COMPUTER ESTIMATION OF DOSAGE OF INTERSTITIAL AND INTRACAVITARY IMPLANTS.

Abstract

THE ADVANTAGES of intracavitary and interstitial radiation therapy have been long acknowledged. In both oral and pelvic carcinomas, which are a considerable fraction of the curable lesions seen by the radiation therapist, the most significant therapeutic contribution toward control is exemplary intracavitary or interstitial radiation therapy (1–6). Neither peroral nor transvaginal intracavitary teletherapy, nor the interstitial injection of colloidal radioactive suspensions has been shown to be other than a poor substitute for carefully planned and executed intracavitary applicator or interstitial needle treatment of the same conditions. In spite of technical advances, i.e., afterloading (7), Ir192 (8), etc., the use of these forms of radium or isotope therapy has not progressed commensurately with other advances in radiation therapy. This is most probably because of the therapist's difficulty in learning the technics and dose distribution of implants without subjecting a number of patients to less than adequate therapy while expert skills and judgment are being acquired. While there seems to be a marked swing back to interstitial and intracavitary therapy from dependence on external radiation alone (9–11), it is probable that this transition to the methods and the concepts inherent in short focal distance therapy (and thus implicit in intracavitary and interstitial treatment) would be more rapidly achieved if a critical evaluation of implants were possible. This should permit correlation of dosage with successor failure to assist in training the beginner and in correcting the misapplication of the expert. Because of the difficulty and confusion concerning the dose distribution in the region of an intracavitary device or in the volume of the sources of an interstitial application, the need for meaningful expression of dosage estimation has been without adequate solution until recently. This need is concerned with the cases undergoing treatment, with the planning of future cases, and, more particularly, with the instruction and teaching of the art and technics of interstitial and intracavitary radiation therapy. Though it is simple to remember that if the dose at 1 em from a point source is 80 roentgens per hour, the dose at 2 em is 20 roentgens per hour, and the dose at 4 em is 5 roentgens per hour, it is difficult to convey in a meaningful form the inhomogeneity of dose that occurs in the region of an applicator consisting of multiple linear sources. The tremendous difficulty of imagining this dose distribution arises from the inability to think “inverse square.” In a given radium application, it is not hard to demonstrate by rapid calculations that the dose at two points may vary by a factor of two or more, resulting in overirradiation of one region and underirradiation in the other. It is not easy to recognize this difference from looking at an anterior and lateral view of the implant unless a fairly sophisticated understanding of dose distribution has been developed.