ESTRO 2021 Abstract Book

S1239

ESTRO 2021

Conclusion In the past 50 years, the proportion of RT articles significantly decreased compared to other cancer therapies. Also, the pace of publication was multiplied by 6 between 1985-2019 for the 10% most productive last authors. Researchers now face fiercer competition as shown by the increasing proportion of first authors that will never publish as a last author. Additionally, RT research is extremely unequally distributed globally. Since research activity is inherent to delivery of high quality clinical care, this contributes to the global inequity of RT services. Africa and South America represent 23% of the world’s population yet contributed to ~3% of RT articles. North America and Europe contributed to 2/3 of research output despite being home to <15% of the world’s population - Learning from these trends is crucial for the future not only of RT research but also of effective, equitable cancer care. PO-1512 Daily adaptive MR-guided SBRT for oligometastatic lymph-nodes:feasibility and preliminary experience M. Rigo 1 , F. Cuccia 1 , V. Figlia 1 , N. Giaj-Levra 1 , R. Mazzola 1 , L. Nicosia 1 , F. Ricchetti 1 , D. Gurrera 1 , A. De Simone 1 , S. Naccarato 1 , G. Sicignano 1 , R. Ruggieri 1 , F. Alongi 1 1 IRCCS Sacro Cuore Don Calabria Hospital, Advanced Radiation Oncology, Negrar di Valpolicella, Italy Purpose or Objective 1.5T MR-linac allows precision radiation dose delivery due to high identification power of targets and surrounding healthy tissue. Moreover, the plan of care can be daily adapted in response to mutations in lymph- nodes position and spatial relationship to organs-at-risk (OARs) at every treatment session. Daily online plan adaptation could enable safe dose escalation of the SBRT. This study describes the implementation and initial experience of MR-guided SBRT for oligometastatic lymph nodes on the 1.5T MR-linac and evaluates treatment time, patient compliance, acute toxicity and target coverage, including an initial assessment of dose escalation. Materials and Methods Patients were treated on a 1.5T MR-linac (7MV, FFF). At each fraction a 3D T2- or T1-weighted (T2w or T1w) MR-sequence was acquired on which the CTV and OARs contours were checked and eventually adapted after a deformable registration from the pre-planning MR scan. Based on the new contours a full online replanning was performed after which a new 3D T2w or T1w MR-sequence was acquired for position verification. A step- and-shoot intensity modulated radiotherapy (IMRT) plan usually with 10 - 11 fields was delivered. Treatment- related adverse effects were assessed according to CTCAE v 5.0. Results Fifty patients with a total of 78 lymph-node oligometastases were treated with a median dose of 35 Gy in 5 fractions. Of these, for 22 lymph-nodes a dose escalation until 45 Gy in 5 fractions was performed. All 255 fractions were delivered on the MR-linac with a clinically acceptable treatment plan. Median in-room time per fraction was 36 minutes (range 24 - 67). No fractions were canceled or interrupted due to patient intolerance or toxicity. CTV coverage after dose escalation was good on all treatment scans. Conclusion In our experience MR-guided SBRT using daily full online contouring adaptation and replanning on a 1.5T MR- linac for oligometastatic lymph nodes is feasible and burdened with very low toxicity rates, even in the dose