TESTICULAR DOSES IN DEFINITIVE RADIATION-THERAPY FOR LOCALIZED PROSTATE-CANCER

Purpose: To measure the dose received by the unshielded testes during a conventional course of 18 MV photon radiotherapy for localized prost ate cancer and to identify the factors influencing it. Methods and Mat erials: For each of four patients, a wax block containing thermolumine scent chips was attached to the posterior aspect of the scrotum in clo se proximity to the testes on each day of treatment during a full cour se of radical radiotherapy, and dose measurements were obtained. The d istances between the thermoluminescent chips and the beam edge were ve rified by measurement from port films. The accuracy of the in vivo mea surements and the factors influencing the testis dose were studied usi ng a phantom arrangement. Six factors were considered: (a) the relativ e contributions from primary and scattered radiation, (b) the amount o f buildup required for the thermoluminescent chips that monitored test is dose, (c) the position of the testes within the scrotum, (d) field size, (e) distance from the held lower border, and (f) the effect of p ort films. Results: Median daily doses to the testes in four patients ranged from 5 to 7 cGy. Daily doses for the four patients ranged from 4 to 14 cGy. The total dose to the testes over the full course of ther apy ranged from 1.8 to 2.4 Gy. The daily dose depended primarily on th e distance from the field lower border. This was increased by approxim ately 2.5 cGy when a 6 MV port film was taken, The relative contributi ons from primary and scattered radiation were found to be similar. Dos e measurements at the posterior aspect of the scrotum overestimated th e testis dose by approximately 15%. Conclusion: The most important fac tors influencing the dose received by the testis are the distance from the testes to the field lower border and the occasion of a port film. A knowledge of the number of port films and the average distance from the field lower border to the testes allows a reasonable prediction o f testes dose without daily measurement.