Multi-Institutional Outcomes of Stereotactic Magnetic Resonance Image-Guided Adaptive Radiation Therapy (SMART) With Median Biologically Effective Dose of 100 Gy10 for Oligometastases

Purpose/objective(s) Randomized data show an overall survival (OS) benefit of stereotactic ablative body radiation therapy (SABR) in addition to chemotherapy (CT) in selected patients with oligometastasis (OM). While ablative dose can be safely delivered to some lesions, others in proximity to some organs at risk (OARs) such as bowel may be treated with non-ablative dose to limit toxicity. Stereotactic magnetic resonance image-guided adaptive radiation therapy (SMART) may facilitate the delivery of ablative dose for OM lesions, especially those adjacent to historically dose-limiting OARs, which may improve long-term disease control. Materials/methods The RSSearch Registry was queried for OM patients (1-5 metastatic lesions) treated with SMART. Patients with Results 101 patients with 114 OM lesions were treated on a 0.35T-MR Linac at 2 institutions between 2018-2020. Median age was 62 years (range 23-89), and 99% were ECOG 0-1 performance status. The primary tumor was definitively managed in 34.7%, most commonly in the lung (30.7%), colon/rectum (22.8%), cervix/uterus/ovary (13.9%), or breast (5.9%). About 50% received CT for OM prior to SMART. SMART was delivered to 1 (87.7%) or 2 (10.5%) lesions, mostly involving abdominal/pelvic lymph nodes (40.4%), liver (17.5%), lung (17.5%), or adrenal gland (11.4%). All were treated without fiducial markers and with real-time tissue tracking and automated beam gating, typically in breath hold. The median prescribed RT dose was 45 Gy (range 24-60Gy) in a median 5 fractions (range 3-15). The median biologically effective dose (BED10) was 100 Gy10 (range 41.4-180). The median PTV was 14.5 cm3 (range 1.5-567.8). Median follow-up was 10 months (range 3-25) from SMART. One-year LC, FFDP, PFS, and OS were 88%, 38.2%, 36.1%, and 90.6%, respectively. Median PFS and OS were 9 months and not reached, respectively. No acute/late grade 3+ toxicities were recorded. Conclusion To our knowledge, this represents the largest analysis of SMART for OM. A median prescribed BED10 of 100 Gy10 resulted in excellent early LC and no significant toxicity, likely facilitated by continuous intrafraction MR visualization, breath hold delivery, and online adaptive replanning. These outcomes are noteworthy given the predominance of OM, especially LNs, treated with dose escalation in proximity to gastrointestinal OARs; such lesions are notably underrepresented in SABR OM trials. Prospective evaluation of dose-escalated SMART for OM is warranted.