Abstract PS5-10-11: Metachrone contralateral breast cancers: Differences in detection mode, Nottingham Prognostic Index, ER, PR and HER2

Abstract Background: Risk factors for contralateral (CL) breast cancers are well studied, not the tumor characteristics if metachrone diagnosed. We studied mode of detection, tumor characteristics and similarities in receptor status in consecutive women with a metachrone CL breast cancer. Methods: We performed a retrospective study comparing demographics, tumor characteristics (Nottingham Prognostic Index) and histologic type, ER, PR (≥1% is pos) and HER2 status (amplified is pos) in women with a > 3m diagnosed CL breast cancer in UZ Leuven between 2000-2023. We also evaluated patient-related variables which could influence changes in tumor type between lesion 1 (prior tumor) and lesion 2 (metachrone tumor). The Mann-Whitney U test was used for continuous variables, the Chi2 test for categoric variables, the Wilcoxon signed rank test for coupled data and Logistic regression for variables predicting changes between lesions. The analyzed variables were : histological subtype (lobular (ILA) vs non-lobular (non-ILA)), time between two diagnosis, age and BMI at diagnosis, age at menarche, number of pregnancies, age at1st term pregnancy, menopausal status, familial risk, germline carrier status, use of hormones. Statistical analysis were done using SAS System for Windows version 9.4. Significance was defined if p<0.05. Results: We selected 289 cases in accordance to our inclusion criteria. Mean age was 54.9y (SD 10.5y); mean BMI 25.8 kg/m2 (SD 5 kg/m2); age menarche 13.3y (SD 1.6y); 1st full term pregnancy 26.2 yrs (SD 5.1y)). There was a mean delay of 8.9 yrs (SD 5.2y) between diagnosis of lesion 1 vs 2. Lesion 2 was more likely screen-detected (82.3% vs 41.5%; P <0.001) with a lower NPI (4.14 vs 3.76; P=0.029). The histologic subtype (Table) of lesion 2 was predicted by that of lesion 1 (P= 0.014). The ER-status of the CL tumor was significantly predicted by the ER-status of the 1st; CL tumor was ER-neg in 32.4% of 68 patients with an initial ER-neg lesion while the ER-status in the CL tumor was ER-pos in 85.5% of 221 patients with an initial ER-pos lesion (P < 0.001); change in ER-status was affected by later age at menarche (OR 1.239; 95% CI 1.030; 1.490). Similar findings were observed for PR-status with a P= 0.034 but none of the tested variables affected these changes. The change in HER-2 status was not significant; 88.9% of 243 cases were HER2 neg if initial HER2 neg and 17.1% of 35 cases were HER2 pos if initial HER2 pos; P= 0.302.[table:Columns refer to histologic subtype in tumor 1; rows in tumor 2 . Reported P-values are two-sided] Conclusion: CL tumors can appear beyond 10 years independently of initial hormonal receptor status. They are more likely screen-detected with a better NPI prognosis. Although histological subtype and receptor status change in a large proportion, lobular type and hormonal receptor status predicted respectively the histological subtype and receptor status of CL lesions. Citation Format: E. Bloemen, A. Laenen, S. Han, C. Remmerie, M. Van Houdt, G. Floris, M. Keupers, C. Van Ongeval, V. Celis, H. De Boodt, A. Coessens, R. Prevos, Y. Van Herck, F. Derouane, C. Weltens, H. Janssen, A. Baten, J. Verhoeven, C. Desmedt, K. Van Baelen, A. Smeets, I. Nevelsteen, H. Wildiers, N. Patrick. Metachrone contralateral breast cancers: Differences in detection mode, Nottingham Prognostic Index, ER, PR and HER2 [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-10-11