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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 V100%/volume), PTV D90% and D80%, homogeneity index (HI = PTVopt D2%/D98%), prescription isodose/target volume (PITV), low-dose conformity (D2cm = maximum dose at 2 cm from PTVopt/Rx dose), and gradient index (R50%=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 D80%), 50.1 ± 4.2 Gy/49.1 ± 4.7 Gy (PTVopt D80%), 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 (R50%), 1.53 ± 0.23/1.55 ± 0.23 (HI), and 58%±7%/59%±7% (D2cm), 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.
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INTRODUCTION: Stereotactic MR-guided online adaptive radiation therapy (SMART) has demonstrated a superior radiotherapeutic ratio for pancreatic patients, by enabling dose escalation while minimizing the dose to the proximal gastrointestinal organs at risk through online adaptive radiotherapy. The safe delivery of stereotactic body radiation therapy (SBRT) is of particular importance in the reirradiation setting and has been historically limited to CT-based nonadaptive modalities. Herein, we report the first use of online adaptive radiotherapy in the reirradiation setting, specifically for treatment of locally recurrent pancreatic adenocarcinoma through SMART reirradiation (SMART reRT). CASE DESCRIPTION: We describe the treatment of a 68-year-old male who was diagnosed with, unresectable locally advanced pancreatic adenocarcinoma. Initial treatment included FOLFIRINOX followed by 45 Gy in 25 fractions on a helical intensity-modulated radiotherapy (IMRT) device with concurrent capecitabine, followed by a boost of 14.4 Gy in 8 fractions to a on an MR-guided radiotherapy (MRgRT) linac. At approximately 12 months from initial radiotherapy, the patient experienced local progression of the pancreas body/tail and therefore SMART reRT of 50 Gy in 5 fractions was initiated. The technical considerations of cumulative dose for gastrointestinal organs across multiple courses, treatment planning principles, and adaptive radiotherapy details are outlined in this case study. The patient tolerated treatment well with minimal fatigue. CONCLUSIONS: The therapeutic ratio of reirradiation may be improved using daily MR guidance with online adaptive replanning, especially for lesions in proximity to critical structures. Future studies are warranted to assess long-term outcomes of dose escalated MR-guided reRT, define OAR dose constraints for reRT, and assess cumulative dose across the adapted SMART reRT fractions and the original RT plan.