Malignant adenomyoepithelioma of the breast: seven cases illustrating morphological diversity and diagnostic challenges

BACKGROUND: Malignant adenomyoepithelioma (M-AME) is an extremely rare and morphologically diverse breast tumour, posing significant diagnostic and management challenges. We present seven cases of M-AME with different histological compositions to contribute additional data to the existing literature. Through these cases, we review and discuss potential pitfalls and classification challenges in diagnosing M-AME. METHODS AND RESULTS: We performed a retrospective clinicopathological review of seven cases of M-AME from women aged 45-74 years (median 53 years). Each case exhibited distinct architectural patterns and compositions of classic and malignant components: (1) Adenomyoepithelioma (AME) with metaplastic spindle and squamous cell carcinoma; (2) Invasive AME with an aberrant immunohistochemical profile; (3) Invasive AME with an adenomyoepithelial adenosis pattern; (4) Metaplastic spindle cell carcinoma arising from AME with suspicion of M-AME in situ; (5) Atypical AME with late lung metastasis harbouring HRAS p.G13R and in-frame PIK3R1 deletion; (6) and (7) M-AME in situ. CONCLUSION: M-AME encompasses heterogeneous patterns that overlap with the infiltrative-like pattern of classic AME (adenomyoepithelial adenosis form of tubular AME) and metaplastic breast carcinoma. Recognition of the underlying AME architecture, coupled with immunohistochemical and molecular correlation, supports pragmatic classification into in situ or invasive M-AME or conventional breast carcinoma arising in the context of AME. Together with clinical, radiological, and molecular correlations, this approach can help guide appropriate management.