Radiation Safety, Dosimetry, and Regulatory Compliance
What the exam tests
This domain tests ALARA reasoning, dose quantities and limits, external and internal dosimetry, and the regulatory rules that govern release, records, and shipping. Calculation items often use inverse-square, half-value layer, decay, and release criteria.
Core rules
- Absorbed dose is in gray (Gy); equivalent and effective dose are in sievert (Sv). Use Sv for protection limits, Gy for deterministic tissue dose.
- ICRP occupational effective dose is 20 mSv/y averaged over 5 years, with 50 mSv in any single year; public 1 mSv/y; lens 20 mSv/y; skin/extremities 500 mSv/y. US NRC limits differ in detail, so read the stem's jurisdiction.
- ALARA controls are time, distance, and shielding. Inverse square: exposure falls with the square of distance from a point source. Shielding is described by HVL: each HVL halves the dose rate.
- Patient release after therapy commonly requires ≤0.05 mSv/h at 1 m or ≤5 mSv effective dose to another person, plus written instructions.
- Monitor external dose with TLD/OSL badges; assess internal contamination with bioassay; keep records and investigate exceedances.
Worked example
A point source gives 8 mSv/h at 1 m. At 4 m, dose rate = 8 × (1/4)² = 0.5 mSv/h. To reach 0.1 mSv/h, rearranged distance = 1 × √(8/0.1) = 8.9 m. If distance is fixed, add shielding: at 0.5 mSv/h, two HVL of lead reduce it to 0.125 mSv/h, close to the target. The reasoning shows why distance is usually cheaper than shielding but must be practical.
Traps that cost marks
- Mixing mSv and μSv, or Gy and Sv.
- Applying inverse square to an extended source or through a barrier without correction.
- Assuming one HVL of lead is adequate for 511 keV PET photons.
- Releasing a patient without documenting instructions or pregnancy status.