Photon beam energy dependent single-arc volumetric modulated arc optimization

Purpose: The purpose of this work was to present a new single-arc mixed photon (6&18MV) VMAT (SAMP) optimization framework that concurrently optimizes for two photon energies with corresponding partial arc lengths. Methods and materials: Owing to simultaneous optimization of energy dependent intensity maps and corresponding arc locations, the proposed model poses nonlinearity. Unique relaxation constraints based on McCormick approximations were introduced for linearization. Energy dependent intensity maps were then decomposed to generate apertures. Feasibility of the proposed framework was tested on a sample of ten prostate cancer cases with lateral separation ranging from 34 cm (case no.1) to 52 cm (case no.6). The SAMP plans were compared against single energy (6MV) VMAT (SE) plans through dose volume histograms (DVHs) and radiobiological parameters including normal tissue complication probability (NTCP) and equivalent uniform dose (EUD). Results: The contribution of higher energy photon beam optimized by the algorithm demonstrated an increase for cases with a lateral separation >40 cm. SAMP–VMAT notably improved bladder and rectum sparing in large size cases. Compared to single energy, SAMP–VMAT plans reduced bladder and rectum NTCP in cases with large lateral separation. With the exception of one case, SAMP–VMAT either improved or maintained femoral heads compared to SE–VMAT. SAMP–VMAT reduced the nontarget tissue integral dose in all ten cases. Conclusions: A single-arc VMAT optimization framework comprising mixed photon energy partial arcs was presented. Overall results underline the feasibility and potential of the proposed approach for improving OAR sparing in large size patients without compromising the target homogeneity and coverage.