Effect of radiation protraction in hypofractionated radiotherapy.

Purpose: To evaluate how protracted delivery of radiation affects radiobiological properties of hypofractionated radiotherapy.

Methods: The utilized approach is based on the concept of biologically effective dose (BED). The linear-quadratic model replete with a protraction factor is used to describe changes in biologically effective dose in normal tissue (BED ) caused by varying number of fractions under the condition of fixed BED in the treatment target (BED ). In the derivations, we study the influence of fraction time (T) and associated repair of sublethal damage of irradiated cells on BED .

Results: We have analytically derived conditions under which hypofractionation lowers BED , in a parallel or serial organ at risk (OAR) in the presence of radiation protraction, as compared to standard fractionation. In the considered examples, maximum value of BED in the spinal cord decreased with decreasing number of fraction when T was relatively short (e.g., T = 5 min). In contrast, in the case of long fraction times of 30 and 45 min, maximum BED in the cord increased with decreasing number of fractions. In the case of lung cancer, the average BED in the lung increased with decreasing number of fractions. The maximum increase in BED varied between 4% for T = 1 min and 19% for T = 22 min.

Conclusion: In the case when repair of sublethal damage occurs faster in the target than in the affected OAR, shortening fraction time helps lower BED in hypofractionated regimen as compared to standard fractionation. In contrast, in the case when repair rate is higher in the OAR than in the target, long fraction times can be radiobiologically beneficial for hypofractionated radiotherapy. Consequently, comparison of different fractionation schemes should take into account both repair rates in the target and OAR, and fraction time.