2530 - Magnetic Resonance-Guided Stereotactic Body Radiotherapy for Reirradiation in the Abdomen

T. Howard¹, D. Ferguson¹, Z. Han², J. E. Leeman², and R. van Dams³; ¹Department of Radiation Oncology, Brigham and Women’s Hospital, Dana-Farber Cancer Institute and Harvard Medical School, Boston, MA, ²Department of Radiation Oncology, Dana-Farber Cancer Institute/Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, ³Department of Radiation Oncology, Brigham and Women’s Hospital/Dana-Farber Cancer Institute, Boston, MA

Purpose/Objective(s): Reirradiation in the abdomen poses a challenge due to both interfraction and intrafraction motion of nearby organs at risk (OARs). We hypothesize that magnetic resonance-guided stereotactic body radiotherapy (MRgSBRT) using daily online adaptation as well as real-time tumor tracking improves the safety of dose escalated treatment by minimizing grade 3+ gastrointestinal toxicities. Materials/

Methods: We performed a single-institution retrospective review of 44 patients who received a total of 53 courses of MRgSBRT as reirradiation within the abdomen on a 0.35-T MR linear accelerator from October 2019 to October 2023. Toxicities were assessed using CTCAE v5. Overall survival (OS) and local control (LC) were estimated using the Kaplan-Meier method.
Results: Median age at time of reirradiation was 67 years (Range: 24-88 years). Eighteen different primary histologies were treated, with the most common including renal (9), pancreatic (7), and prostate (7) cancers. The most common sites for reirradiation were lymph nodes (25), liver (8), and adrenal glands (6). A majority (74%) of MRgSBRT courses overlapped with prior SBRT, while the remainder overlapped with prior conventional (19%) or palliative (7%) RT doses. The median BED₁₀ for the prior course was 72 Gy (R: 28-132 Gy). The median time interval between courses was 11 months (R: 0-156 months). The median dose for MRgSBRT reirradiation was 40 Gy (R: 25-54 Gy); 89% of MRgSBRT reirradiation courses were prescribed to a dose of at least 35 Gy. A single course of 54 Gy was delivered in 3 fractions, while all others were treated in 5 fractions. All cases involved online adaptive replanning. The median follow-up following MRgSBRT re-irradiation was 12 months (R: 1-38 months). One-year LC and OS were 87% and 72%, respectively. Grade 3+ toxicity was seen in two (5%) patients. One patient developed self-limited gastrointestinal bleeding following three courses of SBRT (first CT-guided SBRT, remainder MRgSBRT) for pancreatic cancer without a source identified on endoscopy. A second patient who received two courses of MRgSBRT for a Klatskin tumor developed fatal biliary obstruction in the setting of global progression; this was favored to be due to local tumor growth, but we are unable to exclude treatment toxicity.
Conclusion: MRgSBRT allows for dose escalation and good local control in cases of abdominal reirradiation with acceptable toxicity. This retrospective study supports the development of a prospective clinical trial of MRgSBRT reirradiation in the abdomen.