Residency · Residency · Nuclear Medicine
Occupational Radiation Safety and ALARA
Introduction
Occupational radiation exposure is an inherent aspect of nuclear medicine practice. The ALARA principle (As Low As Reasonably Achievable) is both a regulatory requirement and a professional obligation, mandating that radiation exposures to workers be minimized through practical means without compromising patient care. Understanding dose limits, monitoring strategies, protective measures, and the regulatory framework governing occupational radiation safety is essential for all nuclear medicine personnel.
Regulatory Dose Limits
Annual Occupational Dose Limits (10 CFR 20.1201)
The total effective dose equivalent (TEDE) limit is 50 mSv (5 rem) per year. The lens of the eye limit is 150 mSv (15 rem) per year. The skin or extremity limit is 500 mSv (50 rem) per year. The individual organ or tissue limit is 500 mSv (50 rem) per year. The ICRP 103 recommendation of 20 mSv per year averaged over 5 years has not yet been adopted by the NRC.
| Category | Annual Dose Limit | Notes |
|---|---|---|
| TEDE (whole body) | 50 mSv (5 rem) | NRC limit; ICRP recommends 20 mSv/yr averaged over 5 yr |
| Lens of the eye | 150 mSv (15 rem) | ICRP recommends 20 mSv/yr |
| Skin or extremity | 500 mSv (50 rem) | Shallow dose equivalent |
| Individual organ/tissue | 500 mSv (50 rem) | — |
| Declared pregnant worker | 5 mSv (500 mrem) total | Entire gestation; evenly distributed |
| Minors (<18 years) | 5 mSv (10% of adult) | Applies to all categories |
| Public | 1 mSv (100 mrem) | Per year |
Special Populations
A declared pregnant worker has a limit of 5 mSv (500 mrem) for the entire gestation period, with efforts to distribute the dose evenly over the pregnancy. Minors under 18 are limited to 10% of adult limits, which is 5 mSv TEDE per year. Members of the public are limited to 1 mSv (100 mrem) per year. Declaration of pregnancy is voluntary, and the worker may revoke it at any time.
ALARA Investigation Levels
Level I is triggered when exposure exceeds 10% of the applicable dose limit, requiring RSO review and documentation. Level II is triggered when exposure exceeds 30% of the applicable dose limit, requiring investigation by both the RSO and management. Actions may include reassignment of duties, additional training, or implementation of engineering controls.
Sources of Occupational Exposure in Nuclear Medicine
Radiopharmaceutical Handling
Dose preparation, including drawing, measuring, and assaying radiopharmaceuticals, is the highest-exposure task in nuclear medicine. Syringe handling during injection contributes significant hand dose. Tc-99m, with its 140 keV gamma, is the most commonly handled isotope. F-18, with its 511 keV annihilation photons, presents a greater shielding challenge due to its higher energy. I-131, with its 364 keV gamma and beta emission, poses additional risks because it is volatile, creating inhalation and contamination hazards.
Patient-Related Exposure
Proximity to injected patients during positioning and imaging is a significant source of exposure. PET patients injected with F-18 or Ga-68 produce higher exposure rates due to the 511 keV photons. Therapy patients receiving I-131 or Lu-177 pose exposure risks during and after treatment. Nursing care of hospitalized therapy patients requires particular attention to radiation safety.
Principles of Radiation Protection
Time, Distance, Shielding
Minimizing the duration of exposure reduces total dose; practicing procedures before working with radioactivity improves efficiency. Distance is governed by the inverse square law, meaning that doubling the distance from a source reduces exposure by a factor of 4. Appropriate shielding materials are selected based on the radiation type and energy.
Shielding Materials
Lead is effective for gamma rays from Tc-99m and I-131 and is used in syringe shields, L-blocks, and aprons. Tungsten is denser than lead and is preferred for high-energy PET syringe shields. Lead glass is used for viewing windows and protective eyewear. Plexiglass or acrylic is used for beta-emitting isotopes such as P-32 and Y-90 and should be placed before lead shielding to prevent bremsstrahlung radiation. Syringe shields reduce hand dose by 80 to 95% depending on the isotope and shield thickness.
Engineering Controls
Fume hoods with charcoal filters are required for volatile iodine preparations. Automated dispensing systems reduce technologist exposure. Remote afterloading is used for brachytherapy sources. Designated hot labs must have appropriate ventilation and shielding.
Personnel Dosimetry
Types of Dosimeters
Optically stimulated luminescence (OSL) dosimeters are the most common whole-body monitor. Thermoluminescent dosimeters (TLD) are used as ring badges for extremity monitoring. Electronic personal dosimeters (EPD) provide real-time dose readout and are particularly useful during training. The whole-body badge is worn at collar level (or under the apron if one is worn), and the ring badge is worn on the dominant hand.
Monitoring Requirements
Personnel monitoring is required when a worker is likely to receive more than 10% of any annual dose limit. Badges are exchanged monthly or quarterly. Records are maintained for the duration of the license and are available to workers upon request. Dosimetry processing laboratories must have NVLAP accreditation.
Bioassay
Bioassay is required for workers handling volatile radioiodine (I-131). Thyroid bioassay must be performed within 72 hours of handling therapeutic quantities. The action level is thyroid uptake greater than 0.02 microcuries of I-131. Whole-body counting may be indicated in specific circumstances.
Contamination Control
Prevention
Gloves, a lab coat, and closed-toe shoes are worn when handling radioactive materials. Absorbent pads are placed on work surfaces. Eating, drinking, and applying cosmetics in radioactive material areas are prohibited. Proper labeling and segregation of radioactive waste are maintained.
Detection and Decontamination
Routine surveys are performed with GM counters or sodium iodide detectors. Wipe tests for removable contamination are analyzed in a well counter or liquid scintillation counter. Decontamination uses mild soap and water; abrasive scrubbing is avoided because it may drive contamination deeper into the skin. All contamination events and corrective actions are documented.
Pregnant Worker Considerations
Declaration of pregnancy is voluntary and must be made in writing. Once declared, the fetal dose limit of 5 mSv for the gestation period applies. The RSO reviews the worker's duties and exposure history and may reassign tasks. A two-badge system is recommended, with one badge at the collar and one at the waist under any shielding. Monthly dosimetry review is performed for declared pregnant workers.
Clinical Pearls
The three fundamental principles of radiation protection -- time, distance, and shielding -- should be applied in combination to minimize occupational dose in every clinical scenario.
PET radiopharmaceuticals (F-18, Ga-68) produce 511 keV photons requiring tungsten syringe shields, as standard lead shields provide substantially less attenuation at these energies.
Thyroid bioassay is mandatory within 72 hours for workers handling therapeutic quantities of volatile I-131 to ensure no significant internal contamination has occurred.
Declaration of pregnancy is always voluntary. Once declared, the fetal dose limit of 5 mSv for the entire gestation period applies, and work duties may be modified accordingly.
References
- U.S. Nuclear Regulatory Commission. "10 CFR Part 20 -- Standards for Protection Against Radiation." NRC Regulations.
- NCRP Report No. 180. "Management of Exposure to Ionizing Radiation: Radiation Protection Guidance for the United States." 2018.
- Siegel JA, et al. "Occupational Radiation Safety in Nuclear Medicine." J Nucl Med Technol. 2019;47(3):169-176.
- ICRP Publication 103. "The 2007 Recommendations of the International Commission on Radiological Protection." Ann ICRP. 2007;37(2-4):1-332.