Purpose: To investigate the clinical feasibility of utilizing a commercial CBCT-guided adaptive radiotherapy system (Ethos) for adaptive SBRT to the prostate and elective pelvic nodes.
Methods: A template of clinical goals was created in Ethos to deliver 40Gy to prostate (P), 36.25Gy to P and proximal seminal vesicles (P+pSV), and 25Gy to the pelvic nodes in 5 fractions with 3 full VMAT arcs, automatically optimized with an Intelligent Optimization Engine (IOE). The template was applied to five high-risk prostate patients without modification to simulate adaptive sessions, since anatomy can change but changes to Ethos planning goals are not editable during on-couch adaptive sessions. Plans were evaluated against NRG-GU009 & GU-010 constraints. Secondary calculations with Mobius3D and 3D gamma comparisons (3%/2mm criteria, 10% threshold) were evaluated.
Results: Prostate volumes ranged from 42cc to 135cc. Rectum and bladder volumes ranged from 35cc to 149cc and 206cc to 507cc, respectively. For all 5 patient plans, IOE produced a plan that met all strict OAR constraints. All other constraints were at least within acceptable variation. The mean V100%Rx was 96.1%, 95.9%, and 99.5% for PTV_4000, PTV_3625, and PTV_2500, respectively. The minimum coverage of all PTVs by prescription dose was 93.9%; PTV coverage was sacrificed when necessary to spare overlapping organs-at-risk. Mean rectum max dose was 3837cGy [range: 3658-3911] (<4120 constraint) and mean bladder max dose was 4183cGy [range: 4163-4228] (<4350 constraint). MU ratios ranged from 4.0 to 6.1 (mean 4.9). All 3D gamma values with Mobius 3D dose calculation were greater than 99.6%.
Conclusion: Clinically acceptable SBRT plans to P+pSV and pelvic nodes were generated and feasibility to successfully create adaptive plans was demonstrated. Patient-specific optimization of the template could improve plan quality with confidence in on-couch adaptive plan fidelity.