Salivary Duct Carcinoma With Rhabdoid and Signet-Ring Cell Features Mimicking Invasive Lobular Carcinoma of the Breast: A Case Report and Review of the Literature.

Salivary duct carcinoma is a rare, aggressive salivary gland malignancy that morphologically and immunophenotypically resembles breast ductal carcinoma. The rhabdoid variant is characterized by discohesive tumor cells with rhabdoid cytomorphology, frequent loss of E-cadherin expression, and morphologic overlap with pleomorphic invasive lobular carcinoma of the breast. We report a 74-year-old man who presented with an enlarging right buccal mass and trismus. Positron emission tomography-computed tomography demonstrated right cervical lymphadenopathy with no evidence of breast or lung primary. Resection revealed a discohesive, infiltrative tumor composed of pleomorphic rhabdoid cells, with scattered cells containing intracytoplasmic mucin imparting a signet-ring morphology, and prominent targetoid perineural invasion. Immunohistochemically, the tumor showed diffuse positivity for keratin 7, gross cystic disease fluid protein 15 (GCDFP15/PIP), androgen receptor, weak GATA3 expression, loss of E-cadherin, and cytoplasmic p120-catenin (CTNND1) staining, a profile closely resembling that of invasive lobular carcinoma. Extensive nodal metastases were identified. Despite multimodal therapy, the disease recurred. This report highlights the importance of recognizing salivary duct carcinoma with rhabdoid features to avoid misdiagnosis and guide appropriate management.