Radiation Protection and Radiobiology
This domain tests three linked ideas: the regulatory dose limits that define acceptable exposure, the practical tools (time, distance, shielding, collimation, filtration) that keep doses ALARA, and the radiobiology that explains why low-dose exposure still carries stochastic risk. Items usually give a scenario and ask for a dose, a limit, or a biologic effect.
Dose limits (NCRP Report No. 116)
- Occupational effective dose: 50 mSv (5 rem) per year; lens 150 mSv; skin and extremity 500 mSv.
- Public: 1 mSv per year. Declared embryo/fetus: 0.5 mSv in any month and 5 mSv over the gestation.
- Students under 18: 1 mSv per year.
- Limits apply to occupational exposure only; they are not harm thresholds and do not govern medical exposure of the patient.
Practical protection
- Time: dose is proportional to exposure duration.
- Distance: inverse square law, I1/I2 = d2 squared / d1 squared.
- Shielding: apron at least 0.5 mm Pb equivalent at 100 kVp, thyroid shield, leaded glasses; primary barriers protect high-workload walls and floors.
- Restriction of the beam lowers patient skin dose and scatter; filtration removes low-energy photons (total at least 2.5 mm Al at 70 kVp and above).
Radiobiology the exam rewards
- Stochastic effects (cancer, hereditary) have no threshold; probability rises with dose while severity does not.
- Deterministic (tissue-reaction) effects have thresholds: transient erythema around 2 Gy, cataracts from roughly 0.5 Gy and lower with fractionation, sterility at higher doses.
- Bergonie and Tribondeau: radiosensitivity rises with mitotic rate and undifferentiation, and falls with maturity. Cell-cycle sensitivity order is M, G2, G1, S.
- Four Rs of fractionation: repair, repopulation, redistribution, reoxygenation. High-LET radiation (alpha particles, neutrons) has a higher RBE and lacks an oxygen enhancement ratio.
Worked example
A technologist stands 1.0 m from a patient during a portable abdomen and measures 0.08 mGy of scatter. Moving to 2.0 m gives 0.08 x (1.0/2.0) squared = 0.02 mGy, a 75% reduction. Halving fluoroscopic time as well brings the total to 0.01 mGy. Reasoning: distance follows the inverse square, time follows linearly, and the two factors multiply.
Traps
- Mixing sievert and rem (1 Sv = 100 rem; 1 mSv = 0.1 rem).
- Assuming a lead apron stops the primary beam; at diagnostic energies it attenuates scatter, not a direct exposure.
- Labeling cataracts or erythema as stochastic, or cancer as deterministic.
- Forgetting that collimation reduces dose rather than raising it.