The spread of a primary neoplasm to the skin is unusual in domestic animals. It is occasionally identified in dogs; less commonly in cats; and rarely in horses, cows, sheep, goats, and pigs. Although all malignant neoplasms are capable of secondary cutaneous involvement, metastatic potential is greatest in mammary gland adenocarcinomas, squamous cell carcinomas, transitional cell carcinomas, transmissible venereal tumors, pulmonary adenocarcinomas, and angiosarcomas. Although appearance is variable, the lesions most commonly are multiple, ulcerated papulonodules. Early cutaneous metastasis is characterized by aggregates of neoplastic cells within superficial and deep dermal vessels. As these lesions evolve, they extend into the dermis and are associated with effacement of adnexa. Generally, it is difficult to distinguish the primary neoplasm based on the morphologic features of a metastatic site. This is because only a small population of cells in the primary tumor have the potential for metastasis, and these cells may have different microscopic features. In cats, pulmonary adenocarcinomas appear to preferentially metastasize to the distal extremities, and when carcinomas are diagnosed on multiple feet, radiographic examination for a primary lung tumor should be performed. Cutaneous metastasis is usually a feature of aggressive tumors and is associated with a guarded to poor prognosis.
Last full review/revision March 2014 by Alice E. Villalobos, DVM, DPNAP