Metaplastic Breast Carcinoma Masquerading as a Benign-Appearing Complicated Cyst: A Case Report and Literature Review

Metaplastic breast carcinoma (MpBC) is a rare and aggressive breast cancer, and its squamous subtype often forms a cystic, necrotic mass that can mimic a benign cyst.We report a 48-year-old woman with a family history of breast cancer and a long history of benign proliferative breast disease, including atypical lobular hyperplasia, who developed a palpable right breast cyst with internal debris (a complicated cyst), classified as Breast Imaging Reporting and Data System (BI-RADS) category 2 on ultrasound.The cyst was sampled four times over eight months.The first three aspirations (months 1, 3, and 7) were all negative for malignancy, although the cyst enlarged to about 4 cm and once yielded blood-stained fluid.Magnetic resonance imaging (MRI) then showed a complex cystic lesion with focal wall thickening (BI-RADS 4A), and a fourth, ultrasound-guided fine-needle aspiration showed atypical cells.Excision revealed a grade 3 metaplastic carcinoma with squamous differentiation in multiple foci with involved margins.The tumor was estrogen receptor (ER) low positive (2%), progesterone receptor (PR) negative, and human epidermal growth factor receptor 2 (HER2) negative, with a Ki-67 proliferation index of 80%, and the patient was referred for chemotherapy and re-excision.A review of published cases showed that squamous and metaplastic breast carcinomas are often first managed as cysts, abscesses or hematomas, and that needle aspiration frequently misses the diagnosis.A cyst that recurs, enlarges, yields bloody fluid or develops wall thickening should undergo core needle biopsy of its wall or solid component rather than serial aspiration, particularly in women with prior atypia or a family history of breast cancer.