Dose outside the treatment field for external irradiation with high-energy X-ray reduction of the dose of the remaining testis in postoperative irradiation of seminoma

Abstract

This study aimed to decrease the radiation dose to the disease-free testis in postoperative irradiation for seminoma patients. We consider the factors influencing the peripheral dose (PD) of 10MV X-ray radiotherapy to be the distance between the caudal edge of the irradiation field and the measuring point, the size of the therapeutic irradiation field, the thickness of the lead shield laid above and lateral to the disease-free testis, and the thickness of the lateral absorber. We measured the scattering radiation dose coming from the accelerator head and that due to irradiation volume. We measured these doses using a testicular phantom as the non-diseased testis. Scattering radiation from the accelerator head mainly contributes to PD, whereas the larger the size of the irradiation field the more the scattering radiation from the irradiation volume contributed to PD. PD changed more at the surface of the phantom than at its center. PD at the testicular phantom could be reduced to less than 1% of the therapeutic dose when it was situated more than 5cm distant from the caudal limit of the irradiation field, the lead shield above the testicular phantom was 7.5cm thick, and the lateral lead shield was 2mm thick. PD is influenced by many factors. It is necessary to clarify the change in PD at the testicular phantom, and it is important to limit the caudal edge of the irradiation field and to lay the lead shield for the attenuation of radiation on the disease-free testis.