ESTRO 2025 - Abstract Book

S429

Clinical - Breast

ESTRO 2025

Individual Patient Data.” Journal of clinical oncology : official journal of the American Society of Clinical Oncology vol. 36,22 (2018): 2288-2296. doi:10.1200/JCO.2017.77.6351 Piroth MD, Baumann R, Budach W, Dunst J, Feyer P, Fietkau R, Haase W, Harms W, Hehr T, Krug D, Röser A, Sedlmayer F, Souchon R, Wenz F, Sauer R. Heart toxicity from breast cancer radiotherapy : Current findings, assessment, and prevention. Strahlenther Onkol. 2019 Jan;195(1):1-12. doi: 10.1007/s00066-018-1378-z. Epub 2018 Oct 11. PMID: 30310926; PMCID: PMC6329735.

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Proffered Paper Local recurrence with and without a tumour-bed boost in the DBCG IMN2 study Anders W. Mølby Nielsen 1,2,3 , Lise B. J. Thorsen 1,3 , Demet Özcan 1,4 , Louise W. Matthiessen 5 , Else Maae 6 , Marie L. H. Milo 7 , Mette H. Nielsen 8 , Trine Tramm 4 , Jens Overgaard 1,3 , Birgitte V. Offersen 1,5,9 1 Department of Experimental Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Oncology, Aarhus University Hospital, Aarhus, Denmark. 3 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 4 Department of Pathology, Aarhus University Hospital, Aarhus, Denmark. 5 Deparment of Oncology, Copenhagen University Hospital Herlev and Gentofte, Copenhagen, Denmark. 6 Department of Oncology, Vejle Hospital, University Hospital of Southern Denmark, Vejle, Denmark. 7 Department of Oncology, Aalborg University, Aalborg, Denmark. 8 Department of Oncology, Odense University Hospital, Odense, Denmark. 9 Danish Center for Particle Therapy, Aarhus University, Aarhus, Denmark Purpose/Objective: In early-stage breast cancer (BC), after breast-conserving surgery (BCS) and whole-breast irradiation, the addition of a tumour-bed boost (TBB) reduces the risk of local recurrence (LR) by around 50%. However, it does not improve overall survival and increases the risk of long-term fibrosis. Consequently, no strict indications exist for the use of TBB. As a result, TBB administration varies extensively between countries and institutions, e.g. Norway (0%) 1 , Denmark (15%) 1 , the Netherlands (37%) 2 , and Germany (85%) 1 . To mitigate this, a LR incidence of 6% at 10 years has been proposed as a threshold where benefits outweigh the potentially detrimental effects of a TBB. Therefore, we aimed to investigate indications for a TBB according to prognostic risk factors in node-positive BC patients from the Danish Breast Cancer Group (DBCG) IMN2 study. Material/Methods: The DBCG IMN2 study investigated internal mammary node irradiation (IMNI) from 2007-14 in 4,541 node-positive BC patients, of whom 2,430 received BCS and irradiation to the residual breast and regional nodes with or without IMNI. Radiotherapy was 3D-CRT, delivered as 48Gy/24Fx until 2009 and 50Gy/25Fx thereafter. According to DBCG guidelines, TBB was given sequentially as 10Gy/5Fx (41-49 years) and 16Gy/8Fx (<41 years or narrow margins). Systemic therapy included anthracyclines, taxanes, aromatase inhibitors, and trastuzumab. Patients with and without a TBB were analysed separately. Prespecified subgroups were known prognostic risk factors. LR was first event of invasive carcinoma in the residual ipsilateral breast, with or without synchronous competing events (regional recurrence, distant metastasis, other malignancies, invasive contralateral BC, and death from any cause). Results: In the BCS cohort (n=2,430), median follow-up was 13.7 years, and the cumulative incidence of LR was 3.6% (95% CI, 2.9-4.3) at 10 years. The corresponding cumulative incidence of contralateral BC was 2.9% (95% CI, 2.2-3.6) at 10 years. In patients 50+ years, 1,871 patients were treated without a TBB and 79 with a TBB. In patients without a TBB and with an ER-/HER2- tumour, the 10-year cumulative incidence of LR was 8.3% (95% CI, 4.6-13.6). No other subgroups exceeded the prespecified threshold of 6% at 10 years, table 1.