P1.17-26 Delivery of Stereotactic MR-Guided Adaptive Radiation Therapy for Peripheral Lung Tumors

Abstract

Stereotactic MR-guided adaptive radiation therapy (SMART) allows delivery of stereotactic ablative radiotherapy (SABR) with high precision (van Sörnsen de Koste JR, 2018). For central lung tumors, SMART with on-table plan adaptation improves target coverage and avoids excessive normal organ doses (Finazzi T, 2019). The benefits of SMART for peripherally located lung tumors are unknown. Between 2016-2019, 23 patients (25 peripheral lung tumors) underwent SMART delivered in 3-8 fractions on an MR-Cobalt-60 system or MR-Linac. Before each fraction, a 17-second breath-hold MR scan was acquired, followed by on-table plan adaptation based on the anatomy-of-the-day, using a planning target volume (PTV) margin of 3 or 5 mm. Breath-hold gated delivery was performed under continuous MR-guidance using an in-room monitor (Fig. 1). For 14 patients, a motion-encompassing internal target volume (ITV) was created from a free-breathing 4DCT scan. Benefits of on-table adaptation were studied by comparing 112 «predicted» plans, which are the baseline plans recalculated on the anatomy-of-the-day, with on-table reoptimized plans. The SMART procedure took a median of 62 minutes on the MR-Cobalt-60 system, and 48 minutes on MR-Linac. Average SMART-PTVs were 15.4 cc (range, [3.1-55.6]). In 14 patients with a 4DCT, SMART-PTVs measured only 53.7% (range, [31.9-75.0]) of PTVs generated from the corresponding 4DCT scans (ITV+5mm). Clinicians chose the reoptimized plan for 91% of fractions. Per fraction, on-table plan adaptation improved prescription dose coverage (V100%) of the PTV from a median of 92.2% in predicted plans, to 95.0% in reoptimized ones, thereby increasing the proportion of fractions delivering a BED10Gy ≥100Gy to 95% of PTV from 90.2% to 100.0%. Using SMART for peripheral lung tumors resulted in smaller target volumes, and on-table plan adaptation ensured delivery of ablative doses to the PTV. Despite longer SABR delivery times, our findings suggest that SMART can be beneficial for some peripheral lung tumors.