Dose-rate in IMRT: An overlooked confounding variable?

IMRT is becoming a widely used means of delivering conformai radiation therapy. However, the time required to administer each daily treatment can be longer than with conventional treatment. Since only the b component of radiation damage is generally repairable, in situations where a-4 is low (e.g. prostate cancer), the b component is relatively large and lower dose-rates may be deleterious to tumor control. In such situations, protracted radiotherapy fractions may be sparing of late normal tissue effects but may also have diminished capacity for tumor control. We calculated the average doserates for IMRT treatment and compared these to the dose-rates in conventional radiotherapy and our treatment schema specifically designed to administer pulsed low dose-rate (PLDR) radiotherapy. Materials/Methods: During the first 6 months of implementation, 15 patients were treated in our clinic using IMRT. The time required to treat each patient was recorded. 25 other patients were treated with a treatment schema specifically designed to administer PLDR radiotherapy (aiming for 6-7 cGy/min). The time required for treatment of each cohort was compared. Treatment duration of conventional treatments were also recorded. Results: During this early phase, average duration of treatment using IMRT was 25 minutes (range 15-35 min). This translated to an average effective dose-rate of 7.2 cGy/min, with a range as low as 5.1 cGy/min. These dose-rates were much lower than conventional average dose-rates of 14-300 cGy/min, and comparable to the PLDR rates, which averaged 6.67 cGy/min. Conclusions: The calculated dose-rates using IMRT were substantially lower than in conventional radiotherapy and were comparable to the dose-rates used in our PLDR clinical investigations. The potential consequences of this diminished dose-rate on tumor control should be considered. Based on our observations, we have been selectively implementing IMRT plans that require shorter amounts of time, especially for our prostate cancer patients, until further study clarifies this issue.