ESTRO 2023 - Abstract Book

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ESTRO 2023

larger than 300 cc than those with photon beam therapy (45.0% versus 20.2%, P=0.021). The median lung V10 was 38.5% (range, 6.9-69.9) in PBT group and 49.2% (range, 4.3-99.3) in photon beam therapy group, respectively (P=0.017). The 2 year OS and PFS rates were 57.2% and 35.6% in PBT group, and 63.3% and 39.8% in photon beam therapy group, respectively (P=0.514 and P=0.679). In the multivariable analysis, DLco, stage, prophylactic cranial irradiation, and durvalumab use were demonstrated as the significant prognostic factors for OS. Grade 2 or higher radiation pneumonitis and esophagitis occurred in 5 (25.0%) and 7 (35.0%) patients following PBT, and 66 (27.3%) and 40 (16.5%) patients following photon beam therapy, respectively (P=0.826 and P=0.062).

Conclusion Although PBT group had more frequently poor lung function and larger CTV, treatment outcomes and radiation-related toxicities were comparable with photon beam therapy in LS-SCLC. Through its lung-sparing ability, PBT could be a useful treatment modality for patients with poor pulmonary function or large extent of disease.

PO-1331 Local control rates after MR-guided single fraction SABR for lung tumors

H. Tekatli 1 , M. Palacios 1 , F. Schneiders 1 , N. Haasbeek 1 , F. Lagerwaard 1 , S. Senan 1

1 Amsterdam UMC, Radiation Oncology, Amsterdam, The Netherlands

Purpose or Objective 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. Materials and Methods 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. Results 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