ESTRO 2021 Abstract Book

S709

ESTRO 2021

(EQD2/3) for the surrounding normal tissue. Kaplan-Meier curves were used for calculation of overall survival (OS), local and distant control rates (LCR, DCR). Figure 1

Results Twenty-seven patients from 8 centers have been amenable for evaluation: 12 with non-small-cell lung cancer, 16 with metastatic lesions. In 17 of the cases, the re-irradiated lesion was the only detectable tumor. The median PTV of the first and second SBRT was 35 cm³ and 29.5 cm³, respectively. The median PTV overlap was 22 cm³ and the median cumulative maximal EQD2/10 ponit-dose was 270.04 Gy. The median dose applied in the first SBRT course was 38.5 Gy to the 65%-isodose over a median of 5 fractions. 40 Gy in 5 fractions was the median prescription for the second SBRT-course. After a median interval of 20.2 months between the two SBRT-courses, the 1-year OS, -LCR and -DCR were 78.3%, 70.3% and 73.8% respectively. Three patients developed grade 1 and one patient grade 2 pneumonitis. No grade >2 toxicity was observed. A higher than median dose to the PTV in the second treatment correlated with improved OS (p = 0.005) and LCR (p = 0.055) and peripheral tumor location had also a significantly positive influence on local control (p = 0.013). Conclusion A second in-field course of SBRT with PTV overlap appears to be safe and achieves reasonable tumor control rates. A higher re-irradiation dose to the PTV is the most important factor for long-time tumor control and survival. PD-0873 Daily online adaption in MR-guided pulmonary SBRT: niche or new standard? S. Regnery 1 , C. Buchele 1 , F. Weykamp 1 , P. Hoegen 2 , T. Eichkorn 1 , T. Held 1 , C. Rippke 1 , S. Klüter 1 , J. Rademacher 1 , J. Debus 1 , S. Adeberg 1 , J. Hörner-Rieber 1 1 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany; 2 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany Purpose or Objective MR-guided SBRT is an emerging technique that combines high soft tissue contrast with the ability of real-time imaging before and during each radiotherapy session. Thus, gated dose delivery and daily plan adaption become feasible. Here, we present a preliminary effort-benefit-analysis of MR-guided pulmonary SBRT based on a prospective database. Materials and Methods We analyzed prospective data of 21 patients (15 male, 6 female; median age 65.4 years [IQR 59.1 – 75 years]) who received MR-guided online-adapted SBRT with gated dose delivery to 23 primary and secondary pulmonary lesions. SBRT fractionation was chosen according to tumor location: mostly 5 x 10 Gy for peripheral, 8 x 7.5 Gy for central and 10 x 5 - 6 Gy for ultracentral tumors. Before treatment start, all patients received on-table