Magnetic resonance (MR-)guided stereotactic ablative radiotherapy (SABR) was performed for patients with lung tumors in whom treatment delivery was challenging due to tumor location, motion or pulmonary comorbidity. As stereotactic MR-guided adaptive radiation therapy (or SMART) is a novel approach, we studied clinical outcomes in these high-risk lung tumors.
Fifty consecutive patients (54 lung tumors) underwent SMART between 2016-2018 for either a primary lung cancer (29 patients), or for lung metastases (21 patients). Eligible patients had risk factors which could predispose to toxicity, including a central tumor location (n = 30), previous thoracic radiotherapy (n = 17), and interstitial lung disease (n = 7). A daily 17-second breath-hold MR scan was acquired in treatment position, and on-table plan adaptation was performed using the anatomy-of-the-day. Gated SABR was delivered during repeated breath-holds under continuous MR-guidance.
All but one patient completed the planned SMART schedule. With daily plan adaptation, a biologically effective dose (BED) ≥100Gy to 95% of the planning target volume was delivered in 50 tumors (93%). Median follow-up was 21.7 months (95%CI, 19.9-28.1). Local control, overall and disease-free survival rates at 12 months were 95.6%, 88.0% and 63.6%, respectively. Local failures developed in four patients: two after re-irradiation for a recurrent lung cancer, and in two patients with a colorectal metastasis. Overall rates of any grade ≥2 and ≥3 toxicity were 30% and 8%, respectively. Commonest toxicities were grade ≥2 radiation pneumonitis (12%) and chest wall pain (8%). No grade 4-5 toxicities were observed.
Use of MR-guided SABR resulted in low rates of high-grade toxicity, and encouraging early local control, in a cohort of high-risk lung tumors. Additional studies are needed in order to identify patients who are most likely to benefit from the SMART approach.

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