Mucin-rich salivary duct carcinoma with signet-ring cell feature ex pleomorphic adenoma of the submandibular gland: a case report of an unusual histology with immunohistochemical analysis and review of the literature.

Salivary duct carcinoma (SDC) is a highly aggressive salivary gland carcinoma, and the mucin-rich variant of SDC (mSDC) is extremely rare. We report herein one case of salivary mSDC, showing predominantly signet-ring cell features. The patient was an 84-year-old Japanese woman, who noticed swelling in the left submandibular region. The tumor consisted of two components: one was of mSDC, which contained numerous signet-ring cells in large mucinous lakes, whereas the other was of preexisting pleomorphic adenoma (PA), which showed a hyalinized nodule. mSDC was markedly positive for mucin staining. Signetring cells in the mSDC component were immunopositive for androgen receptor, gross cystic disease fluid protein-15, MUC1, MUC2, MUC5AC, MUC5B, and MUC6, but negative for Her-2 and myoepithelial markers. This case was diagnosed as mSDC with the signet-ring cell feature ex PA. We discuss herein the significance of the signet-ring cell feature in mSDC and the mucin pattern of mSDC.