Answer to Question #636 Submitted to "Ask the Experts"

Q

I had three head CT scans on 10/11/00 and one on 10/14/00 (total of four head CT scans in one week). How much exposure did I get in rad and what are the long-term effects of this? I am very worried that I might get cancer. Thanks.

A

The effective dose equivalent for a head and body CT exam is 111 mrem (from Bushberg, The Essential Physics of Medical Imaging, Baltimore, MD: Williams & Wilkins; 1994). Assuming this value and considering the number of CT exams, your effective dose equivalent from these studies was about 450 mrem (4x111=444 mrem). Regulations do not limit doses resulting from the intentional exposure of patients during diagnostic or therapeutic procedures. However, regulations do limit the doses that individuals may receive in the course of employment (when their job responsibilities involve exposure to radiation). These occupational dose equivalent limits have been set to prevent certain health effects like cataracts and skin reddening (which are examples of "deterministic effects") and to reduce the risk of cancer and genetic effects (which are "stochastic effects"). The annual occupational effective dose equivalent limit (to reduce the risk of stochastic effects) is 5000 mrem. Clearly your estimated dose from the CT exams (approximately 450 mrem) is well within this limit. If you would like information specific to your exams (information that would consider the specific CT scanner and the specific imaging techniques), you may want to ask your referring physician to contact the radiologist, medical physicist, or radiation safety officer (RSO) at the facility where you had your studies. You may request a dose estimate and, upon receiving that information, you may want to discuss further with these individuals the radiation risks. Just keep in mind, however, that with medical procedures, it is believed that the benefits of the diagnostic evaluation (i.e., getting the needed information about the medical condition) outweigh the risks associated with the radiation exposure. Carmine Plott, CHP, Ph.D.