PET Radiation Safety

Robert E. Reiman, MD, ABNM

Radiation Safety / OESO

Duke University Medical Center

Academy of Molecular Imaging

Topics to Consider

• General Regulatory / Practice Considerations

• Why is PET Different?

• External Radiation Hazards

• Measures to Reduce Personnel Dose

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

General Requirements: Records

• Shipping and Receiving

• Personnel Dosimetry

• Area Surveys

• Trash Surveys

• Public Dose Limit Compliance

General Requirements: Radiation Signs

> 100 mrem/hr> 500 rem/hr Hot Lab, Scanner Areas

General Requirements: Personal Dosimeters

Wear with the label on the palmar (inside) surface of the hand

Wear at the chest or waist

General Requirements: Survey Instruments

General Requirements: Survey Meter QA

• Meters OFF when not in use

• Operation check with each use

• Regular battery and high-voltage checks

• Annual calibration

Good Hot Lab Procedures

•Cover work surfaces•Use correct pipetting technique•Wash hands frequently

Things NOT To Do in the Lab

•Don’t Drink•Don’t Eat•Don’t Smoke•No cosmetics

Why is PET Different?

• PET radionuclides have higher Exposure Rate Constants than “traditional” nuclear medicine radionuclides.

• Photon energies are higher.

• Half-lives are shorter.

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.

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

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

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.

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

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.

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

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

FDG PET: Sources of External Radiation to Staff

• Cyclotron

• Fluoride Transport

• FDG Production

• Dose Dispensing / Calibration

• Dose Administration

• Patients

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.

Measures to Reduce Personnel Dose

• Time, Distance and Shielding

• Laboratory Technique

• Administrative and Procedural Controls

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.

Measures to Reduce Dose: Maximize Distance!

Technologists should minimize the time spent in close proximity (less than two meters) from the patient.

15

41.0

0.3 mrem/hr

0.5

1

24 meters

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).

Typical “Shadow” Shield

“Rule of Thumb: Shadow Shield provides maximum reduction of about 1 part in 400

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.

Measures to Reduce Dose: Other Techniques

Mobile Shields Syringe Shields (Tungsten and Lead Glass)

Tongs to Maximize Distance

Measures to Reduce Dose: Procedural Controls

• Automated dose dispensing and Calibration (“Unit” Dose)

• Elimination or automation of “flush” during patient administration

• Rotation of personnel

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.

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.

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.