S219
Clinical - Breast
ESTRO 2026
Poster Discussion 861 Major Adverse Cardiac Events after breast radiotherapy: A 12-year multicentre cohort study with cardiac substructure NTCP modeling Jeffrey C.H. Chan 1 , Jeffrey C.F. Lui 1 , Julian Tito Tong 2 , Jonan C.Y. Lee 3 , James C.H. Chow 1 , Gavin T.C. Cheung 4 1 Department of Clinical Oncology, Queen Elizabeth Hospital, Kowloon, Hong Kong. 2 Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Kowloon, Hong Kong. 3 Department of Diagnostic and Interventional Radiology, Queen Elizabeth Hospital, Kowloon, Hong Kong. 4 Department of Oncology, United Christian Hospital, Kowloon, Hong Kong Purpose/Objective: Adjuvant radiotherapy (RT) after breast cancer (BC) surgery improves treatment outcome but incidental cardiac irradiation increases the risk of cardiotoxicity. Evidence suggests that cardiac substructure dose may be more crucial than mean heart dose (MHD) but long- term data are scarce. This study aimed to identify the specific cardiac substructure dose parameters most strongly associated with major adverse cardiac events (MACE), and to develop a normal tissue complication probability (NTCP) model for individualized risk prediction. Material/Methods: All female BC patients from two tertiary oncology centres who received postoperative RT between 2011- 2013 with available 3D dose plan were included. The heart and relevant cardiac substructures (LA, RA, LV, RV, LAD and RCA) were retrospectively contoured on planning CT using deep-learning auto-segmentation tool followed by manual verification. Radiation dose- volume parameters were extracted in EQD2 ( α / β = 3 Gy).MACE was defined as cardiogenic death, myocardial infarction, coronary revascularization, unstable angina or heart failure. Multivariable competing risk analysis was performed to identify the best dosimetric predictor for MACE. An NTCP model was developed to estimate the 10-year excess MACE risk attributable to radiation with internal bootstrap validation. Results: A total of 633 patients with a median age of 54 years (IQR: 47-63) were included, with their baseline characteristics shown in table 1. Most common fractionation schedules were 42.56Gy/16Fr or 50Gy/25Fr. Twenty-six (4.1%) patients were ever– smokers. Pre-existing diabetes, hypertension, hyperlipidaemia or prior history of cardiac events were noted in 49 (7.7%), 104 (16.4%), 29 (4.6%) and 37 (5.8%) patients, respectively. The median MHD for the entire cohort was 0.7Gy (IQR: 0.2-2.9 Gy) and 2.7 Gy (IQR: 1.5- 3.9 Gy) for left or bilateral breast cases. All patients
and 2024 in the Netherlands. Eligible tumours were ≤ 1cm (T1a/T1b) grade 1 or 2, or ≤ 2cm (T1c) grade 1 with radical surgical resection. The Chi2-test was used for categorical data. All data was derived from the Netherlands Cancer Registry. Results: A total of 22.601 patients were included, of whom 15.451 were aged 50-69 years old and 7.150 were aged ≥ 70 years old. Most tumours were pT1b (55.3%), grade I (69.4%) and progesterone receptor-positive (83.5%). Morphologically, no special type was most predominant in all age groups (>75%). Endocrine therapy was administered to 3.6% of all patients.Stratified by 10-year age groups within patients that received RT, whole breast RT (WBRT) without boost was most common in all age groups (>64%), followed by partial breast irradiation (PBI) (>20%). Over time, application of WBRT without boost decreased from 79.1% (2014-2016) to 46.6% (2023- 2024, p<0.001), while PBI increased from 4.1% (2014- 2016) to 49.3% (2023-2024, p<0.001). Rates were comparable between younger and older patients. Although RT type was similar across age groups over time, changes in administration of RT were seen. In patients aged 50-69, there was a slight decline from 98.1% (2014-2016) to 91.5% (2023-2024, p<0.001). However, among patients ≥ 70 years, the use of RT declined from 95.0% (2014-2016) to 66.3% (2017- 2019) and further to 46.4% (2020-2022), increasing again to 66.8% (2023-2024, p<0.001). Conclusion: Over the past decade, changes in administration of adjuvant RT after BCS were observed in clinically low- risk women aged ≥ 50. Treatment with RT was more frequent in younger patients. WBRT remained the most common RT type in both groups, but its use decreased over time, while PBI increased. Omission of RT became more frequent, particularly in older patients and was largely driven by participation in the national TOP-1 study on omission of RT after BCS [1]. However, after the TOP-1 inclusion period ended, RT was still omitted in about one third of older patients, indicating an ongoing and broader shift toward treatment de-escalation in this population. References: [1] BOOG Study Center. TOP-1, BOOG 2016- 01, https://www.boogstudycenter.nl/studies/studie- overzicht/top-1/ [accessed 10 November 2025]. Keywords: Radiotherapy trends, elderly, The Netherlands
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