A case series of adenomyoepithelioma of the breast with a case of malignant adenomyoepithelioma misinterpreted as polymorphous adenocarcinoma.

Background: Adenomyoepithelioma (AME) of the breast is a rare tumor composed of epithelial and myoepithelial cells, typically exhibiting benign behavior. However, malignant transformation (MAME), local recurrence, and distant metastases have been reported. Polymorphous adenocarcinoma (PmA), an exceptionally rare breast carcinoma, can mimic MAME morphologically, complicating diagnosis and management. Due to their rarity, standardized management guidelines remain undefined highlighting the need for further research and comprehensive evaluation. Case Description: This study presents nine cases of AME diagnosed at a single institution diagnosed with AME, including one initially misdiagnosed with PmA but later reclassified as MAME. The patients aged 31-61 years with a varied clinical presentation including palpable masses (n=3), lesions detected on mammography (n=4), microcalcifications (n=1), and nipple discharge (n=1). Imaging findings revealed predominantly solid masses with lobulated contours on ultrasound (US) and variable enhancement patterns on magnetic resonance imaging (MRI). Immunohistochemical (IHC) analysis, including p63, CK5/6, and calponin, was essential for confirming diagnoses. The highlight case involved a 54-year-old woman initially diagnosed with PmA based on histopathological evaluation. However, upon retrospective review, the lesion was reclassified as MAME due to predominant myoepithelial differentiation. The patient received surgical excision and radiation therapy and remained disease-free after 78 months of follow-up. AME typically presents as a unilateral, centrally located mass, with recurrence rates of 11.1% reported. MAME, characterized by high mitotic rates and cellular atypia, requires aggressive management, while PmA, a low-grade carcinoma, often exhibits perineural invasion and triple-negative status. Conclusions: This case series underscores the diagnostic challenges and importance of accurate differentiation between AME, MAME, and PmA, as misdiagnosis can significantly impact treatment and prognosis and highlighting the importance of IHC and molecular profiling. Given the potential for recurrence and metastasis, complete excision with negative margins remains the mainstay of treatment. This series contributes to the limited literature on these rare tumors, emphasizing the critical role of meticulous pathological evaluation and multidisciplinary approaches to improve patient outcomes.