Abstract Background: Tamoxifen remains a cornerstone in the treatment of early-stage hormone receptor-positive breast cancer and is also used as prophylaxis in high-risk women. However, its use has been associated with an increased risk of venous thromboembolism (2-8%). In Greece, the prevalence of thrombophilic factors (inherited or acquired) is approximately 15%, and these factors may contribute to thrombosis when additional risk factors are present. Methods: We conducted a three-year retrospective analysis, from April 2022 until March 2025, of medical records from breast cancer patients in our department who were evaluated by a hematologist and underwent thrombophilia screening before tamoxifen initiation. Results: Among fifty-nine patients, eighteen (31%) were found to have clinically significant inherited thrombophilic factors, while another twenty (34%) had high-risk acquired thrombophilic conditions. Twelve patients (20%) had no detectable thrombophilic factors, and nine (15%) had non-clinically significant mutations that did not require therapeutic intervention. Following hematological assessment, treatment plans were modified in thirty-eight out of fifty-nine patients (64%). Specifically, four patients received tamoxifen with the addition of aspirin, twenty-five were treated with tamoxifen combined with prophylactic oral anticoagulation (apixaban or rivaroxaban), and in nine cases, an aromatase inhibitor was prescribed instead of tamoxifen due to high thrombotic risk, in combination with antiplatelet or anticoagulant therapy. Conclusions: Pre-treatment hematological evaluation and thrombophilia screening are considered of significant clinical value in patients considered for tamoxifen therapy. Based on their individualized thrombotic risk profile, a substantial proportion of patients may benefit from antithrombotic prophylaxis or a change in hormonal therapy. We intend to further evaluate these findings with a prospective cohort of these patients. Citation Format: S. Gkoura, I. Binas, E. Aravantinou - Fatorou, T. Bilidas, I. Evmorfiadis, C. Poziopoulos, E. Klouva, K. Koutsoukos, G. Oikonomopoulos, G. Karkaletsos, E. Vasili, G. Tsakalos, A. Vassias, G. Klouvas, M. Stathoulopoulou, V. Venizelos, S. Epameinondas, C. Christodoulou, E. Galani. Thrombophilia Screening Prior to Tamoxifen Therapy in Early-Stage Hormone Receptor-Positive Breast Cancer: A Three-Year Retrospective Study 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 PS3-09-12.
Gkoura et al. (Tue,) studied this question.