Session Item

Single fraction (SF) stereotactic radiotherapy (SABR) is an effective treatment in patients with peripheral lung tumors. Most published experience with SF-SABR has been using CT-based treatment planning, with often on-couch cone-beam CT guidance. MR guided adaptive radiotherapy is a recent development, and there are concerns about the accuracy of target definition, treatment planning and MR-guided SABR delivery. We report on clinical outcomes following MR guided SF-SABR for lung tumors.

Details of patients with peripheral lung tumors undergoing MR guided SF-SABR at a single institution since 2018 were accessed from a prospective institutional database. This study was approved by our Medical Ethics Review Committee. Treatment simulation is performed using a 17-s breath-hold 3D MR based on a true FISP sequence acquisition on a 0.35 T MR scanner. Tracking performance was assessed visually during MR simulation. GTV contouring was first performed on a breath-hold planning CT scan, followed by contours on a rigid co-registered 3D MR scan by the same clinician. Any discrepancies in GTV’s between imaging techniques were reviewed by a second clinician to reach a consensus. SABR plans were generated using step-and-shoot IMRT beams, and normalized to cover 95% of the PTV by the prescription dose. A PTV Dmax of up to 140% was used. On-table contour editing was performed if considered clinically relevant. A 5 mm PTV margin, and a 3 mm gating window, were used. SABR was delivered using automatic beam gating during repeated breath-holds, and under continuous visualization of the tracked GTV in a sagittal MR plane.

A total of 51 consecutive patients were treated between 2018 and 2022. The majority (69%) had a primary NSCLC. One patient was treated for two tumors in the same lobe. Median PTV was 11.2 cc (range 3.9-53.5). The SF dose was 20 Gy (2%), 24 Gy (4%), 28 Gy (4%), 30 Gy (31%) or 34 Gy (59%). Details of treatment duration were available for 38 patients and measured from the patient entered the changing room to the end of delivery. Median duration for the period 2018-2020 was 119 minutes (range 65-178), and 90 minutes (range 60-192) for the period 2021-2022. After a median follow-up of 18.7 months (95% CI 14.8-22.5), 2-year OS was 81%, ranging from 89% for primary lung tumors to 60% for metastases. All but 4 patients were eligible for assessment of local control. Local recurrences occurred in 2 patients (4%), manifesting at 9.4 months and 37.3 months post-SABR, respectively. Acute toxicity within 90 days post-SABR was observed in 2 patients (1 patient with grade 2 radiation pneumonitis and 1 with severe temporary back pain due to the duration of SABR delivery).

The early results of MR guided SF-SABR delivered in breath-hold reveal high tumor control rates, indicating that the use of automatic beam gating with a 3 mm gating boundary is safe for patients with peripheral lung tumors.