Spontaneous malignant myoepithelioma in the parotid gland of an aged female Crl:CD (SD) rat

An aged female Crl:CD (SD) rat developed a subcutaneous tumor in the left occipital region. Histologically, the tumor was predominantly composed of spindle-shaped cells proliferating in a loosely cohesive fascicular arrangement in a basophilic myxoid matrix with a minor population of epithelioid cells. Metastases were detected in the cervical lymph nodes, kidneys, lungs, and the pleural cavity. Duct-like structures, considered to be entrapped normal salivary ducts, were scattered within the tumor. Immunohistochemically, the tumor cells were diffusely positive for vimentin, focally positive for alpha-smooth muscle actin and p63 in the spindle-shaped population, and positive for cytokeratin AE1/AE3, exclusive to the epithelioid cell population, whereas S100, chromogranin A, and myoglobin were not expressed in any of the tumor cells. Ultrastructurally, spindle-shaped tumor cells exhibited microfilaments, desmosome-like junctions, and cytoplasmic processes. These features were consistent with the diagnosis of malignant myoepithelioma, most likely originating from the parotid gland.