CLINICALLY USEFUL CALCULATIONS TOF THE DOSE DISTRIBUTION FROM MULTIPLE RADIATION SOURCES.

Abstract

Carcinoma of the cervix is commonly treated by a combination of intracavitary radium and external high-energy x-ray or cobalt-60 gamma radiation. Standard values of dose are essentially preselected, i.e., 6,000 R to Point A in two seventy-two-hour radium applications two weeks apart and 3,000 R by external radiation. More description, however, is not only needed, but is essential to the understanding of what an “ideal” radium application should be and what the actual application will achieve. The radium application is designed to achieve a bell-shaped zone of irradiation by the loadings selected for the tandem and ovoids. Certain values in the determination of dose are predetermined. The total dose to be given by intracavitary radium is known, and the tolerance values for bladder and rectum can be established within limits. Almost any loading in any configuration will produce the standard dose in the three-day application. There is some difficulty in determining where this bell-shaped zone of radiation lies in relation to the anatomy. The real problem is dose distribution. Where should the standard dose be delivered and is it in fact delivered where the clinician intends ? What variation in distribution can be accepted as satisfactory? Such questions have been answered in part. The whole development of dosimetry for intracavitary radium has been toward providing more information about the distribution of zones of radiation. The Manchester System relied on predetermined loadings used with various sizes and combinations of tandems and ovoids to give an ideal dose distribution. The loadings were designed to deliver a pre-calculated dose of 1,000 R per day at a point designated as Point A. Generally, right and left Points A lie 2 cm above the level of the cervical os and 2 cm lateral from the midline. Point B is 3 cm lateral to Point A. Symmetrical applications are not always achieved, however, and in actuality the critical spacing and interrelation of the sources necessary for an ideal application are not always possible. If the symmetrical application is not achieved, the problem in dosimetry is still the determination of where the 1,000 R per day zone actually is in relation to the individual anatomy. Several points including Point A are commonly selected, and calculations are made to determine the dose rate at these points of interest. The calculation requires that the distance be determined from each source to each point for which dose information is desired. This is measured on radiographs in two projections with a correction for distortion due to magnification and requires the careful application of geometry in 3 planes. Once the distance from each source to the point of interest is known, the contribution of each source is determined by the inverse-square law. The determination of dose at even a few points is tedious and time-consuming, and the time required for many calculations soon becomes more valuable than the information obtained.