Effect of dose rate in hypofractionated radiotherapy

Purpose: To evaluate how dose rate affects radiobiological properties of hypofractionated radiotherapy. Methods: This study is based on the linear-quadratic (LQ) model used to determine biologically effective dose (BED). Changes in the biologically effective dose in normal tissue (BEDnt) are studied as a function of number of fractions and dose rate under the condition of fixed BED in the treatment target (BEDtar). Results: In this study we demonstrate that compared to standard fractionation, hypofractionation can either decrease or increase BEDnt depending on the average dose rate. In the considered examples, maximum value of BEDnt in the spinal cord varies monotonically with number of fractions (N-f) when dose rate is sufficiently high so that the corresponding fraction time is much smaller than characteristic repair half-lives for malignant and normal cells. In contrast, in the case of a lower dose rate of 300 MU/min, BEDnt in the cord can vary non-monotonically with N-f. In the later case, there exists optimum number of fractions which corresponds to the minimum BEDnt. It is shown that in the case when radiation induced sublethal damage is repaired faster in the target than in the affected organ at risk (OAR), increasing dose rate helps lower BEDnt. Conclusion: We have demonstrated that, as compared to standard fractionation, hypofractionation can either increase or decrease BEDnt in the OAR depending on the utilized dose rate. Consequently, radiobiological assessment of hypofractionation should take into account dose rate as well as repair rates in the target and OAR.