Abstract PS5-04-30: A Rare Case of Fibroadenoma After Long-Term Estrogen Therapy in a Transgender Woman

Abstract Fibroadenomas are benign breast masses typically present as painless, mobile, well-circumscribed lesions commonly encountered in young women. They are well characterized in this population but not routinely studied in other groups. Fibroadenomas are rare in cisgender men and even less frequently documented in transgender women. Estrogen-based hormone therapy is a fundamental component of gender-affirming treatment for transfeminine individuals, promoting the development of secondary breast tissue. Prolonged exposure to exogenous estrogen may predispose this population to breast pathology not typically observed in cisgender males. We describe the case of a 30-year-old transgender woman who presented with a palpable, mobile breast mass following ten years of continuous oral estrogen therapy. Surgical excision of the breast mass confirmed the diagnosis of a fibroadenoma. With the increasing accessibility of hormonal replacement therapy (HRT) and expanded transgender healthcare services, it is reasonable to expect a rise in breast pathology within this population. HRT-induced breast development facilitates the emergence of breast conditions typically seen in cisgender women, including fibroadenomas and potential malignancy. Despite their rarity in cisgender men and the limited number of reported cases in transgender women, thorough evaluation of any breast lesion in this population remains essential to ensure accurate diagnosis and appropriate management. Citation Format: J. Sahagun, E. A. Marcus. A Rare Case of Fibroadenoma After Long-Term Estrogen Therapy in a Transgender Woman [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 PS5-04-30.