Radiation Safety and Protection
The Radiation Safety and Protection domain tests whether you can apply ALARA, dose limits, and shielding logic to real therapy-department situations rather than merely recite numbers. Questions probe the three pillars (time, distance, shielding), the regulatory framework (NRC 10 CFR 20 and 35, NCRP 147/151, ICRP 103), and practical controls such as surveys, badges, posting, and source handling.
Dose limits to hold in memory
- Occupational effective dose: 20 mSv/y averaged over 5 consecutive years, never more than 50 mSv in a single year (ICRP). US NRC 10 CFR 20 still states 5 rem/y total effective dose equivalent.
- Lens: 20 mSv/y averaged (ICRP 103); skin and extremities 500 mSv/y.
- General public: 1 mSv/y. Declared pregnant worker: 0.5 mSv per month and 5 mSv over the gestation.
- ALARA is a legal requirement: dose must be as low as reasonably achievable, with economic and social factors weighed.
The three pillars, quantified
- Time: dose = dose rate x exposure time. Ir-192 decays with a 73.8-day half-life, so a current source strength matters more than the label.
- Distance: inverse square; doubling distance quarters the dose.
- Shielding: transmission B = P d squared / (W U T), where W is weekly workload at 1 m, U the use factor, T the occupancy factor. NCRP 151 design goals are 0.1 mGy/wk for controlled and 0.02 mGy/wk for uncontrolled areas.
Worked example
A 6 MV linac has W = 500 Gy/wk at 1 m, primary use factor U = 0.25, T = 1, and the barrier is 6 m from the isocenter. Unshielded weekly dose at the barrier is 500 x 0.25 x 1 / 36 = 3.47 Gy/wk. For an uncontrolled area (P = 0.00002 Gy/wk), B = 0.00002 / 3.47 = 5.8 x 10 to the minus 6. Log10(1/B) = 5.24 TVLs, and at roughly 0.33 m of concrete per TVL for 6 MV you need about 1.7 m of primary concrete. That is why primary barriers dominate vault design and why secondary barriers use leakage plus scatter instead.
Traps that cost marks
- Mixing mGy (absorbed dose) with mSv (effective dose) in shielding or badge questions.
- Using the distance to the isocenter when the question gives the distance from the barrier face.
- Applying a use factor to secondary (leakage and scatter) barriers.
- Applying occupational limits to the public or to a declared pregnant worker.