ESTRO 2021 Abstract Book

S1254

ESTRO 2021

bone metastases in a multicenter panel and compare MRI and CT in GTV delineation. Materials and Methods

After the institutional review boards approved the study we analyzed anonymous CT and MRI obtained from ten non-spine bone metastases. Six radiation oncologists in our center using CT and MRI to delineate ten GTVs of non-spine bone metastases. Non-spine bone metastases locations were pelvis and skull. The cases show different characteristics: blastic and lytic metastases, different primaries (lung, breast, prostate, rectum, urotelial and colangiocarcinoma). In both CT and MRI, the GTV volumes were compared. The index of agreement was evaluated according to Landis and Koch. Results The GTV volume as defined on MRI was in all cases larger or at least as large as the GTV volume on CT. The median GTV volume on MRI was 3,15 cc (0.027–70.64 cc), compared to 2,8 cc on CT (0.075–77.95 cc) (p=0.25). Variance and standard deviation between observers in CT and MRI were the minor (576,26 vs 722,15, and 24 vs 26,9 respectively). The index of agreement was insignificant (0.03) between CT and MRI in all patients. Conclusion In cases with well-defined tumor CT is easier and reproductible. In cases with no defined tumor in CT we need other tools as MRI can be complementary. The interobserver variability in target delineation of non-spine bone metastases in this study is noteworthy. Training tools and expert consensus are needed for the delineation of non-spine bone metastases. We are studying the role of PET in the delineation of non-vertebral bone metastases. PO-1531 Photon and particle beam radiotherapy for atrial fibrillation: a systematic review J. Franzetti 1,2 , S. Volpe 1,2 , C. Piccolo 3 , M. Pepa 1 , F. Cattani 3 , A.M. Camarda 1 , G. Piperno 1 , C. Carbucicchio 4 , V. Catto 4 , B.A. Jereczek-Fossa 1,2 1 European Institute of Oncology IRCCS, Division of Radiation Oncology, Milan, Italy; 2 University of Milan, Department of Oncology and Hemato-Oncology, Milan, Italy; 3 European Institute of Oncology IRCCS, Unit of Medical Physics, Milan, Italy; 4 Centro Cardiologico Monzino IRCCS, Department of Arrhythmology, Milan, Italy Purpose or Objective Atrial fibrillation (AF) represents one of the most common cardiac arrhythmias. Its main trigger resides in the pulmonary veins, making this site the main target of ablation. The purpose of this study is to collect available evidence on the feasibility and efficacy of both photon radiotherapy (XRT) and particle beam radiotherapy (PBR) in AF, and to provide Radiation Oncologists with a practical overview on this topic. Materials and Methods Research was performed according to the PRISMA criteria. Overall, 325 articles were identified. Only reviews and full articles on both preclinical and clinical studies were included; no data restrictions or language limitations were applied. Selected works were analyzed for comparing target selection, treatment plan details and the accelerator employed. Furthermore, an overview on pre- and post-procedure exams and on the efficacy and side-effects of the treatments was performed. Efficacy was defined as either the presence of a myocardial scar (animals) or the absence of AF recurrence (humans). Results Following duplicates elimination, 21 works published between 2010 and 2020 were included. Twelve and three studies involved XRT and PBR, respectively; only two of the six reviews included both XRT and PBR. The main characteristics of the reviews evaluated are provided in Table 1.

Seven studies enrolled a total of 49 animals, with delivered doses ranging from 15 to 40 Gy. An equivalent number of investigations focused on humans (5 studies in silico and 2 in vivo), with doses varying from 16 to 50 Gy. All but two studies used single-fractionated schedules. Only one work focused on treatment plan delivery in animals. The average follow-up duration was 6 months for animals and 3 years for humans, respectively. The minimum prescription dose for efficacy resulted to be 25 Gy; efficacy was reached in all cases but in one patient, who experienced AF recurrence 6 months later. No acute or late RT-induced toxicity was recorded. Respiratory and cardiac target motion management were the most critical issues. Details for the included studies are provided in Table 2.