Unusual, fatal neck cancer in a foal: immunohistological investigations and differential diagnosis.

Neck masses may represent a diagnostic challenge for equine veterinarians, since they may involve different structures with a similar clinical appearance. Whereas neck masses in foals mostly result from non-neoplastic conditions, including various congenital abnormalities, neoplastic cervical masses have been rarely documented. This report describes the unusual case of a rapidly growing neck cancer in a four-month-old colt, resulting in a fatal outcome. Gross and immunohistological findings were most consistent with a poorly differentiated thymic carcinoma with sarcomatoid features and multifocal neuroendocrine differentiation. To the best of our knowledge, thymic tumors involving the cervical component of the equine thymus have not been reported so far. Although thymic tumors are rare, especially in the cervical region, the present findings serve to remind clinicians that when a mass occurs in the horse neck, thymic tumors should be included in the differential diagnosis.