Effect of radiation protraction on BED in the case of large fraction dose

PURPOSE: To investigate the effect of radiation protraction on biologically effective dose (BED) in the case when dose per fraction is significantly greater than the standard dose of 2 Gy. METHODS: By using the modified linear-quadratic model with monoexponential repair, the authors investigate the effect of long treatment times combined with dose escalation. RESULTS: The dependences of the protraction factor and the corresponding BED on fraction time were determined for different doses per fraction typical for stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT). In the calculations, the authors consider changes in the BED to the normal tissue under the condition of fixed BED to the target. CONCLUSION: The obtained results demonstrate that simultaneous increase in fraction time and dose per fraction can be beneficial for SRS and SBRT because of the related decrease in BED to normal structures while BED to the target is fixed.