Session Item

Single-fraction (1-fxn) SABR is rarely utilized, potentially due to the absence of real-time imaging on CT-guided linacs, limited CBCT soft tissue resolution, and concerns of geographic miss. MRI-linacs (MRLs) provide real-time imaging, visual confirmation of geographic accuracy, and on-table adaptive radiation therapy (ART) that may facilitate broader adoption of 1-fxn SABR. However, the feasibility of delivering 1-fxn SABR on an MRL within a reasonable amount of time has not been previously evaluated.    

We developed a prospective feasibility and tolerability trial (NCT04939246) of 1-fxn SABR delivered on a 0.35T MRL for primary/metastatic lesions of the lung (30-34 Gy; BED10 = 120-149.6 Gy), liver (35-40 Gy; BED10 = 157.5-200 Gy), pancreas (25 Gy; BED10 = 87.5 Gy), adrenal gland (25 Gy), kidney (25 Gy), and abdominal/pelvic lymph nodes (LNs) (25 Gy). Each fxn was delivered in breath-hold (BH); oxygen (O2) by nasal cannula to facilitate BH was recommended. Continuous intrafraction soft tissue tracking and automatic beam gating were required. Primary objectives include: 1) completing treatment in 90 minutes of entering the treatment room (total in-room time) and 2) acute grade 3+ toxicity <15%. Secondary objectives include 1-year LC, 1-year OS, QOL, and late grade 3+ toxicity. The study opened in June 2021 with an accrual goal of 30 patients. We conducted an unplanned interim analysis of the primary objectives to inform whether to open this trial at a second institution.

20 patients have been treated with 21 fxns to 22 lesions in the lung (n=9; 40.9%), liver (n=5; 22.7%), adrenal gland (n=4; 18.2%), abdominal LNs (n=3; 13.6%), and pancreas (n=1; 4.5%). 2 patients (10%) had multiple lesions, 1 with 2 lung metastases sequentially treated with separate plans and another having 2 LN metastases concurrently treated with a single plan. Median GTV and PTV volumes were 4.45 cc (range, 1.02-40.46) and 15.85 cc (range, 5.29-95.81), respectively. 5 patients (25%), all with abdominal targets, needed ART to meet OAR constraints. 18 (90%) received O2. Median total in-room and treatment delivery times were 56 minutes (range, 40-195) and 33 minutes (range, 22-144), respectively. Total in-room time was 90 minutes for 19 fxns (90.5%). >90 minutes elapsed for 2 patients, 1 with nausea who needed a break and another who had difficulty reproducing the treatment position; both completed SABR on the same day. Median follow-up is 5.4 months (range, 0-15.3). 1 patient (5%) experienced acute grade 3 toxicity (chest pain) possibly related to lung SABR that resolved spontaneously within 1 day.

We demonstrate encouraging early outcomes of 1-fxn SABR delivered in BH on a 0.35T MRL. Should feasibility and tolerability be confirmed upon trial completion, additional studies would be warranted to evaluate this novel treatment strategy especially among patients with 2+ metastatic lesions for which multi-fxn SABR might otherwise be delivered over several weeks.