Polymorphous adenocarcinoma-like tumor of the breast: the first case report from Japan.

Polymorphous adenocarcinoma-like (PmA-like) tumor of the breast is a rare salivary gland-like neoplasm, with few cases reported globally. We present the first case of a PmA-like tumor with a fibrous pseudocapsule from Japan, including radiological findings. A 40-year-old premenopausal woman presented with a painful mass in the right breast. Mammography revealed a high-density oval mass with predominantly circumscribed margins, while ultrasonography showed a hypoechoic oval mass with heterogeneous internal echoes and vascularity. Both imaging modalities suggested malignancy, prompting a vacuum-assisted biopsy. Histopathological evaluation demonstrated diverse growth patterns and positivity for cytokeratin (CK) 7 and Bcl-2. Initially diagnosed as an unusual ductal proliferation, malignancy could not be excluded, leading to lumpectomy. The resected tumor measured 26 mm, exhibited a fibrous pseudocapsule with focal disruption, and showed varied histological patterns, including solid, cribriform, glandular, and cord-like structures. Immunohistochemistry revealed expression of CK7, S100, vimentin, CK5/6, E-cadherin, Bcl-2, p63, and smooth muscle actin (SMA), with no expression of p40, estrogen receptor, progesterone receptor, HER2, c-Kit, or androgen receptor. These findings, including penetration beyond the capsule, supported the diagnosis of an invasive PmA-like tumor of the breast. Following surgery, the patient underwent additional resection and sentinel lymph node biopsy due to a positive margin. No residual tumor or nodal metastases were found. The patient declined adjuvant therapy and remains recurrence-free after 26 months of follow-up. PmA-like tumors of the breast present diagnostic challenges due to their rarity and diverse histopathological features. Further studies are necessary to characterize their clinical behavior and guide management strategies.