ESTRO 2020 Abstract book

S551 ESTRO 2020

versus BCS HR 1.03; 95% CI 0.60 - 1.75) between the surgery type. Conclusion In breast cancer patients who underwent NAC and post- operative radiation therapy, BCS and mastectomy are effective with similar survival outcomes regardless of surgery type PO-0946 Clinical impact of FFF-VMAT combined with DIBH for breast cancer radiotherapy D. Mychko 1 , A. Zverava 1 , S. Siamkouski 1 , P. Dziameshka 2 1 Minsk city Oncological Center, Radiotherapy department, Minsk, Belarus ; 2 N.N. Alexandrov National Cancer Centre of Belarus, Radiotherapy, Lesnoy, Belarus Purpose or Objective Application of flattening filter free (FFF) beams in radiotherapy (RT) has promise of potential beam-on time (BOT) reduction especially for RT techniques demanding active patients’ involvement in the process. The current study is focused on comparative evaluation of deep inspiration breath-hold (DIBH) 3D-CRT, IMRT, VMAT, FFF- VMAT techniques. Material and Methods 80 left-sided breast cancer female patients were included in the study. The patients underwent physical examination and echocardiography before and after RT as well as NT- proBNP serum level was measured. 3D-CRT, IMRT, VMAT, FFF-VMAT plans (50Gy in 2Gy per fx, 5fx/week) were generated for each patient. Dosimetric criteria for PTV and OARs were evaluated for 320 plans applying QUANTEC and ICRU recommendations. Based on planning statistics BOT was also measured and analyzed. Statistical tests (Friedman and Nemenyi) were conducted on comparisons (p < 0.05). Results As all created plans were satisfying DVH dose constraints for PTV and OARs, the main parameter used to estimate impact on DIBH treatment was BOT per fraction (table 1).

Taking into account the average breath-holding cycle duration (30s) FFF-VMAT and 3D-CRT were defined as equal regarding BOT. IMRT implementation increased BOT by 74.8%, 65.9% and 54.2% (р<0.001) comparing with 3D-CRT, FFF-VMAT and VMAT and prolonged treatment session from 7 to 4 breath-holding cycles respectively. Advantage of FFF-VMAT over VMAT and IMRT at least for one breath- holding cycle was revealed and dose delivery time reduction by 24.7% and 65.9% (р<0.001) respectively was showed. For 14 patients FFF-VMAT was chosen to be a treatment option because of lower OAR doses and BOT compared to other RT techniques. The patients were under observation from 3 to 9 month after RT. The signs of G2 skin toxicity and cardiac function deterioration were not found. Conclusion BOT should be taken into account for RT session time optimization and active breathing control treatment tolerance improvement. FFF-VMAT usage leads to dose delivery time decrease with no evidence of plan quality degrading and early toxicity increasing. PO-0947 Dosimetric evaluation of organs at risk in APBI patients treated with multicatheter interstitial BT M. Maçães 1 , S. Pinto 1,2 , A. Pereira 1,2 , J. Lencart 1,2 , P. Fernandes 3 , L. Trigo 3 1 Portuguese Oncology Institute of Porto IPO Porto, Medical Physics, Porto, Portugal ; 2 IPO Porto Research Center CI-IPOP, Medical Physics - Radiobiology and Radiation Protection Group, Porto, Portugal ; 3 Portuguese Oncology Institute of Porto IPO Porto, Brachytherapy Department, Porto, Portugal Purpose or Objective The present work consist of a dosimetric evaluation of organs at risk (OARs) according to the recent GEC-ESTRO recommendations (1) in sixty one APBI patients treated with multicatheter interstitial brachytherapy (MIBT) using Between February 2017 and December 2019, 61 APBI patients (32 patients with left-sided and 29 with right- sided breast tumors) were treated with MIBT technique. After lumpectomy, a post-operative multicatheter implant was performed with a prescription dose of 32Gy (4Gy/fraction in 5 days with a minimum interval between fractions of six hours). After catheter insertion, the patients underwent a CT scan (2mm slices), and treatment plans were obtained with TPS Oncentra MasterPlan v4.1. After the target volumes and the OAR, including skin, lung ipsilateral, ipsilateral non- target breast and heart (in cases of left-sided tumors), were delineated and the dose-volume histograms were evaluated. Results Of 61 patients, 52% had a left sided lesion and 48% had the tumor in the right breast. Regarding the tumor location in breast, we did not find a relation between the quadrant and the dose received by OAR. a HDR Ir-192 source. Material and Methods

Statistical analysis showed significant differences between all compared techniques for this parameter (р<0.001). BOT difference between compared RT techniques was measured to evaluate the FFF-RT treatment session duration benefit. Medians of BOT difference value (s) for 3D-CRT vs. IMRT, VMAT and FFF-VMAT are listed in table 2.