ESTRO 2021 Abstract Book

S1578

ESTRO 2021

accuracy. The efficiency of this tool is reduced for small target volumes.

PO-1851 Comparison of 3 techniques for left-sided breast irradiation with or without internal mammary nodes L. Bogers 1 , A. Petoukhova 1 , J. Penninkhof 2 , M. Mast 3 , P. Poortmans 4 , M. Hoogeman 2 , H. Struikmans 5 1 Haaglanden Medical Center, Medical Physics, Leidschendam, The Netherlands; 2 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands; 3 Haaglanden Medical Center, Radiation oncology, Leidschendam, The Netherlands; 4 Iridium Netwerk and University of Antwerp, Radiation Oncology, Antwerp, Belgium; 5 Haaglanden Medical Center, Radiation Oncology, Leidschendam, The Netherlands Purpose or Objective The 15-years results of the EORTC 229922-10925 phase-III trial showed a significant decrease in breast cancer mortality, less breast cancer recurrences and limited toxicity without increased non-breast cancer-related mortality after internal mammary chain (IMC) RT.[1] These findings seem counterintuitive since a higher mean heart dose (MHD) in the IMC group was expected to coincide with a higher incidence of cardiac disease and/or cardiac death. To better understand these results and place them in the context of modern RT, a planning study was performed on target volume coverage and OAR doses for whole breast irradiation (WBI) with versus without IMC RT between the following techniques: 2D-RT as used during the era of the EORTC trial (free-breathing), hybrid IMRT (breath-hold) and robust IMPT (free-breathing). Materials and Methods On free-breathing and breath-hold CT-scans of ten left-sided breast cancer patients, we delineated the ipsilateral CTV- breast, CTV-IMC according to the ESTRO guidelines, the heart (including the pericard) and the left anterior descending coronary artery (LAD) region. The CTV-PTV margin was 5mm. Prescribed dose of 40.05Gy/15 fractions was used. For 2D-RT, WBI consisted of two tangential wedged photon fields and the IMC was planned with a mixed electron/photon(47%/53%) beam technique. Hybrid IMRT included two photon tangential fields (70%) supplemented with IMRT (30%). Both were planned using the Monaco TPS. IMPT plans were created with Erasmus-iCycle using multi-field robust optimization. The IMPT plans consisted of two (WBI) or three (WBI+IMC) beams with 5mm setup and 3% range uncertainty as robust optimization setting. The 2D-RT plans were approved according to the EORTC 22922-10925 study protocol. For modern techniques, treatment plans were accepted if at least 98% of the breast volume and 90% of the IMC volume was planned to receive >95% of the prescription dose. MHD, mean LAD and V5 doses were retrieved for all techniques. Results The target volume dose constraints for the hybrid IMRT and IMPT plans were met for both groups. The WBI target coverage was the lowest for the 2D-RT technique (94%). For 2D-RT including IMC, target coverage was 97% and 83% for the breast and the IMC, respectively. MHD for WBI-only was <2Gy for all techniques. For WBI+IMC, MHD was <4Gy for the modern techniques but significantly higher for 2D-RT. Significant reduction in mean LAD and V5 dose was observed for WBI+IMC using modern techniques (Figure 1). The V5 LAD reduced from 100% (2D-RT) towards 70% and 20% for hybrid IMRT and IMPT, respectively.

Conclusion This study obtained insight in target coverage and OAR dose for all techniques. The MHD, mean LAD and V5 dose were significantly higher in the earlier WBI+IMC group. This means that cardiac mortality alone is probably not an appropriate endpoint for evaluating the level of radiation induced cardiac disease. [1] Poortmans et al. The Lancet Oncology, 2020, volume 21, issue 12, P1602-1610

PO-1852 Predicting factors for differences between dose algorithms in SBRT lung patients M. Barceló-Pagès 1 , J.M. Chimeno Hernández 1 , A. Latorre-Musoll 1 , P. Gallego Franco 1 , C. Ansón Marcos 1 , N. García Apellaniz 1 , N. Jornet Sala 1 , J. Perez-Alija 1 , M. Adrià Mora 1 , H. Vivancos Bargalló 1 , A. Ruiz Martínes 1 , P. Carrasco de Fez 1