Online adaptive radiotherapy: Assessment of planning technique and its impact on longitudinal plan quality robustness in pancreatic cancer q

Background and Purpose: Planning on a static dataset that reflects the simulation day anatomy is routine for SBRT. We hypothesize the quality of on-table adaptive plans is similar to the baseline plan when delivering stereotactic MR-guided adaptive radiotherapy (SMART) for pancreatic cancer (PCa).Materials and Methods: Sixty-seven inoperable PCa patients were prescribed 50 Gy/5-fraction SMART. Baseline planning included: 3-5 mm gastrointestinal (GI) PRV, 50 Gy optimization target (PTVopt) based on GI PRV, conformality rings, and contracted GTV to guide the hotspot. For each adaptation, GI anatomy was re-contoured, followed by re-optimization. Plan quality was evaluated for target coverage (TC = PTVopt V-100%/volume), PTV D-90% and D-80%, homogeneity index (HI = PTVopt D-2%/D-98%), prescription isodose/target volume (PITV), low-dose conformity (D-2cm = maximum dose at 2 cm from PTVopt/Rx dose), and gradient index (R-50%=50% Rx isodose volume/PTVopt volume). A novel global planning metric, termed the Pancreas Adaptive Radiotherapy Score (PARTS), was developed and implemented based on GI OAR sparing, PTV/GTV coverage, and conformality. Adaptive robustness (baseline to fraction 1) and stability (difference between two fractions with highest GI PRV variation) were quantified.Results: OAR constraints were met on all baseline (n = 67) and adaptive (n = 318) plans. Coverage for baseline/adaptive plans was mean +/- SD at 44.9 +/- 5.8 Gy/44.3 +/- 5.5 Gy (PTV D-80%), 50.1 +/- 4.2 Gy/49.1 +/- 4.7 Gy (PTVopt D-80%), and 80%+/- 18%/74%+/- 18% (TC), respectively. Mean homogeneity and conformality for baseline/adaptive plans were 0.87 +/- 0.25/0.81 +/- 0.30 (PITV), 3.81 +/- 1.87/3.87 +/- 2.0 (R-50%), 1.53 +/- 0.23/1.55 +/- 0.23 (HI), and 58%+/- 7%/59%+/- 7% (D-2cm), respectively. PARTS was found to be a sensitive metric due to its additive influence of geometry changes on PARTS' sub-metrics. There were no statistical differences (p > 0.05) for stability, except for PARTS (p = 0.04, median difference-0.6%). Statistical differences for robustness when significant were small for most metrics (<2.0% median). Median adaptive re- optimizations were 2.Conclusion: We describe a 5-fraction ablative SMART planning approach for PCa that is robust and stable during on-table adaption, due to gradients controlled by a GI PRV technique and the use of rings. These findings are noteworthy given that daily interfraction anatomic GI OAR differences are routine, thus necessitating on-table adaptation. This work supports feasibility towards utilizing a patient-independent, template on-table adaptive approach.(c) 2023 The Author(s). Published by Elsevier B.V. Radiotherapy and Oncology 188 (2023) 109869 This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).