Radiation Safety, Dosimetry, and Regulations
Dose limits, medical events, and shielding
ALARA governs practice: minimize time, maximize distance, and shield according to photon energy. Dose rate follows the inverse-square law for a point source, so doubling distance quarters the rate; for an extended source such as a patient the approximation overestimates the benefit at short distances. Lead half-value layer is about 0.2–0.3 mm at 140 keV but roughly 4 mm at 511 keV, which is why PET shielding needs much more lead or uses tungsten.
U.S. occupational limits are 50 mSv annual total effective dose, 500 mSv to extremities and skin, and a lens limit that has historically been 150 mSv with newer standards moving toward 50 mSv. The public limit is 1 mSv; a declared pregnant worker is limited to 0.5 mSv in any month and 5 mSv over gestation; minors are limited to 1 mSv. Sealed sources are leak-tested every 6 months with a 185 Bq (0.005 µCi) wipe-test threshold.
A written directive is required for therapeutic radiopharmaceuticals and for certain diagnostics such as I-131 above a specified activity. A medical event generally means the wrong patient, radiopharmaceutical, or route, a therapeutic dose deviation of 20% or more, or a diagnostic effective dose of 0.5 Sv (5 Sv to an organ). Recordable events are a separate, lower bar.
Traps: applying the 20% therapeutic rule to a diagnostic injection; forgetting I-131 therapy release is exposure-rate based (about 0.07 mSv/h at 1 m) plus written instructions; confusing CTDIvol (per-slice) with DLP (length-integrated).
Worked example: a source reads 0.28 mSv/h at 0.5 m. By inverse square, the rate at 1 m is 0.07 mSv/h, which just meets the release criterion.