Magnetic Resonance-Guided Stereotactic Body Radiotherapy (MRgSBRT) with Daily Online Plan Adaptation for Reirradiation in the Abdomen.

Abstract

Purpose: Reirradiation in the abdomen poses a challenge due to both interfraction and intrafraction motion of the target and nearby organs at risk (OARs). We hypothesized that magnetic resonance-guided stereotactic body radiotherapy (MRgSBRT) with daily online adaptation allows for safe and effective dose-escalated treatment by minimizing grade 3+ gastrointestinal (GI) toxicities.

Materials/methods: We performed a single-institution retrospective review of 38 patients who received a total of 44 courses of MRgSBRT reirradiation within the abdomen. Clinical outcomes included local control, overall survival, and GI toxicities assessed using CTCAE v5. OAR metrics of original and adapted plans were compared to assess the added value of daily adaptation.

Results: Fourteen different primary histologies were treated, with the most common including pancreatic (18.2%), renal (18.2%), and prostate (15.9%) cancers. The most common site for reirradiation was abdominal lymph nodes (61.3%). A majority (70.4%) of MRgSBRT courses were preceded by prior SBRT. The median and modal prescribed dose of MRgSBRT reirradiation was 40 Gy in 5 fractions (BED₁₀=72); 87% of courses were treated to a BED₁₀ of at least 59.5. Across 218 total fractions, daily adaptation improved PTV coverage by a mean of 4.1% (95% CI: 3.1 - 5.1%, p<0.0001) and met 100% of hard luminal GI OAR 0.03cc constraints that would have been exceeded without adaptation in 53.2% of fractions. The median follow-up after MRgSBRT reirradiation was 15.8 months. One-year local control and overall survival were 89.5% and 74.9%, respectively. Grade 3+ toxicity possibly related to MRgSBRT reirradiation was observed following two (4.5%) courses.

Conclusion: MRgSBRT allows for dose escalation and good local control in cases of abdominal reirradiation with acceptable toxicity. Daily adaptation provided substantial benefit in meeting safety goals and improving coverage. This retrospective study supports the development of a prospective clinical trial of MRgSBRT reirradiation in the abdomen.