A Method for Estimating the Average Bone-Marrow Dose from Some Fluoroscopic Examinations

The difficulties inherent in determining the x-ray dose to the active bone marrow in human beings are manifoldly increased in the case of x-ray exposure during fluoroscopic examination. In the course of fluoroscopy, a large number of discrete exposures to fields of varying size and body location are made over the region of interest. In addition, some scanning with any one of the component exposures can be expected. The particular combination and duration of exposures will, in general, vary widely from examination to examination and from one radiologist to another. There is little uniformity in the distribution of bone marrow in the regions of the body commonly studied by fluoroscopy. In order to evaluate the average bone marrow dose, therefore, it is necessary to know how the radiation is distributed in relation to the active marrow. For proper determination of radiation exposure, a system of monitoring which yields information about both total exposure and exposure distribution is required. This information must then be translated into tissue dose distribution in the body. Blatz and Epp (1) have proposed that photographic films be used for monitoring fluoroscopic examinations. A large film placed on the fluoroscopic table, interposed between the patient and the x-ray tube, will yield a record of the entrance dose distribution in terms of film density. The radiation dose is related to film density by a previously measured dose-density curve for the film and x-ray quality employed. The present paper will describe a method for relating the incident dose distribution to the average active bone-marrow dose. The method consists essentially in dividing the incident exposure field into contiguous “subfields” and then calculating a set of constants (“subfield parameters”) which will relate the average entrance dose within each of these divisions to an average marrow dose. Summation of the contributions from all subfields yields the average marrow dose for the entire examination. The subfield parameters presented in this paper were derived for application to postero-anterior exposures in a gastrointestinal series. They should, however, be applicable to other postero-anterior exposures in which the trunk of the body is involved. In this laboratory, it is intended that this scheme be used in conjunction with the film monitoring system mentioned above. For the sake of completeness, a brief discussion of the use of film is included. Film Monitoring System Kodak Commercial Film, which is available in 14 × 17-inch “Ready Packs,” was chosen for this study. It has a useful range from about 0.1 to 100 r for x-rays of 2 mm. Al h.v.l. Two films, taped side by side on the fluoroscope table to measure 17 × 28 inches, more than cover the area of interest in most examinations.