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 nt ) caused by varying number of fractions under the condition of fixed BED in the treatment target ( BED tar ). In the derivations, we study the influence of fraction time ( T ) and associated repair of sublethal damage of irradiated cells on BED nt . Results We have analytically derived conditions under which hypofractionation lowers BED nt , 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 nt 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 nt in the cord increased with decreasing number of fractions. In the case of lung cancer, the average BED nt in the lung increased with decreasing number of fractions. The maximum increase in BED nt 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 nt 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.