Identification of patients with locally advanced pancreatic cancer benefitting from plan adaptation in MR-guided radiation therapy

Background and purpose: MR-guided radiation therapy (MRgRT) with daily plan adaptation is a novel but time-and resource-intensive treatment for locally advanced pancreatic cancer (LAPC). We analyzed the benefit in target coverage and organ-at-risk (OAR) sparing of daily plan adaptation in 36 consecutive LAPC patients treated with MRgRT to 40 Gy in 5 fractions. Materials and methods: Adaptive planning was assessed for 180 fractions by comparing non-adapted plans with re-optimized plans using (a) GTV coverage and OAR high-doses, and (b) compliance with institutional objectives for GTV coverage and high-dose OAR constraints. Using these criteria, plan adaptation for each fraction was characterized as not needed, beneficial, or no benefit. Decision-tree analysis was performed to identify subgroups most likely or not to benefit from routine plan adaptation. Results: The percentage of plans fulfilling institutional constraints increased from 43.9% (non-adapted plans) to 83.3% after online plan adaptation, with significant improvements in GTV coverage and lower V33Gy OAR doses. Adaptive re-optimization was found to be not needed in 80 fractions (44.4%), beneficial in 95 fractions (52.8%) and of no benefit in 5 fractions (2.8%). Decision-tree analysis identified a grouping based on distance from tumor to OAR of <= 3 mm and GTV size, respectively, to be the major determinants for the benefit of daily plan adaptation. Conclusion: MRgRT with daily plan adaptation for LAPC was of benefit in approximately half of fractions, improving target coverage and OAR sparing. Plan adaptation appeared to be relevant mainly in cases where the GTV to adjacent OAR distance was <= 3 mm. (C) 2018 Elsevier B.V. All rights reserved.