Salivary duct carcinoma of the mandibular gland in a Bardigiano pony

Salivary gland tumors are uncommon in horses, and almost exclusively represented by sporadic cases listed in equine tumor reviews. A monolateral swelling in the right mandibular area in a fourteen-year-old intact male Bardigiano pony was sampled for cytology, and a presumptive diagnosis of low-grade carcinoma was made. A 14 cm roundish subcutaneous mass, firm and well-circumscribed, was surgically removed and identified as the rostral portion of the mandibular gland; cut section showed an encapsulated whitish-yellow glandular-like tissue, composed by multiple irregular lobules separated by fibrous stroma. Histopathology revealed nests and islands of moderately pleomorphic epithelial cells frequently arranged around small duct-like structures, and lined by myoepithelial cells. Several inflammatory cells (neutrophils) and large plump mononuclear cells with eosinophilic intracytoplasmic globules were frequently observed in interstitial and luminal spaces. Tumor cells stained positively for cytokeratins AE1/AE3, 5/6, 14 and negatively for cytokeratin 19, vimentin, MAC-387, S-100, NSE, sm-actin, desmin and laminin; globular material within the plump cells was strongly PAS+ (with and without diastase), PTAH+, NSE+, vimentin+, and negatively to cytokeratins, MAC-387, S-100, actin, desmin, and laminin. These findings, along with transmission electron microscope observations, suggested the diagnosis of salivary gland carcinoma arising from the nonkeratinized squamous epithelium of the terminal portion of the excretory duct.