Outcomes of simultaneous, adaptive, MRI-guided liver SBRT in patients with heavily pretreated colorectal cancer and four or more liver metastases.

160 Background: About one-third of colorectal cancer (CRC) patients (pts) develop metastases, most commonly in the liver (CRLM). Ablative treatment of multiple liver lesions is difficult and often not done with conventional CT-based radiation therapy, but feasible with MRI-guided adaptive radiation. This study reviews our experience treating pts with high burden CRLM using MRI-guided stereotactic body radiation therapy (MRgRT). Methods: This study identified pts with 4 or more CRLM treated with MRgRT between 7/2019 and 4/2024, all of whom had progressive disease during or after standard-of-care therapy. Ablative radiation was delivered to all sites of disease with no concurrent systemic therapy. All with Child-Pugh score 5-6 (class A) liver function. Local control was defined as no progression in any treated lesion on a per-patient basis. Descriptive statistics summarized the data. A Cox proportional hazards model was used to identify predictors of survival, while the Kaplan-Meier method was used to estimate survival functions. Statistical analysis was performed using SPSS. Results: Seventeen pts with 110 lesions were included, with a median follow-up of 10.7 months (range, 1.7–36.9). The majority (n=15) received systemic therapy in the year before MRgRT. The median number of prior lines of systemic therapy was 2 (range, 1–5). Eight pts (47.1%) had nodal and/or lung progression in addition to CRLM, and 4 pts (23.5%) had at least one prior course of MRgRT. A median of 6 liver lesions (range, 4–11) were prescribed a median dose of 60 Gy (range, 60–80), each delivered in 5 fractions. Median total gross tumor volume per pt was 23.8 cm 3 (range, 0.97–60.1). Seventy-four fractions were available for radiation delivery analysis, of which 55 (74.3%) were adapted. At least one target was modified in 51 (92.7%) of the adapted fractions. Acute toxicity occurred in 7 pts, all with self-limited fatigue and nausea. Progression in Child-Pugh classification from A to B occurred in three pts 3-6 months post-MRgRT. Late toxicity occurred in 1 patient with mild, possibly radiation-related peripheral biliary dilation. Two pts developed malignant biliary strictures. Per-patient local control was 50.8% and 34.8% at 1 and 2 years, respectively, while liver control was 28.7% at 2 years. Six-month systemic therapy-free survival was 31.9%. Two-year overall survival (OS) was 43.9%, with a median of 17.1 months (range, 8.6–25.7). The size of the largest tumor correlated with OS (HR 1.23, p=0.014). Conclusions: In this small series of CRC pts with 4+ CRLM, MRgRT is well-tolerated and shows promise as astrategy to consolidate disease and delay the immediate need for systemic therapy changes. Patient selection is critical to identify those most likely to benefit, though the overall impact on survival and disease trajectory remains uncertain.