Radiation risk to the patient in diagnostic radiology
The medical physicist must be able to assess the radiation risk to both patients and staff in diagnostic radiology. The medical physicist must also have an appreciation of the benefits of diagnostic radiology procedures and be able to compare these benefits and risks to other risks, e.g., the risks and benefits of radiation from an abdominal CT examination to the risks of morbidity and mortality from abdominal surgery. In order to do this an understanding of radiobiology and epidemiology, dosimetry, and audit techniques is required. The medical physicist must also be able to provide advice on these risks to staff and patients, and on strategies for radiation risk reduction.
In addition to the concept of benefits versus risk, it is important to understand relative and absolute risk, and the concept of justification. Justification is even more important when one is considering a research study involving ionizing radiation or a screening program, e.g., mammography screening.
Most concern in diagnostic radiology is focused on stochastic risk. The effective dose provides a limited method to estimate the stochastic risk, but the uncertainties in this estimate must be appreciated and communicated. The limitations of the effective dose for use in diagnostic radiology patient doses must be understood, including the variability of risk with age, dose rate, etc. Other important topics include cancer risk estimation from applying BEIR V risk data to organ doses that are usually known through Monte Carlo techniques, variations in risk based on radiography projection (eg. PA vs AP projection), typical doses and risks from medical exposures, and the contribution of population collective dose from medical exposures.
Deterministic effects, though limited in diagnostic radiology, are important since they can produce significant morbidity in patients undergoing interventional procedures and, in limited number of cases, diagnostic procedures. (See Supplemental References.)
Three groups of people require special consideration: 1) Patients who are or might be pregnant; 2) Pediatric patients; and 3) Research subjects. The medical physicist must be able to provide consultation about the risks to the fetus for a patient known to be, or that might be, pregnant. Consequently, it is necessary to calculate the potential dose to the fetus and the associated risk. Pediatric patients are not just small adults. Their sensitivity to radiation dose is higher due to the rapidly dividing cells throughout their bodies and their long life expectancy which allows significant time for the expression of stochastic effects. Research subjects require special consideration since their exposure to radiation results in risk with either no or lower than normal benefits.
Introduction to References
ICRP 103 provides the foundation for risk assessment and optimization. The Biological Effects of Ionizing Radiation Reports from the U.S. National Research Council have, for many years, provided the risk estimates which are relied upon by most medical physicists throughout the world. Software, such as that available from STUK, is necessary to determine specific organ doses from various diagnostic projections.
The New York Times article, in the supplemental references, provides insight into two situations where deterministic effects (erythema and epilation) were caused by excessively high doses of radiation. The AJR articles provide an excellent overview of deterministic effects (including color photographs), and a review of 73 cases and recommendations for minimizing dose to patients from interventional procedures.