S236
Clinical - Breast
ESTRO 2026
Wilhelmina Hospital Nijmegen, Nijmegen, Netherlands. 12 Dept. of Pathology, Antoni van Leeuwenhoek Hospital, Amsterdam, Netherlands. 13 Dept. Epidemiology, GROW Research Institute for Oncology and Developmental Biology, Maastricht University Medical Centre+, Maastricht, Netherlands Purpose/Objective: The 5-year follow-up results of the NSABP-B51 study [1] and the RAPCHEM cohort [2] suggested that locoregional radiation therapy (RT) can be adapted to the ypN-status in cT1-2N+ breast cancer (BC) patients after primary systemic treatment (PST). However, long- term data is not yet available. Hereby, we present the 10-year results of the RAPCHEM study, evaluating the long-term safety of adapting locoregional RT to the ypN-status, in terms of locoregional recurrence (LRR) rate and LRR including synchronous metastasis (LRR*) rate. Time till LRR* (T-LRR*) was also studied. Material/Methods: Patients with cT1-2N+M0 ( ≤ 3 suspicious nodes on imaging, cN+ histologically confirmed) BC were included in this national prospective registry study, from January 2011 to January 2015. Treatment involved PST, followed by breast surgery (lumpectomy or mastectomy), combined with axillary surgery (axillary lymph node dissection (ALND), or sentinel lymph node biopsy (SNLB) and/or removal of marked axillary lymph nodes (MARI)). Based on ypN-status, three risk groups with corresponding RT strategy were defined: low-risk group (ypN0): whole breast RT (WBRT) after lumpectomy and no RT after mastectomy; intermediate-risk group (ypN1): WBRT or chest wall RT, and if no ALND performed, RT of axilla levels 1-2; high-risk group (ypN2+): WBRT or chest wall RT, and RT of axilla levels 3-4 (with/without internal mammary nodes (IMN) after ALND and levels 1-4 (with/without IMN) after SN/MARI. The primary endpoint was 10-year LRR rate. The T-LRR* was divided into early ( ≤ 5 years) and late (>5 years). Competing risk analysis was used, with cox proportional hazards model to test for differences between BC subtypes and risk groups. Fisher exact test was used to compare early with late LRR*. Results: Of 848 included patients, 838 were eligible for analysis: 291 in the low-, 370 in the intermediate- and 177 in the high-risk group. Twenty-four patients had LRR and forty-one patients had LRR*. The 10-year LRR and LRR* rates are 2.7% and 7.8%, respectively, without significant differences between risk groups. In the whole group, hormone-negative patients had a significantly higher LRR* rate than hormone-positive patients (Table 1). Furthermore, T-LRR* was much shorter (all within 5 years) in TN tumors (Table 2).
of breast radiotherapy dosimetry to date. Linking these real-world dosimetry data with clinical and patient-reported outcomes will be essential to evaluate long-term cardiac and pulmonary morbidity and to advance personalised, data-driven radiotherapy practice in the NHS. References: 1. UK, C.R. Cancer statistics 2024 21.10.2025; Available from: https://www.cancerresearchuk.org/.2. Gannon MR et al. Change in the Use of Fractionation in Radiotherapy Used for Early Breast Cancer at the Start of the COVID-19 Pandemic: A Population-Based Cohort Study of Older Women in England and Wales. Clin Oncol (R Coll Radiol).3. Murray Brunt et al. Hypofractionated breast radiotherapy for 1 week versus 3 weeks (FAST-Forward): 5-year efficacy and late normal tissue effects results from a multicentre, non-inferiority, randomised, phase 3 trial The Lancet, Volume 395, Issue 10237, 4. Bergom C et al. Deep Inspiration Breath Hold: Techniques and Advantages for Cardiac Sparing During Breast Cancer Irradiation. Front Oncol. 2018;8:87. Keywords: Breast Radiotherapy, Cloud Respiratories, Big Data Proffered Paper 1246 Long terms results of radiation therapy de- escalation in cT1-2N1 breast cancer after primary systemic treatment (RAPCHEM, BOOG 2010-03; NCT01279304) Fleur Mauritz 1 , Linda de Munck 2 , Janine Simons 3 , Janneke Verloop 2 , Thijs van Dalen 4 , Paula Elkhuizen 5 , Astrid Scholten 5 , Ruud Houben 1 , A. Elise van Leeuwen 6 , Sabine Linn 7 , Ruud Pijnappel 8 , Philip Poortmans 9,10 , Luc Strobbe 11 , Jelle Wesseling 12 , Adri C. Voogd 13,2 , Liesbeth J. Boersma 1 1 Dept. of Radiation Oncology (Maastro), GROW Research Institute for Oncology and Developmental Biology, Maastricht University Medical Centre+, Maastricht, Netherlands. 2 Dept. of Research and Development, Netherlands Comprehensive Cancer Organisation, Utrecht, Netherlands. 3 Dept. of Radiation Oncology, Erasmus MC, Rotterdam, Netherlands. 4 Dept. of Surgery, Erasmus MC, Rotterdam, Netherlands. 5 Dept. of Radiation Oncology, Antoni van Leeuwenhoek Hospital, Amsterdam, Netherlands. 6 BOOG Study Center, Dutch Breast Cancer Research Group, Amsterdam, Netherlands. 7 Dept. of Medical Oncology, Antoni van Leeuwenhoek Hospital, Amsterdam, Netherlands. 8 Dept. of Radiology, University Medical Centre Utrecht, Utrecht, Netherlands. 9 Dept. of Radiation Oncology, Iridium Netwerk, Wilrijk-Antwerp, Belgium. 10 Faculty of Medicine and Health Sciences, University of Antwerp, Wilrijk-Antwerp, Belgium. 11 Dept. of Surgery, Canisius
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