Which of the following situations results in the highest probability of causing radiation-induced cancer?

• A. 72 milligray (mGy) absorbed dose (D) to the stomach from beta radiation
• B. 98 milligray (mGy) absorbed dose (D) to the gonads from gamma radiation
• C. 302 milligray (mGy) absorbed dose (D) to the brain from x-radiation
• D. 17 milligray (mGy) absorbed dose (D) to the lungs from alpha radiation

Answer: (D)

Radiation-induced cancer risk depends on both how damaging the radiation is to DNA and where it deposits energy. High-LET radiation, like alpha particles, causes dense ionizations along a short path, giving it a much higher biological effectiveness per unit energy than beta, gamma, or X-rays. When alpha-emitting material is inhaled, energy is released directly in lung tissue, making the lungs particularly susceptible to cancer from even relatively small absorbed doses.

To see why this scenario tops the list, compare the effective impact of each exposure. Alpha radiation has a higher radiation weighting factor (about 20) than the other types (about 1). So the 17 mGy absorbed dose to the lungs from alpha radiation translates to a much larger effective dose than the other cases: roughly 340 mSv for the lungs, versus about 98 mSv for gamma to the gonads, 302 mSv for X-rays to the brain, and 72 mSv for beta to the stomach. This higher effective dose in the lungs reflects a greater probability of cancer induction.

In short, the combination of highly damaging alpha energy and deposition in a sensitive organ (the lungs) makes that scenario the most likely to cause radiation-induced cancer despite the seemingly smaller dose.