SUN-128 Presentation of a Benign Phyllodes Tumor in a Transgender Woman on Estradiol and Progesterone Therapy

Abstract Disclosure: K. Yuang: None. S. Khan: None. Introduction: Phyllodes tumors (PT) of the breast are fibroepithelial tumors of stromal origin that account for less than 1% of breast tumors, usually occurring in women with a median age of 45.1 It has been suggested that somatic mutations of fibroadenomas, growth factors produced by breast endothelium, and altered expression of progesterone and estrogen receptors on the epithelium of PT may play a role in PT development.1,2 Case reports of PT in men demonstrate an association with gynecomastia, and use of bicalutamide is noted in several cases, which together suggest a possible role of estrogen in PT development in men.3 Little is known about PT in transgender women, the first case report published only in 2018. Here we discuss an interesting case of a transgender woman who developed PT of the breast after 4 years of hormone therapy. Case: A 24-year-old transgender female presented for evaluation of a firm lump under her left breast, not previously noted on exam one year prior. She had received estradiol for 4 years (oral tablets for 3 years, then switched to intramuscular (IM) injections 1 year prior to presentation; at the visit, she was on 8 mg IM weekly) and progesterone 100 mg daily for 10 months. Spironolactone was initiated 4 years ago and discontinued when she was started on progesterone. Her estradiol levels exceeded 200 pg/mL on only two occasions (levels of 323 pg/mL and 739 pg/mL), the most recent of which was about 10 months prior. On exam, she had a 1-inch palpable mass in the left breast, 4 O’clock region, without lymphadenopathy or nipple discharge. An ultrasound revealed a 7.6 x 4.1 x 6.8 cm mass (BIRADS 4) that was biopsied. Pathology revealed fragments of a fibroepithelial lesion with broad leaf like fonds and an expanded stromal component raising the possibility of a phyllodes tumor. After review at the multidisciplinary breast tumor board, patient underwent mastectomy. Surgical pathology confirmed a benign phyllodes tumor. Conclusion: This case brings into question the role exogenous estrogen and progesterone may have in PT development in transgender women. It further highlights the importance of the awareness of breast pathology and consideration of breast cancer screening in this population. References:1 Parker, S J, and S A Harries. “Phyllodes tumours.” Postgraduate Medical Journal, vol. 77, no. 909, 1 July 2001, pp. 428-435, https://doi.org/10.1136/pmj.77.909.428. 2 Tse, Gary M K et al. “Hormonal receptors expression in epithelial cells of mammary phyllodes tumors correlates with pathologic grade of the tumor: a multicenter study of 143 cases.” American journal of clinical pathology vol. 118,4 (2002): 522-6. doi:10.1309/D206-DLF8-WDNC-XJ8K 3 Dzulhijar, Naufal, & Muhammad Yamsun. "Phyllodes Tumor on Male Breast: A Literature Review." Indonesian Journal of Cancer [Online], 18.2 (2024): 234-240. Web. 1 Feb. 2025 Presentation: Sunday, July 13, 2025