Session Item

Intensity modulated proton therapy (IMPT) for prostate cancer (PCa) is susceptible to inter- and intra-fraction organ movement. Its dosimetric impact can be magnified by hypo-fractionation. In general, proton treatment plans are robustly evaluated against setup and range uncertainty. This study proposes  a method to evaluate the dosimetric impact of inter- and intra-fraction prostate motion in ultra hypo-fractionated IMPT for PCa. Results will be compared against the general robustness evaluation approach.

One representative PCa patient treated in our medical center, presenting typical inter-fraction motion(Figure 2) based on fiducial marker registration between five consequential review CT images, was included in this study. IMPT plans were generated  using two lateral opposed beams and robust optimization settings of 3%/5mm (Raysearch, 8.99). Plans were clinically accepted when fulfilling voxel-wise worst criteria for CTV coverage using 5-mm setup and +/-3% range uncertainty, plans were accepted if the dose parameters were within max 5% deviation of constraints. Each proton beam was splitted into 14 sub-plans in accordance with energy layers and time of beam delivery (one sub-plan per second and per energy layer). Electro-magnetic detected prostate motion was summarized from 26 PCa patients treated in the department of radiation oncology of the University of Pennsylvania, and 8 maximum motion vectors (diagonal directions) covering 95% range of prostate movement were created, with max motion vector of 5.4mm. In each fraction of every scenario, a synthetic CT was created from the corresponding review  CT and randomly selected prostate motion vector with random duration of movement using deformable image registration with controlling ROIs in RayStation 7.99 Research. Figure 1 shows one example treatment simulation of prostate motion trajectory in five fractions of treatment. In total, 20 treatments were simulated based on 40 synthetic CTs. For each treatment simulation, the five resulting fractional doses were warped back and accumulated dose was presented at the planning CT.

The nominal, voxel wise worst-case and accumulated dose of the simulated scenarios is presented in Table 1. In terms of target coverage, simulated accumulated doses were higher than the voxel wise worst-case dose in 19 out of 20 simulations. For OARs, all simulated accumulated doses were lower than the voxel wise worst-case dose. The largest difference between nominal vs. voxel wise worst vs. simulated accumulated dose was found for the posterior rectal wall (1898 cGy vs. 3026 cGy vs. 2675 cGy, respectively), highlighting the importance of plan evaluation using a voxel-wise worst case evaluation.

Evaluation of ultra hypofractionated IMPT for PCa by a voxel wise worst-case approach using 5-mm setup and 3% range uncertainty is found to be robust for the anticipated target coverage and OAR dose for at least 95% of intra-fraction prostate movement found in clinical practice.