pet radiation safety robert e. reiman, md, abnm radiation safety / oeso duke university medical...
TRANSCRIPT
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PET Radiation Safety
Robert E. Reiman, MD, ABNM
Radiation Safety / OESO
Duke University Medical Center
Academy of Molecular Imaging
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Topics to Consider
• General Regulatory / Practice Considerations
• Why is PET Different?
• External Radiation Hazards
• Measures to Reduce Personnel Dose
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General Requirements: Annual Dose Limits
• Total effective dose equivalent to whole body: 5 rem
• Lens of eye: 15 rem
• Sum of deep-dose and committed dose equivalents to all other tissues and extremities: 50 rem
• Fetus: 0.5 rem
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General Requirements: Records
• Shipping and Receiving
• Personnel Dosimetry
• Area Surveys
• Trash Surveys
• Public Dose Limit Compliance
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General Requirements: Radiation Signs
> 100 mrem/hr> 500 rem/hr Hot Lab, Scanner Areas
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General Requirements: Personal Dosimeters
Wear with the label on the palmar (inside) surface of the hand
Wear at the chest or waist
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General Requirements: Survey Instruments
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General Requirements: Survey Meter QA
• Meters OFF when not in use
• Operation check with each use
• Regular battery and high-voltage checks
• Annual calibration
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Good Hot Lab Procedures
•Cover work surfaces•Use correct pipetting technique•Wash hands frequently
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Things NOT To Do in the Lab
•Don’t Drink•Don’t Eat•Don’t Smoke•No cosmetics
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Why is PET Different?
• PET radionuclides have higher Exposure Rate Constants than “traditional” nuclear medicine radionuclides.
• Photon energies are higher.
• Half-lives are shorter.
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Why PET is Different: Exposure Rate Constants
• The “Exposure Rate Constant” of a radionuclide is the exposure rate (roentgens per hour) measured at one centimeter from a source with activity of one millicurie.
• For positron emitters, ERC is about 6 R/hr per millicurie at one centimeter.
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Higher Exposure Rate Constants
Radionuclide ERC (R/hr/mCi at 1 cm)
Fluorine-18 6.0
Indium-111 3.4
Gallium-67 1.1
Technetium-99m 0.6
Thallium-201 0.4
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Higher Exposure Rate Constants
Radionuclide Admin. Act. (mCi)
Exp. Rate
(mR/hr at 1 m)
Fluorine-18 12.0 4.0
Technetium-99m 30.0 0.6
Gallium-67 10.0 0.4
Indium-111 0.5 0.06
Thallium-201 4.0 0.05
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Why PET is Different: Photon Energy
• Photon energy is 0.511 MeV for positron emitters.
• This higher photon energy is more difficult to shield (using lead) than “traditional” nuclear medicine radionuclides.
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Higher Photon Energy
Radionuclide TVL (mm)
Fluorine-18 13.7
Gallium-67 4.7
Indium-111 2.2
Technetium-99m 0.9
Thallium-201 0.9
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Why PET is Different: Half-Life
• The half-lives of radionuclides used in PET imaging are much shorter (minutes-hours) than those of “traditional” radionuclides (hours-days).
• This leads to cumulated doses that are lower than you might expect, given the very high ERC.
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Shorter Half-Life
Radionuclide Half-Life
Gallium-67 3.26 days
Thallium-201 3.04 days
Indium-111 2.83 days
Technetium-99m 6.02 hours
Fluorine-18 109.8 minutes
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Shorter Half-Life
Radionuclide Admin. Activity (mCi)
Cum. Dose at
1 m (mrem)
Gallium-67 10.0 26.6
Fluorine-18 12.0 5.5
Indium-111 0.5 3.9
Technetium-99m 30.0 3.3
Thallium-201 4.0 2.9
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FDG PET: Sources of External Radiation to Staff
• Cyclotron
• Fluoride Transport
• FDG Production
• Dose Dispensing / Calibration
• Dose Administration
• Patients
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Types of External Exposure
• Positrons: Non-penetrating. Most are stopped in glassware, syringes, patient; etc. However, energetic positrons have formidable ranges in air.
• Annihilation Photons: Penetrating. Energy = 511 KeV. “Tenth-value Layer” in lead is 1.37 cm.
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Measures to Reduce Personnel Dose
• Time, Distance and Shielding
• Laboratory Technique
• Administrative and Procedural Controls
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Measures to Reduce Dose: Minimize Time!
• Total radiation dose is the product of dose rate and duration of exposure.
• For a given exposure rate, less time means less dose.
• So – perform tasks quickly but safely.
• Try not to spend unnecessary time around the patient.
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Measures to Reduce Dose: Maximize Distance!
Technologists should minimize the time spent in close proximity (less than two meters) from the patient.
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15
41.0
0.3 mrem/hr
0.5
1
24 meters
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Measures to Reduce Dose: Shielding
Positrons can be stopped by 2 - 5 mm Lucite. Gammas require a high-Z material. Neutrons require high hydrogen content (paraffin or the “waters of hydration” in concrete).
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Typical “Shadow” Shield
“Rule of Thumb: Shadow Shield provides maximum reduction of about 1 part in 400
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X-ray Aprons -- No Protection at 511 KeV
100 KeV: Transmission = 4.3 %
511 KeV: Transmission = 91.0 %
The “lead” aprons used in diagnostic radiology have about 0.5 mm lead equivalent. These are protective at energies under 100 KeV, but are nearly useless against annihilation photons.
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Measures to Reduce Dose: Other Techniques
Mobile Shields Syringe Shields (Tungsten and Lead Glass)
Tongs to Maximize Distance
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Measures to Reduce Dose: Procedural Controls
• Automated dose dispensing and Calibration (“Unit” Dose)
• Elimination or automation of “flush” during patient administration
• Rotation of personnel
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Prevention of Unintentional Fetal Exposure
• Good History (includes asking direct question “Are you pregnant?”)
• Common-sense Assessment of Risk of Pregnancy (age, surgical hx, contraception)
• Beta HCG
• Cannot prevent all unintentional exposures.
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Fetal Doses (rads)
mCi Early 3 Mo. 6 Mo. 9 Mo.
FDG 10 1.0 0.63 0.35 0.30
MDP 30 0.68 0.60 0.30 0.27
Nuclear Medicine procedure doses courtesy: Russell J, Sparks R, Stabin M, Toohey R. Radiation Dose Information Center, Oak Ridge Associated Universities.
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In Summary...
• PET personnel exposures have the potential to be higher than in “standard” settings.
• Doses can be minimized by time/distance/shielding measures.
• Special administrative and engineering measures can further reduce dose.