Dosimetric comparison of linac-based modulated techniques for hippocampal sparing whole brain radiotherapy

Abstract

Intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques were compared in terms of their dosimetric quality, treatment efficiency, and delivery accuracy for hippocampal sparing prophylactic whole brain radiotherapy. Ten previously treated patients were selected for this study. All plans were prescribed to deliver 30 Gy in 10 fractions to 90% of the target volume. RTOG 0933 recommendations were applied for treatment planning. Plans were compared based on the organ at risk (OAR) sparing, homogeneity and conformity indexes, monitor unit (MU), and beam on time (BOT). Delivery accuracy of the plans was also compared. VMAT plans had better homogeneity index and conformity index than IMRT plans. In terms of hippocampus sparing, VMAT plans were superior to other plans. Since brainstem, optic nerves, and chiasm were in the PTV, their doses were nearly equal to each other for both techniques. So, there were no statistical differences between techniques. Although both eyes were not in the PTV, there was no significant dose difference between techniques. However, due to the posterior gantry angles of IMRT plans, lens doses were lower in IMRT plans than those in VMAT plans. The VMAT technique had lower MU and BOT values than the IMRT technique. In terms of delivery accuracy, VMAT plans were superior than IMRT plans. VMAT plans provide better target volume coverage, homogeneity, conformity, and hippocampus sparing when compared with IMRT plans. VMAT plans are also the best in terms of treatment efficiency since they require a much smaller number of MUs and thus a shorter treatment time than IMRT plans.