The Dosimetric Effect Of Prostate Motion In Flattening Filter Free Vmat Treatments Using Extreme Hypofractionation

To evaluate the performance of two treatment planning systems (TPS), using radiotherapy beams both with and without flattening filter, for extreme hypo-fractionated prostate cancer treatment. The TPSs were evaluated regarding plan quality and robustness with respect to prostate motion. With hypofractionation, the treatment time per fraction is increased, making prostate motion more significant. Flattening filter free treatments can decrease the treatment time due to the higher dose rate, and therefore reduce the effect of prostate motion. Prostate cancer patients treated with extreme hypofractionation (7x6.1 Gy=42.7 Gy) in the experimental arm of the randomized Scandinavian HYPO-RT-PC trial were used in this study. The TPSs used were Varian Eclipse (Varian Medical Systems) and RayStation (RaySearch Laboratories). The treatment plan comparison was made using the Pareto front method. Several VMAT plans were optimized with varying importance assigned to rectal sparing and PTV coverage objectives. The rectal volume receiving more than 90%, (Rectum, V90%), of the prescribed dose was plotted as a function of the PTV under dosage, (PTV, V95%). Two different beam qualities were used on each TPS, 10MV with flattening filter (FF) and 10MV flattening filter free (FFF). All plans were made for a Varian TrueBeam accelerator. The Pareto optimal plans, where one objective cannot be improved without deteriorating the other, formed a front for each TPS and beam quality. Three plans for each Pareto front were selected for measurement from three different regions in the fronts. These were defined as follows: PTV more important than Rectum (PTV>>Rect), Rectum more important than PTV (Rect>>PTV) and the knee of the Pareto front. The latter is the point where the target coverage starts to drastically decrease without significant improvement in rectum dose. The measurements were performed using the Delta4 Phantom+ (ScandiDos) and motion was simulated using the HexaMotion device with realistic motion patterns. The measurements were evaluated with gamma analysis using the local clinical protocol at our clinic (90% pass rate at 3%/2mm). The Pareto front plan comparison showed very small differences between the two TPSs and beam qualities. Eclipse had slightly better target coverage in the PTV>>Rect region but RayStation had lower rectum doses in the Rect>>PTV region. All plans fulfilled the dose-volume constraints of the trial protocol. However, all fronts showed that the rectum dose could be decreased even more without losing target coverage. The static measurements showed 100% pass rate for all plans. For the in-motion measurements, the FF plans had a pass rate below the clinically acceptable 90% pass rate while the FFF plans were above 90%. The beam-on time was decreased from 2.5 min to 1 min. Both TPSs delivered plans with good quality for both FF and FFF beams. The FFF plans proved to be clinically acceptable even with prostate motion for both TPSs.