ESTRO 2025 - Abstract Book

S485

Clinical - Breast

ESTRO 2025

4. Yang K, et al. Optimal radiotherapy for patients with internal mammary lymph node metastasis from breast cancer. Radiat Oncol. 2020 Mar 3;15(1):16.

1531

Mini-Oral Risk factors of local breast cancer recurrence after mastectomy in the Netherlands: a retrospective nationwide cohort study Thomas Stindt 1 , Marissa van Maaren 1,2 , Birgitte V Offersen 3 , Sabine Siesling 2 , Dominique van Uden 4 , Philip Poortmans 5 , Liesbeth J Boersma 6,7 , Orit Kaidar-Person 8,9,10 1 Department of Health Technology and Services Research, Technical Medical Centre, University of Twente, Enschede, Netherlands. 2 Department of Research and Development, Netherlands Comprehensive Cancer Organisation, Utrecht, Netherlands. 3 Department of Experimental Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark. 4 Department of Surgery, Canisius Wilhelmina Hospital, Nijmegen, Netherlands. 5 Department of Radiation Oncology, Iridium Netwerk, Wilrijk-Antwerp, Belgium. 6 GROW- Research Institute for Oncology and Developmental Biology, Maastricht University, Maastricht, Netherlands. 7 Department of Radiation Oncology, MAASTRO, Maastricht, Netherlands. 8 GROW- Research Institute for Oncology and Developmental Biology, Maastricht UniversityUniversity, Maastricht University, Netherlands. 9 School of Medicine, Tel Aviv University, Tel Aviv, Israel. 10 Breast Radiation Unit, Sheba Medical Center, Ramat Gan, Israel Purpose/Objective: Despite the supposed complete removal of glandular breast tissue after mastectomy, some breast cancer (BC) patients experience a local recurrence (LR) after surgery. As most literature focuses on locoregional recurrence combined, knowledge about isolated LR is limited. This study aims to identify potential risk factors associated with the development of LR after mastectomy. Material/Methods: In this retrospective nationwide cohort study, women without prior BC diagnosed with primary unilateral nonmetastatic breast cancer treated with radical mastectomy between 2012 and 2016 were selected from the Netherlands Cancer Registry. Variables were selected based on univariable Cox regression before inclusion in multivariable cause-specific Cox regression to estimate the hazard ratios (HR) and 95% confidence intervals (95%CI) of LR rates. Separate models were constructed for patients with and without primary systemic therapy (PST). Collinearity and variable interactions were assessed using variance inflation factors and inspecting interaction variables, respectively. The goodness of fit was assessed using Cox-Snell residuals. Missing data were imputed. Results: In total, 22,115 patients were included: 17,082 in the non-PST group and 5,033 in the PST group. In the non-PST group, 345 (2%) had LR within the median follow-up of 7.8 years. In the PST group, 80 (1.6%) had LR within the median follow-up of 6.7 years. A total of 8,040 patients had postmastectomy RT, 11,643 had chemotherapy, and 15,715 had endocrine therapy (ET). In the non-PST group (table 1), multivariable Cox regression showed statistically significant protective effects for LR for low socioeconomic status, detection through screening, differentiation grade 1 compared to 2, targeted treatment when HER2 positive compared to HER2 negative, receipt of RT, and receipt of adjuvant chemotherapy. Risk-increasing effects were age 50-59 years compared to 60-69, location in the inner quadrants compared to the outer quadrants, ≥pT2 compared to pT1, ≥pN1 compared to pN0, immediate breast reconstruction, and no endocrine treatment while PR or ER positive. In the PST group (table 2), multivariable Cox regressionshowed statistically significant protective effects for pathologic complete response, positive HER2 status, and receipt of RT. Risk-increasing effects were cT4 compared to cT2, the presence of a DCIS component, and receiving no endocrine treatment while ER or PR positive. No collinearity or interactions were detected and all models showed adequate goodness of fit.