MR-Guided Single-Fraction Stereotactic Ablative Body Radiation Therapy for Extracranial Metastases: The Miami Cancer Institute Experience

Summary: Stereotactic ablative body radiation therapy (SABR) is usually delivered over multiple fractions but rarely in a single fraction (SF). This may be due to concerns about toxicity and geographic miss using CT-guided linacs. MR-linacs (MRLs) have advanced capabilities including superior soft tissue imaging, continuous intrafraction imaging, automatic beam gating, and online adaptive radiation therapy (oART). However, published clinical outcomes of MR-guided SF-SABR are scarce. A single institution retrospective analysis was performed of patients treated for extracranial metastases with MR-guided SF-SABR on a 0.35 Tesla MRL. The prescribed dose was based on target location: lung (30-34 Gy; BED10=120-149.6 Gy10), liver (35-40 Gy; BED10=157.5-200 Gy10), adrenal gland (25 Gy; BED10=87.5 Gy10), and abdominal or pelvic lymph node (25 Gy). Study objectives were to describe treatment times, local control (LC) as per RECIST v1.1, overall survival (OS), and toxicity (CTCAE v5.0). Kaplan Meier method was used for time to event analysis. 41 patients were treated to 46 lesions with SF-SABR; 36 (87.8%) had 1 lesion while 5 (2.2%) had 2 lesions. Median age was 68 years and 21 patients (51.2%) were female. Most had the primary tumor definitively managed (n=34; 82.9%) and systemic therapy (n=25; 61.0%) for metastatic disease prior to SF-SABR. Time interval from initial cancer diagnosis to SF-SABR was median 25.0 months (range, 1.8-102.1 months). The most common primary tumor type was colorectal (n=21; 51.2%), lung (n=11; 26.8%), and esophageal (n=3; 7.3%). Tumor histology was predominantly adenocarcinoma (n=27; 65.9%) and non-small cell carcinoma (n=8; 19.5%). Treated lesions were in the lung (n=18; 39.1%), liver (n=15; 32.6%), adrenal gland (n=7; 15.2%), and lymph nodes (n=6; 13.0%). Median GTV and PTV volumes were 4.2 cc (range, 1.0-83.3 cc) and 15.9 cc (range, 5.3-112.1 cc), respectively. oART was used for 17 lesions (37.0%). Median total delivery time was 38 minutes (range, 15-144 minutes) and median total in-room time was 65 minutes (range, 39-195 minutes). Median follow-up after SF-SABR was 16.0 months (range, 6.0-29.0 months). Median LC and OS were both not reached while 2-year LC and OS were 96.9% (95% CI: 90.8-100.0%) and 67.6% (95% CI: 51.3-83.8%), respectively. The only patient who had local failure was prescribed 30 Gy for a colorectal lung metastasis that was surgically salvaged. On multivariate analysis, factors associated with worse OS included colorectal primary tumor type (HR 5.439; 95% 1.147-25.792; P=0.033) and higher GTV volume (HR 1.090; 95% CI: 0.997-1.192; P=0.059). One patient experienced (2.4%) an acute grade 3 adverse event (pericarditis); there was no acute grade 4 or late grade 3 toxicity. This is the largest analysis to date showing MR-guided SF-SABR is safe, feasible, and achieves exceptional LC even for radioresistant tumors such as colorectal cancer. These data support routine use of SF-SABR especially for multiple metastases. Michael Chuong (he/him/his), MD (Presenting Author) - MIami Cancer Institute; Roberto Herrera, BA (Co-Author) - Miami Cancer Institute; Kathryn Mittauer, PhD (Co-Author) - Miami Cancer Institute; Carolina Rojas, BA (Co-Author) - Miami Cancer Institute; Muni Rubens, MBBS, PhD, MPH (Co-Author) - Miami Cancer Institute; Adeel Kaiser, MD (Co-Author) - Miami Cancer Institute; Noah Kalman, MD, MBA (Co-Author) - Miami Cancer Institute; Nema Bassiri, PhD (Co-Author) - Miami Cancer Institute; Antonio Ucar, MD (Co-Author) - Miami Cancer Institute; Santiago Aparo, MD (Co-Author) - Miami Cancer Institute; Fernando De Zarraga, MD (Co-Author) - Miami Cancer Institute; Sarah Joseph, MD (Co-Author) - Miami Cancer Institute; Paul Kaywin, MD (Co-Author) - Miami Cancer Institute; Federico Albrecht, MD (Co-Author) - Miami Cancer Institute; Alonso Gutierrez, PhD, MBA (Co-Author) - Miami Cancer Institute; Rupesh Kotecha, MD (Co-Author) - Miami Cancer Institute