A preliminary study on the effect of modulated photon radiotherapy (XMRT) optimization for prostate cancer treatment planning

Abstract

This preliminary work compared a new optimization technique, modulated photon radiotherapy (XMRT), with IMRT in a treatment planning study for a cohort of eight prostate cancer patients. XMRT differs from IMRT in that it allows the typically fixed beam energy to vary such that the optimizer finds a dose distribution by simultaneously optimizing photon beamlet fluence and energy. Plans were comprised of a seven-coplanar beam arrangement, with IMRT restricted to 6 MV while XMRT used both 6 and 18 MV beams. Both IMRT and XMRT optimization were based on a linear programming model with partial-volume constraints implemented via the conditional variable at risk (cVaR) approach. XMRT and IMRT provided similar coverage to 95% of the target (PTV) with the prescribed dose (78 Gy), however XMRT improved the target dose homogeneity. XMRT was able to reduce the dose to a large volume (p <0.05) of the rectum, bladder, and femoral heads, particularly in the low-dose region (≤ 40 Gy). Further, XMRT provided an improvement in the high dose-region of the bladder with a lower near maximum dose, D2%, Bladder, and reduced volume receiving at least 80 Gy, V80, Bladder (p <0.05). Overall, a significant (p<0.05) decrease in certain healthy organ dosimetric parameters was observed using XMRT. This preliminary study provides an impetus for further use of XMRT in a larger cohort clinical evaluation.