Signalment: 10-year-old male neutered Labrador Retriever
History: 7mm black cutaneous mass on the dorsal hind end that owners just noticed a few days ago.
Description = The smears are highly cellular and nucleated cells are seen in cohesive sheets. The cells are basaloid to ovoid and have a variable volume of medium purple cytoplasm that occasionally contains a small amount of punctate black pigment granules consistent with melanin. Anisocytosis / anisokaryosis are moderate and N/C ratios are variable.
Interpretation = Melanin containing tumor
Comments = The samples contain sheets of moderately pleomorphic, pigmented epithelial cells, confirming a melanin containing tumor. Possible rule outs may include a pigmented tumor of adnexal origin (e.g. pigmented trichoepithelioma, trichoblastoma or pilomatricoma, etc), or a dermal melanoma. Histopathology is recommended for appreciation of tissue architecture for further characterization of origin and biologic behavior.
Description = Haired skin: Extending from the dermis and into the subcutis, there is a well-circumscribed, nonencapsulated, moderately cellular, polycystic mass present. The mass is composed of basaloid epithelial cells piling up to two dozen cell layers deep with central cystic spaces composed of necrotic cells and cellular debris supported by a moderately abundant fibrous stroma. The basaloid epithelial cells have indistinct cell margins, scant to moderately abundant cytoplasm with variable amounts of cytoplasmic pigmentation, oval nuclei, finely stippled chromatin, and 1 to 2 nucleoli. Within the superficial dermis, melanomacrophages are noted.
Interpretation = Apocrine ductal carcinoma
Comments = An apocrine ductal carcinoma rather than a melanoma is noted. This is characterized by the basaloid epithelial cells forming cystic spaces. These can be pigmented or unpigmented. In this instance, this mass is pigmented. Apocrine ductal carcinomas are uncommon neoplasms reported in the dog and cat. The peak incidence is between 6 to 13 years of age. These neoplasms are relatively slow growing and most are amenable to surgical excision with wide margins. Metastases are uncommon.