The present report describes a rare case of spontaneous tumor of the salivary gland in a male Sprague-Dawley rat. The clinically confirmed mass rapidly developed in the cervical region between 19 and 21 weeks of age, and the animal was subsequently euthanized. At necropsy, a well-circumscribed nodule approximately 7 × 6 cm in diameter was found at the site of the salivary gland. The cut surface of the nodule was lobulated and soft and had a pinkish tan fish-flesh appearance. One large cyst (approximately 3 × 2 cm in size) containing reddish fluid was also present in the nodule. Histopathologically, the tumor, with a partially lobulated structure, was surrounded by a thin fibrous capsule. The majority of tumor cells formed a diffuse solid sheet structure that mainly consisted of small ovoid or spindle-shaped cells. In the tumor periphery, some cells were arranged in nest-like structures. Small duct-like structures lined with a monolayer of cuboidal epithelial cells resembling an intercalated duct or large polygonal clear cells with a myoepithelial component were also observed. Mitotic figures and necrotic foci were frequently observed in solid areas. Immunohistochemically, the tumor cells were positive for cytokeratin, epithelial membrane antigen, vimentin, p63, α-smooth muscle actin and calponin. The cells were negative for calcitonin, synaptophysin and chromogranin A. On the basis of these findings, the tumor was diagnosed as an epithelial-myoepithelial carcinoma originating from the luminal epithelial cells and myoepithelial cells in the submandibular gland.
Keywords: Sprague-Dawley rat; epithelial-myoepithelial carcinoma; immunohistochemical features; submandibular gland.