Radiology Physics – Deterministic effects

For radiology residents preparing for the ABR Core Examination, memorizing the endless numbers that they need to know can seem like a daunting task. One section of the test which is particularly dense with these numbers is the physics section. In preparing for the 2015 administration next week, I thought it would be nice to have a series of radiology physics related articles with the key numbers that may be tested on. To start it off, deterministic effects.

Deterministic effects are adverse effects from radiation that occur only once a certain threshold is reached. The severity of deterministic effects increases as the dose of exposure increases. Because of an identifiable threshold level, appropriate radiation protection mechanisms and occupational exposure dose limits can be put in place to reduce the likelihood of these effects occurring. Below is a graphical representation and comparison of deterministic effects in “A” and stochastic effects in “B,” which we will address a later date.

Dose response for the health effects of radiation. A: Adverse tissue reactions (deterministic effects). B: Cancer and heritable effects (stochastic effects).

The main deterministic threshold doses that radiology residents should know are listed below:

Cataracts are an ongoing area of research. The numbers previously claimed 2.5 Gy threshold does for cataracts, although some studies suggest 0.5 Gy is a better threshold. Previously it was thought that acute exposures require less overall dose to induce a cataract compared to a chronic fractionated dose; however, according to the 2011 International Commission on Radiological Protection the threshold dose for radiation-induced eye cataracts is now considered to be around 0.5 Gy for both acute and fractionated exposures.