Abstract
A solid mass approximate 20cm in diameter appeared in the head of a male 16-year-old thoroughbred horse was pathologically diagnosed as poorly differentiated squamous cell carcinoma without cancer pearl formation. The neoplastic cells showed alveolar structure of neoplastic prickle cells with prominently atypical nuclei, frequent mitosis and unicellular keratinization. Some of the cells were cytokeratin positive, and electron microscopy revealed abundant free ribosomes, and less amount of tonofilaments and desmosomes in the tumor cells.
