverse of the thickness ratio. (ρc /τ ).

CompensatorsTwo-dimensional compensators

Thickness varies, along a single dimension only.

Can be constructed using thin sheets of lead, lucite or aluminum.

Compensators• Three-dimensional compensators• 3-D compensators are designed to measure tissue

deficits in both transverse and longitudinal cross sections.

• Various devices are used to drive a pantographic cutting unit.

• Cavity produced in the Styrofoam block is used to cast compensator filters.

• Medium density materials are preferred to reduce errors.

• Various systems in use for design of these compensators are:

• Moiré Camera.• Magnetic Digitizers.• CT based compensator designing systems.

Compensating wedges:Compensating wedges are useful where the contour can be

approximated with a straight line for an oblique beam.• Three important differences between compensating wedges

and wedge filters are:1.Standard isodose curves, can be used 2.No wedge transmission factors are required.3.Partial field compensation can be done.

Set up• At the filter-surface distance calculated ≥ 20 cm. • Nominal SSD measured from a plane perpendicular to beam

axis touching the highest point in the contour.• In SAD technique the depth of the isocentre is measured from

the same elevated point only.

Beam Spoilers• Special beam modification device where shadow trays

made from Lucite are kept at a certain distance from the skin.

• Based on the principle that relative surface dose increases when the surface to tray distance is reduced.

• First used by Doppke to increase dose to superficial neck nodes in head and neck cancers using 10 MV photon beams.

• Also used in TBI to bring the surface dose to at least 90% of the prescribed TBI dose.

Wedge filters• A beam modifying device, which causes a progressive decrease in intensity

across the beam, resulting in tilting the isodose curves from their normal positions.

• Degree of the tilt depends upon the slope of the wedge filter.• Material: tungsten, brass. Lead or steel.• Usually wedges are mounted at a distance of 15 centimeters from the skin

surface.• The sloping surface is made either straight or sigmoid in shape.• A sigmoid shape produces a straighter isodose curve.• Mounted on trays which are mounted on to the head of the gantry.

Normal Field

Types of wedge systems:1. Individualized wedge .2. Universal wedge.3. Dynamic wedges4. Virtual wedges 5. Pseudo wedges• The two dimensions of wedges are important – “X” or width and “Y” or length.• All wedges are aligned so that the central axis of the beam is at the central axis of the

wedge.• If the X dimension of field is longer then we can’t use the wedge without risking a hot spot!! • Wedge angles used are: 60°, 45°, 30° & 15°.

Definition of wedge angleWedge angle : Line drawn through two points a quarter of a field size on either side of central axis which lie on the isodose contour that intersects that central axis at a 10 cm depth.

Wedge filtersThe wedge isodose angle (θ) is the

complement of the angle through which the isodose curve is tilted with respect to the central ray of the beam at any specified depth.

This depth is important because the angle will decrease with increasing depth.

The choice of the reference depth varies:10 centimeters.1/2 - 2/3rd of the beam width.At the 50% isodose curve (kV).

θ

Wedge angle (φ): the angle between the plane of the ground and the point at which the central axis crosses the 50% isodose line.

Hinge angle (Θ) = 180−2φ

Individual wedges• Individual wedges are useful in Cobalt beams • Using bigger wedges than necessary will reduce output of the machine →

increased treatment time.• The width (W) of the wedge is fixed and important. • The same wedge can however be used for fields with lesser lengths or breadths. • The wedge systems available are:• 6W ( x 15)• 8W ( x 15)• 10W ( x 15)• All systems have the following four angles 15°, 30°, 45°, 60°.

Universal wedges• Universal wedges are designed so that the same wedge can be used

with all field sizes.• This is useful as it saves time.• However not suitable for cobalt beams because of excessive

reduction of beam output with smaller fields.

• Come in one size of 20 x 30 cms (except 60°).• Wedge angles used are: 60°, 45°, 30° & 15°.

• Thus the 2 factors on which the wedge angle is chosen are:

• The hinge angle.• The wedge separation

• The wedge angle that will make the isodose curves parallel to each other and the hinge angle bisector is obtained using the equation.

• Hinge angle = 90-Wedge angle/2Θ = 90 – φ / 2

Dynamic wedge:• Treatment delivery technique in which wedged dose profile is created electronically, by the

sweeping action of an independent collimator jaw from open to closed position while the beam is on.

• Because of the collimator motion, different parts of the field are exposed to the primary beam for different lengths of time. This creates wedged dose gradient across the field.

• Dose rate is modified according to jaw position

• Dose is delivered throughout the treatment.

