Answer to Question #4568 Submitted to "Ask the Experts"

Q

I am a clinical investigator in a bioequivalence centre. At the time of screening or clinical examination, we are taking chest x-ray exams of healthy male volunteers. After how many days can x-ray exams be done in the same individual so that the volunteer is not overexposed to x rays?

A

Spacing of standard radiographic imaging studies over a period of days has little or no impact on reducing the very small radiation risk to a patient or volunteers in a research study. This is particularly true with respect to a limited number of low-dose equivalent procedures such a chest radiographs. Clinically, it is not uncommon to see multiple radiographs taken daily on gravely ill patients in intensive care units. In the case of occupational radiation exposure, we use the calendar year as the dose equivalent accumulation period and assume the spacing within that year generally does not affect the risk assessment.
In clinical medicine, the frequency of the examination is dictated by the condition of the patient and by the physician's need to assess that condition. Failure to perform the exam, when needed for effective patient treatment, may result in greater risk than the radiation exposure. If radiographic imaging is absolutely required to establish the validity of a research study's conclusions, failure to incorporate a sufficient number, and adequate frequency, of radiographic imaging studies in the protocol, to the extent that the conclusions of the study are invalidated, removes any benefit from the research and makes all of the exposure unwarranted.

With the exception of angiography and fluoroscopy, it is very difficult, although perhaps not impossible, to cause demonstrable radiation damage, i.e., nonstochastic effect, to an organ system of a patient from medical imaging studies. This is true even in the case of multiple imaging studies of the same patient unless the number of studies done is truly excessive. More generally, the concern in medical radiographic imaging is limiting the risk of stochastic radiation effects, such as radiogenic cancers or genetic insults, in a population irradiated for medical reasons.

With regard to radiography in the United States, there are no regulatory limits for the radiation doses intentionally delivered to a specific patient for purposes of medical diagnosis, treatment, or medical research. Radiation outputs of radiographic systems may have regulatory limitations for specified conditions but those limits are not applied to specific patients. The entrance skin exposures to very obese patients undergoing radiography may realistically be expected to exceed the typical state limitations for entrance skin exposures for standard radiographic projections.

A medical provider's assessment of the appropriateness of radiation exposure to a patient requires a subjective evaluation of the potential benefit to the patient in relation to the small additional radiation risk resulting from the imaging study. In the case of medical research, society rather than the individual research volunteer may be the recipient of any realized benefit that is used to justify the radiation risk to the volunteer.

Selecting a benchmark for an appropriate radiation dose to a volunteer in a research study is subjective. Some researchers use the occupation dose equivalent limits as their benchmark for acceptable radiation risk in research studies. Once an effective dose equivalent benchmark is selected, the effective dose equivalent from the radiographic studies can be compared to it to determine the appropriateness of the number and nature of the radiographic procedures required of a volunteer.

Wayne A. Wiatrowski, PhD, FACR, CHP
Associate Professor, Department of Radiology
University of Texas Health Science Center at San Antonio

Editor's Note: Information on typical radiation doses and the quantification of risks to human subjects can be found at the RADAR Medical Procedure Radiation Dose Calculator and Consent Language Generator Web site.