Session Item

Magnetic Resonance Image-guided Radiation Therapy (MRgRT) offers improved soft tissue visualization compared to CT-based techniques, intra-fraction real-time fiducial-free tracking, and ability to adapt the treatment plan, facilitating dose-escalated radiotherapy (RT) for abdominal and pelvic tumors. In this study, we report our institutional MRgRT toxicity results.

Consecutive patients treated on a 0.35T MR-linac for primary and metastatic tumors in the abdomen and pelvis from April 2018 to September 2022 were evaluated. All were treated without fiducial markers and with real-time MR-based soft tissue tracking and automated beam gating, allowing for intra- and inter-fraction visualization of both the target and the critical organs-at-risk (OARs). The biologic effective dose with α/β of 10 Gy (BED10) was calculated for each treatment. The Common Terminology Criteria for Adverse Events (CTCAE) v5.0 criteria was used to score treatment-related toxicities by the treating physicians. Any toxicity within the first 3 months from completion of MRgRT was scored as “acute” and thereafter was scored as “late.”

335 patients with a median age of 68 years (range, 28-94) met the inclusion criteria. The most common treatment sites were pancreas (n=157, 47%), abdominopelvic lymph nodes (LNs) (n=70, 21%), adrenal glands (n= 48, 14%), and liver/hepatobiliary tract (n=44, 13%). All patients had ECOG 0-1 performance status. 208 (62%) patients were treated for primary tumors and 127 (38%) were treated to metastatic sites. The median prescribed RT dose was 50 Gy (range, 20-76 Gy) in 5 fractions (range, 1-30) over 7 days (range, 1-50 days), the median prescribed BED10 dose was 100 Gy10 (range, 34-158 Gy10), and 184 (55%) patients were treated with 100 Gy10 or higher. 1190 of 1893 prescribed fractions (63%) required plan adaptation. The median follow-up was 11 months (range, 1-52 months). Acute grade 2 or 3 toxicity occurred in 52 (16%) and 6 (2%) patients, respectively. Late grade 2 or 3 toxicity occurred in 24 (7%) and 6 (2%) patients, respectively. Grade 2+ toxicity rates were not different for daily compared to every other day treatments (24% vs. 17%, p=0.3). For 159 patients with more than 1 year follow-up (median 22 months), the 1-year rate of Grade 2+ toxicity was 15% (95% CI: 10.6-19.4%). There were no acute or late grade 4 toxicities, and 1 acute grade 5 toxicity was observed in a pancreatic cancer patient post Whipple procedure, possibly related to RT.

Dose-escalated RT delivered on a 0.35T MR-linac has been very well tolerated in this large series of consecutively treated patients, with acute or late grade 3+ toxicity being rare even with daily treatment delivery.  This is notable given our frequent use of ultra-hypofractionation, frequent use of the adaptive workflow, and ablative dosing in most patients to sites close to gastrointestinal OARs.