In radiobiology, how is a stochastic effect defined?

• A. When a biological response is based on the probability of occurrence rather than the severity of the change.
• B. It requires a defined dose threshold.
• C. It is immediate and dose-related.
• D. It affects only skin tissue.

Answer: (A)

Stochastic effects are defined by their probabilistic nature: radiation exposure changes the chance that an adverse effect will occur, but does not predict how severe that effect will be in an individual. Classic examples are cancer and heritable mutations, where the likelihood increases with dose, but once the effect happens, the severity is not determined by the dose. There is no safe threshold for these effects, which is why they’re described in terms of probability rather than a fixed dose level triggering a definite outcome. This is why the correct statement emphasizes that the response is based on the probability of occurrence rather than the severity. Deterministic effects, by contrast, have a threshold and show increasing severity with dose, and stochastic effects can involve many tissues, not just skin.