2806 - Cumulative EQD 2 Dose Reporting from an Adaptive Reirradiation Phase I Trial of Ablative 5-Fraction Stereotactic MR-Guided Radiotherapy for Abdominal and Pelvic Tumors

Purpose/Objective(s): Reirradiation (reRT) is often limited to palliative doses resulting in suboptimal long-term efficacy. MR-guided linacs (MRL) with online adaptive radiotherapy (oART) may facilitate ablative dose escalation for reRT. We previously reported clinical outcomes from the first prospective trial of ablative, 5-fraction MR-guided reRT (NCT06397573) demonstrating no grade =3 toxicities and 100% 1-year freedom from local progression and overall survival. Herein, we report cumulative gastrointestinal (GI) EQD₂ doses and hypothesize they exceed contemporary reRT thresholds.

Materials/Methods: Ten patients were prescribed reRT of 50 Gy in 5 fractions (BED=100 Gy₁₀) on consecutive days on a 0.35 T MRL with continuous cine MRI, automatic gating, and oART. Dosimetric eligibility included =1 prior RT course, =6 months prior RT with prescription dose >40 Gy EQD₂, and =50% isodose overlap among the prior and reRT plans. Prior and reRT doses were converted to EQD₂ using a/ß of 3 Gy. Rigid registration of the prior CT/MR to the reRT MR using the spinal vertebrae was performed to sum EQD₂ doses. Cumulative EQD₂ trial constraints included: kidneys Dmean <20 Gy, liver Dmean <30 Gy, and spinal canal D0.03cc =60 Gy. GI organ-at-risk (OAR) constraints, defined for reRT plan alone, were V33Gy <0.5 cc and V35Gy <0.03 cc. Cumulative EQD₂ for GI OARs were reported and compared to cumulative thresholds from a 2025 systematic review of GI abdominal malignancies from the international reirradiation collaborative group (ReCOG).

Results: ReRT was delivered at a median of 13.3 months after prior RT to 50 Gy (range 40-70) in 5 fractions (range 5-30). ReRT treatment sites included pancreas (50%), abdominopelvic lymph node (40%), and liver (10%). 80% of patients had GI OARs within 3 mm of the gross tumor volume. Mean (max) cumulative EQD₂ doses are listed in the table below for each GI OAR, along with respective ReCOG thresholds. Dmax exceeded 110 Gy ReCOG threshold in 50%, 67%, 70%, and 40% of stomach, duodenum, small bowel, and large bowel cases, respectively. D1cc, D2cc, and D5cc exceeded thresholds in 38%, 50%, and 38% of stomach cases. Overall, 8/10 patients exceeded =1 ReCOG threshold.

Conclusion: Ablative MR-guided adaptive reirradiation resulted in cumulative GI EQD₂ doses exceeding contemporary reRT thresholds, including >250 Gy Dmax for select OARs, without grade =3 toxicity. MR-guided oART may expand cumulative GI dose tolerances and merits validation in larger studies.