Abstract PS2-04-08: Incidence of second primary tumors and second primary breast cancers events in patients with metastatic breast cancer: results from the Austrian AGMT_MBC-Registry

Abstract Background: Second primary tumors (SPTs) occur in approximately 10% of patients following an initial cancer diagnosis and are associated with increased cancer-related mortality. Ongoing advancements in systemic therapies have led to a growing population of both breast cancer (BC) survivors and patients living with metastatic disease. Consequently, individuals with BC face a heightened risk of developing SPTs and second primary breast cancers (SPBC), with a reported 25-year cumulative incidence of contralateral BC of about 10%. Here we present real-world data on SPTs and SPBC in patients with metastatic breast cancer (MBC) derived from the Austrian Study Group of Medical Tumor Therapy (AGMT) MBC-Registry. Patients and methods: The AGMT_MBC-Registry is a multicenter, nationwide, ongoing retrospective and prospective registry capturing data from MBC patients across Austria. For this analysis, only patients with documented information on the presence or absence of SPTs were included. The analysis of SPBC was limited to patients with known tumor location and diagnosis date. To avoid misclassification, no distinction was made between local recurrences and ipsilateral SPBC. Bilateral BC was defined as cancer diagnosed in both breasts within 90 days. Results: As of June 26, 2024, the AGMT_MBC-Registry included 2,850 patients. Among 2,630 evaluable patients, 225 (8.6%) were diagnosed with a SPT, with 12% (27/225) of these having more than one SPT. Most SPTs were diagnosed after the initial BC diagnosis but prior to the development of metastatic disease. A total of 260 malignancies were reported: 88% were solid tumors and 12% hematologic. The most frequent solid tumors were colorectal cancer, melanoma, and lung cancer, while non-Hodgkin lymphoma and acute leukemia were the most common hematologic malignancies ( Table 1 ). Among 2,576 evaluable patients, 533 (20.7%) experienced a SPBC: 145/533 (27.2%) contralateral, 287/533 (53.8%) ipsilateral, and 101/533 (18.9%) initially presented with bilateral BC. Conclusion: In this real-world analysis of patients with MBC, 9% were diagnosed with SPT and 21% experienced a SPBC. These findings highlight the critical role of histopathological verification of suspicious lesions and metastases and underscore the importance of continued awareness for SPTs - not only among BC survivors but also in patients with metastatic disease Table 1: Citation Format: V. Castagnaviz, S. P. Gampenrieder, A. Pichler, W. Herz, R. Pusch, C. Dormann, C. Suppan, M. Sandholzer, T. Winder, S. Heibl, L. Scagnetti, C. Schmitt, A. F. Zabernigg, D. Egle, C. Hager, P. Pichler, F. Roitner, J. Andel, K. Strasser-Weippl, R. Bartsch, M. Hubalek, M. Knauer, C. F. Singer, G. Rinnerthaler, R. Greil. Incidence of second primary tumors and second primary breast cancers events in patients with metastatic breast cancer: results from the Austrian AGMT_MBC-Registry [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 PS2-04-08.