Experience and uncertainty analysis of CT-based adaptive radiotherapy for abdominal treatments

Background: Online adaptive radiotherapy (ART) allows for daily replanning of treatment plans with adjustments according to current day anatomy. The purpose of this work is to present our methodology for using CT-based ART applied to abdominal cases along with our experience with this treatment. We additionally aim to estimate some of the uncertainties associated with the adaptive process. Methods and Materials: Analysis was performed on patients with abdominal targets (N = 41, 205 fractions), treated on a CT-based adaptive treatment unit; treatment sites were divided into 3 categories: pancreas, liver, and other (e.g., lymph nodes). Statistics regarding contouring time, planning target volume (PTV) coverage, and organ-at-risk (OAR) sparing are presented. Contouring uncertainty was estimated by expanding critical OARs and recalculating dose, and auto-registration uncertainty was estimated by adjusting the registration between the cone beam computed tomography scan and the dose cloud and recalculating dose. Results: Coverage for the planning optimization PTV (PTV_Opt) for adaptive plans was on average 94.7 ± 0.4 %, while for scheduled plans it was on average 92.0 ± 0.6 %. The average decrease in OAR maximum dose by using the adaptive plans was 11.6 ± 1.0 %. Contouring time was on average 23 ± 0 min. Uncertainty estimates for PTV V100% were on average 0.6 ± 0.4 %; combined uncertainties for maximum OAR dose were on average 4.6 ± 0.4 %. Conclusion: Adaptive therapy on average led to plans with improved PTV coverage or OAR sparing, and our workflow allowed for treatment to be completed within a reasonable timeframe. The benefit of adaptive therapy largely outweighed estimates of uncertainty.