ESTRO 2020 Abstract book

S281 ESTRO 2020

treatment sessions. Gated SABR was delivered during repeated breath-holds, using a planning target volume (PTV) margin of 5 mm, and an isotropic boundary of 3 mm around the gross tumor volume (GTV) as a gating window. Visual patient feedback was provided during delivery using an in-room monitor (Fig. 1, A), depicting the continuously tracked GTV (green) and the gating window (red) in a sagittal MR plane (Fig. 1, B-C). The acquired real-time MR images were analyzed to determine precision and efficiency of gated delivery as described previously (van Sörnsen de Koste JR, Int J Radiat Oncol Biol Phys 2018).

Results Fourteen patients underwent MR-simulation but 6 were considered unsuitable, mostly due to poor tracking of small tumors. Subsequently, eight suitable patients (median age 75 years; range 65-80) underwent a single fraction of 34 Gy on the MR Linac (Table 1). Treatment indications were a primary lung cancer (n = 6) or lung metastasis (n = 2). The median GTV and PTV were 2.9 cm 3 (range, 1.8-4.3) and 10.1 cm 3 (7.5-13.9), respectively. The total in-room procedure required a median of 111 minutes (range, 74-150), with irradiation comprising 37 minutes (28-59). A geometric analysis of SABR delivery in the first 5 patients revealed a mean duty cycle efficiency of 60.6% (range, 33.9-85.3), and mean GTV coverage by the PTV during beam-on was 99.6% (99.0-100.0). Pre-treatment online plan reoptimization improved PTV coverage (V 34Gy ) by an average of 5.5% (from 89.5% using baseline plans, to 95.0% in reoptimized ones), although this had only limited impact on GTV and organ at risk doses in this cohort.

Conclusion Single-fraction lung SABR using MR-guidance is feasible, and it allows high-precision delivery. However, not all evaluated tumors were suitable for SABR delivery on the MR Linac, indicating that further improvements in imaging are needed to ensure tumor tracking in all patients. OC-0469 MR-guided online adaptive radiotherapy for muscle invasive bladder cancer: First UK experience A. Hunt 1 , I. Hanson 2 , A. Dunlop 2 , L. Bower 1 , H. Barnes 3 , J. Chick 2 , T. Herbert 3 , R. Lawes 3 , H. McNair 1 , A. Mitchell 2 , J. Mohajer 2 , T. Morgan 1 , G. Smith 3 , S. Nill 2 , U. Oelfke 2 , R. Huddart 1 , S. Hafeez 1 1 Institute of Cancer Research- The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, London, United Kingdom ; 2 Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, The Joint Department of Physics, London, United Kingdom ; 3 The Royal Marsden NHS Foundation Trust, Radiotherapy, London, United Kingdom Purpose or Objective The Elekta Unity (Elekta AB, Stockholm, Sweden) combines a linear accelerator with a 1.5T magnetic resonance (MR) scanner enabling daily online adaptive MR-guided radiotherapy. We describe the first UK experience of this technology for the treatment of muscle invasive bladder cancer (MIBC). Material and Methods