Purpose: To evaluate target coverage differences between adapted and non-adapted treatment plans for patients receiving adaptive radiation therapy to their prostate and proximal seminal vesicles.
Methods: The D95 of the target structure was recorded for each session for both the adapted and non-adapted plans. All treatments were delivered on a Varian Ethos. For each treatment session contours were generated to reflect the anatomy on that day, including the target structures. The daily change in volume from baseline for the PTV, rectum, and bladder was also recorded.
Results: Of the first 204 sessions delivered to 6 different patients, the average PTV D95 of the adapted plans was 99.7% of the prescription dose while the scheduled plans had an average D95 of 88.4%. Additionally, 198 of the 204 sessions had superior target coverage with the adaptive plan over the scheduled plan. The 6 sessions in which the scheduled plan provided better coverage were all from the same patient. During these 6 sessions the bladder was, on average, 63cc smaller than at the time of simulation while the rectum was 33cc larger. Comparatively, the bladder was 24cc underfilled and the rectum 12cc larger, on average, during all other sessions for this patient. During these 6 sessions the adapted plan respected all specified normal tissue limits while the non-adapted plan exceeded the rectum and/or bowel limits.
Conclusion: While adapted plans offered better target coverage in approximately 97% of treatment sessions there were instances in which the system intentionally under-covered the target in order to respect normal tissue limits as a result of variable bladder and rectum filling. This underscores the need to maintain a reproducible setup despite having adaptive capabilities.