The influence of the dose-rate of 106Ru-plaques on tumor response was investigated in an experimental tumor-host system (rhabdomyosarcome R1H of the rat). A highly significant difference in tumor response was observed between tumors treated with the identical total dose (25 Gy) but varying dose rates (1.3 Gy/h vs 6.0 Gy/h). All tumors treated with the low-dose irradiation were locally controlled, whereas only one local tumor was controlled by higher dose irradiation. These unexpected results may be explained by the short overall treatment time with high dose rates, which does not allow sufficient reoxygenation of hypoxic tumor cells. In addition to the total dose, the dose rate should be included in treatment planning for choroidal melanoma brachytherapy, and analysis of the clinical data on this topic seems to be necessary in order to develop new therapy concepts.
