Tracking OAR Volume and DVH Variability in the Initial Cohort of Pelvic Patients Treated with CBCT-Guided, Online Adaptive Therapy

Purpose/Objective(s) We have recently commissioned and began treatment using a novel, CBCT-guided, online adaptive radiotherapy (ART) platform. We have evaluated the volumetric and dosimetric variation in our initial cohort of patients. Materials/Methods We have tracked our 6 initial online adaptive pelvis patients for 208 delivered fractions. A custom script was developed to extract volume and dosimetry data from our second check software. Target and organ-at-risk (OAR) volumes were gathered for every fraction. Structure volume variations were computed relative to the reference plan for each fraction and averaged per patient. Dose-volume histogram (DVH) data was collected for the primary treatment plan for our four prostate patients, all treated to 45 Gy in 25 fractions. The range of OAR DVH values at 20, 30, and 40 Gy and PTV V45 Gy over the primary course of each patient's treatment was computed for both scheduled and daily adapted plans. Results OAR variability was very patient dependent. Bladder volumes showed the largest variation, with most patients unable to replicate their bladder filling from simulation. Mean deviations in bladder volume ranged from -47% to +12%, with standard deviations >20% for two-thirds of patients. Rectal volume was also variable, but to a lesser extent, with mean variations ranging from -31% to +16%. DVH data demonstrates that ART produces a much tighter distribution of DVH curves compared to the scheduled plan for all structures while maintaining PTV coverage to within 1% of the reference value. The variability of V40Gy, V30Gy, and V20Gy showed reductions of 13.9%, 29.8%, 30.9% for bladder and 52.6%, 48.9%, and 24.6% for rectum relative to the scheduled plan, respectively. Using only the scheduled plan would result in reductions of up to 8% of target coverage compared to the reference plan. Conclusion We have observed considerable OAR interfraction volume variations during the course of pelvic radiation that can be highly patient dependent. Use of ART maintains adequate target coverage while reducing the variability in dose delivery to these nearby OARs. Combined with the large variability in OAR filling, we have demonstrated that ART enables clinicians to have a higher likelihood of delivering their intended doses to targets and OARs compared to standard-of-care radiotherapy.