Metastatic Digital Chondrogenic Melanocytic Tumour in a Dog

A 9-year-old intact male Brazilian Mastiff dog with a 1-year history of progressive severe swelling in all four limbs and significant locomotory difficulty was euthanized due to unresponsiveness to medical management. Macroscopically, the distal phalanx of the 5^th digit of the left hindlimb was replaced by a cutaneous, non-ulcerated, 3.0 cm diameter, multilobulated, black, firm nodule. The cortical bones of the appendicular skeleton were thickened and partially effaced by infiltrative coalescing nodules (0.1-1.0 cm diameter). The lungs, heart, pleura, mesentery, adrenal glands and kidneys were infiltrated by similar nodules (0.5-3.5 cm diameter). Histological evaluation of the masses revealed a non-encapsulated malignant neoplasm composed of spindloid to polygonal cells that contained variable amounts of intracytoplasmic melanin and were arranged in haphazardly interlacing streams and bundles supported by scant fibrovascular stroma with myxoid and chondroid matrix formation. Neoplastic cells had intense cytoplasmic labeling for S100 and 10% had moderate cytoplasmic immunoreactivity for Melan-A. To the authors' knowledge, this is the first description of a canine metastatic digital chondrogenic melanocytic tumour with strongly suggestive chondroid differentiation of neoplastic melanocytes.