• In VARIAN dynamic wedge, one Y jaw moves, while other Y jaw and X jaws stand still during treatment.

Flattening filters• A beam flattening filter reduces the central exposure rate relative to that near the

edge of the beam.• Used for Linear accelerators.• Due to the lower scatter the isodose curves are exhibit “forward peaking”.• The filter is designed so that the thickest part is in the centre.• Material: copper or brass.• Penetrating power should not increase as this will alter the PDD as well as reduce the

flattening.• In cobalt beams:• The beam is almost monoenergetic.• Source emits uniform radiation all around.

Flattening filters• The beam flatness is specified at 10 centimeters.• The extent of flatness should be ± 3% along the central axis of the beam at 10

centimeters.• Should cover 80% or more of the field, or reach closer than one centimeter from

the edge.• There is usually over flattening of isodoses, near the surface. This results in

production of “horns” or hot spots.• No point parallel to the surface should receive a dose > 107% of the central axis

dose.• Because of the thinner outer rim, the average beam energy is lower at the

periphery as compared to the centre

Bolus• A tissue equivalent material used to reduce the

depth of the maximum dose (Dmax).• Better called a “build-up bolus”.• A bolus can be used in place of a compensator

for kilovoltage radiation to even out the skin surface contours.

• In megavoltage radiation bolus is primarily used to bring up the buildup zone near the skin in treating superficial lesions.

BolusThe thickness of the bolus used varies according to the energy of the radiation.In megavoltage radiation:• Co60 : 2 - 3 mm• 6 MV : 7- 8 mm• 10 MV : 12 - 14 mm• 25 MV: 18 - 20 mmProperties of an ideal bolus:• Same electron density and atomic number.• Pliable to conform to surface.• Usual specific gravity is 1.02 -1.03

BolusCommonly used materials are:• Cotton soaked with water.• Paraffin wax.Other materials that have been used:• Mix- D (wax, polyethylene, mag oxide)• Temex rubber (rubber)• Lincolnshire bolus (sugar and mag carbonate in form of spheres)• Spiers Bolus (rice flour and soda bicarb)Commercial materials:• Superflab: Thick and doesn't undergo elastic deformation. Made of synthetic oil gel.• Superstuff: Add water to powder to get a pliable gelatin like material.• Bolx Sheets: Gel enclosed in plastic sheet.

Breast Cone: • A beam modifying and directing device used for a

tangential field therapy.

Advantages:• Directs beam to the central axis of the area of interest,

where a tangential beam is applied to a curved surface.• Helps position, the patient with an accurate SSD.• Endplate provides compensation, enhances surface dose

and presses down the tissue.• Effective shielding of lungs.

Penumbra trimmers: Refers to the region at the edge of the beam where the dose-rate changes rapidly as a function of

distance from the beam axis.Types:• Transmission penumbra: Transmission through the edge of the collimator block.• Geometrical penumbra : Finite size of the source.• Physical penumbra: Lateral distance between to specified isodose curves at a specific depth (90% &

20% at Dmax). • Takes scattered radiation into account.Penumbra width depends upon:• Source diameter.• SSD.• Depth below skin.• Source to diaphragm distance (inversely)

Electron beams:• Electron field shaping is done using lead cutouts.• For a low-energy electrons (<10 MeV), sheets of lead, less than 6

mm thickness are used.• The lead sheet can be placed directly on the skin surface.• Shields can also be supported at the end of the treatment cone if too

heavy at the cost of greater inaccuracies.• Design is easier, because the size is same as that of the field on the

patients skin.• To avoid variation in output and electron scatter, jaws cannot be

used to collimate electron beams.• An electron beam cone is therefore used to provide the collimation.• A primary collimator is provided close to source – defines the

maximum field size.• A secondary collimator, near the patient defines the treatment field.

Scattering foil: • A device to widen the thin pencil beam (3 mm) of electrons.• Metallic plates of tin, lead or aluminium are used.• Disadvantages:• Beam attenuation.• Generation of bremsstrahlung radiation.• Advantages:• Less prone to mechanical errors.• Less expensive.• Requires less instrumentation.• Nowadays dual foil systems are used, which compare well with

scanning beam systems.

Conclusion:• Beam modification increases conformity allowing a higher dose

delivery to the target, while sparing more of normal tissue simultaneously.

• Megavoltage radiotherapy is better suited for most forms of beam modification due to it’s favourable scatter profile.

• However any beam modification necessitates a close scrutiny of every phase of the planning and treatment process.

THANK YOU