Purpose: The Elekta Unity MR-Linac utilizes 1.5 T magnetic resonance (MR) images and software tools to create an adaptive plan prior to each patient treatment. This study aims to investigate the impact of adaptive planning on SBRT prostate treatments.
Methods: In this preliminary retrospective planning study, two adaptive planning methods were investigated for a single prostate patient following the PACE-B SBRT protocol (3,625 cGy delivered in 5 Fractions). The first method mimicked the time constraints of online adaptive planning and allowed only a single optimization attempt. The second method mimicked an offline planning approach and allowed for multiple optimization runs in an attempt to maximize the treatment plan quality. Both adaptive planning methods were then compared to a Versa HD VMAT plan calculated on the daily MRI. All dose calculations on MRI used bulk density overrides, and all fractions were added using a DVH addition technique to determine final DVH metrics.
Results: All organ-at-risk DVH metrics met the PACE protocol requirements for all planning methods evaluated, but PTV coverage was determined to be 76% in the VMAT plan which was not modified for anatomical changes. Prostate and proximal seminal vesicle coverage was 95% and 92% respectively for the VMAT plan but was 99.9% for both adaptive planning methods with PTV coverage normalized to 95% for both adaptive planning approaches. It was found that the offline adaptive plan approach yielded additional OAR sparing for the rectum and bladder compared to the online plan approach.
Conclusion: PTV coverage was significantly improved with an adaptive planning strategy, as were the prostate and proximal seminal vesicle coverage. Both the adaptive and non-adaptive VMAT plan met all OAR constraints of the PACE-B protocol, but it was found that the time constraints related to online adaptive planning may lead to sub-optimal OAR sparing.