Purpose: To measure the dose received by the unshielded testes during a conventional course of 18 MV photon radiotherapy for localized prostate cancer and to identify the factors influencing it.
Methods and materials: For each of four patients, a wax block containing thermoluminescent chips was attached to the posterior aspect of the scrotum in close proximity to the testes on each day of treatment during a full course of radical radiotherapy, and dose measurements were obtained. The distances between the thermoluminescent chips and the beam edge were verified by measurement from port films. The accuracy of the in vivo measurements and the factors influencing the testis dose were studied using a phantom arrangement. Six factors were considered: (a) the relative contributions from primary and scattered radiation, (b) the amount of buildup required for the thermoluminescent chips that monitored testis dose, (c) the position of the testes within the scrotum, (d) field size, (e) distance from the field lower border, and (f) the effect of port 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 therapy ranged from 1.8 to 2.4 Gy. The daily dose depended primarily on the distance from the field lower border. This was increased by approximately 2.5 cGy when a 6 MV port film was taken. The relative contributions from primary and scattered radiation were found to be similar. Dose measurements at the posterior aspect of the scrotum overestimated the testis dose by approximately 15%.
Conclusion: The most important factors 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 of testes dose without daily measurement.