Low-grade adenosquamous carcinoma of the breast is a rare variant of metaplastic mammary carcinoma. It shows indolent behavior contrary to the usual aggressive nature of metaplastic carcinomas and has a good prognosis despite being triple negative. Recurrence rates tend to be high and a consequence of incomplete excision. Although this variant has an infiltrative growth pattern, owing to its bland cytologic features, it is liable to be confused with benign sclerosing adenotic breast lesions. We present here a case of a 55-year-old postmenopausal female, who presented with a painless, mobile, hard, and nontender lump in the lower outer quadrant of the left breast, with normal overlying skin and nipple-areola complex. No associated axillary lymphadenopathy was seen. On mammography, a high-density mass of architectural distortion, characterized as BIRADS category 4C, was found. Core-needle biopsy showed haphazard glands lined by a double layer of epithelium and nests of squamoid cells arranged in an infiltrative fashion within a fibromyxoid stroma. On immunohistochemistry, tumor cells showed a lack of expression of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 receptor and were positive for CK 5/6 and CK7. There was counterintuitive, but characteristic positivity for myoepithelial markers calponin and CD 10 around the neoplastic nests and stromal cells expressed smooth muscle myosin. Subsequently, the patient underwent a wide local excision with free margins and sentinel lymph nodes were negative for tumor deposits. This patient remains well and free of recurrence well into follow-up.
Keywords: Adenosquamous; breast; immunohistochemistry; metaplastic carcinoma; variant.