[Theoretical considerations of dose-rate effect for clinical application: a study of pulsed brachytherapy]

Pulsed brachytherapy is starting to be used for patients with various kinds of carcinoma, in the expectation of fair treatment results without increasing the late radiation-induced damage. We calculated the early and late biologically effective dose (BED) of this modality to predict early and late radiation damage based on the linear-quadratic model (L-Q model). The way to set the values of alpha/beta and mu (factor of repair) in the L-Q model remains controversial. The value of alpha/beta has been fixed at 3Gy for the late effect and 10Gy for the early effect in recent studies. In this study, we applied these alpha/beta values to calculate the BED, however, the effects of its alteration ranged from 0.5Gy through 15Gy on BED were also analysed. We assumed mu = 0.5 (T1/2 = 1-2 hours) in this study, but we also tried to examine the effects of its variation. Our results indicate that pulsed brachytherapy is a safe treatment procedure even if the dose-rate ranges from 1Gy/hr through 100Gy/hr, but the fractionation number should be increased when using a very high dose rate. Pulsed brachytherapy should be applied cautiously for tissues with a small mu value, such as the spinal cord, because an extremely high late BED would be expected in such cases.