CT-Based Online Adaptive Prostate SBRT Improves Target Coverage and Reduces Rectal Dose

Abstract

<h3>Purpose/Objective(s)</h3> Stereotactic body radiation therapy (SBRT) is an emerging treatment modality for clinically localized prostate cancer (PCa). Online daily adaptive radiotherapy (ART) could potentially improve the therapeutic ratio of prostate SBRT by accounting for inter-fraction variation in target and OAR volumes. To-date no group has evaluated the clinical utility of a novel AI-augmented CT-based ART system for prostate SBRT. In this study we hypothesized that adaptive prostate SBRT plans would result in improved target coverage and lower dose to OARs versus the unadapted treatment plan. <h3>Materials/Methods</h3> Eight patients with unfavorable intermediate or high-risk PCa treated with 5-fx whole-pelvis prostate adaptive SBRT were reviewed. Patients were treated to the elective nodes (2500 cGy) with simultaneous boosts to the prostate/seminal vesicles (3625 cGy) and prostate (4000 cGy). 4 of 8 had rectal hydrogel spacer. For each Fx, CBCT was acquired and OARs (rectum, bladder, bowel, sigmoid, femurs) were segmented/deformed using AI. CTVs were rigidly registered. Volumes were adjusted manually and PTV expansions added. Adaptive treatment plans were developed based on the contoured targets and OARs and dose to these volumes for the adapted vs. initial plans were compared. The V100 (% PTV receiving prescription dose) and the D0.03 cc between scheduled and adapted treatment plans were compared using a Student's t-test, with significance threshold of <i>P<0.05</i>. <h3>Results</h3> Eight patients successfully completed 40 Fx's of adaptive RT as scheduled. Daily adaptation resulted in a statistically significant mean improvement in PTV V100 for all targets: [11.9% ± 2.1% for PTV 4000 (p <0.0001); 7.9% ± 1.5% for PTV 3625 (p <0.0001); and 6.1% ± 1.6% for PTV 2500 (p= 0.0007)]. Mean rectal D0.03 was significantly reduced by 45.4 cGy ± 10.8 cGy (p= 0.0002) per fraction (227 cGy/5 fractions) compared to the initial plans. There were no statistically significant differences between scheduled and adapted D0.03 to the bladder, sigmoid, or bowel (Table). No patients experienced acute CTCAE grade ≥3 GI/GU adverse events (median F/U 9.5 months). All statistically significant differences were maintained in the presence and absence of rectal hydrogel spacer. <h3>Conclusion</h3> CT-based online adaptive SBRT resulted in statistically significant and clinically meaningful improvements in PTV coverage and D0.03cc dose to the rectum. A trial evaluating CT adaptive prostate SBRT is underway.