PURPOSE: The influence of the dose-rate on tumor response in radiotherapy with 106-Ruthenium eye plaques has been investigated in an experimental tumor system. MATERIALS AND METHODS: The identical total dose was applied within three different overall treatment times (42 hr, 192 hr and 312 hr), corresponding to dose-rates of 6.0 Gy/hr, 1.3 Gy/hr and 0.8 Gy/hr. RESULTS: The therapeutic outcome of brachytherapy varied significantly between the three groups of animals treated with different dose-rates. At a dose-rate of 1.3 Gy/hr all tumors were locally controlled, but no local control was observed when a dose-rate of 6.0 Gy/hr was delivered. 0.8 Gy/hr was less effective than 1.3 Gy/hr but more effective than 6.0 Gy/hr. CONCLUSION: These results were unexpected but they might be explained by an incomplete reoxygenation if the overall treatment time is too short (42 hr, dose-rate 6.0 Gy/hr) and by proliferation of tumor cells under treatment if the overall treatment time is too long (312 hr, 0.8 Gy/hr).
