Pathomorphology of malignant melanoma in a canine spleen

A case of malignant melanoma was recorded in a four year old female Rottweiler. Grossly, four subcapsular smooth nodules of varying sizes were observed on the parietal surface and one smooth nodule on the visceral surface. On section, the nodule was soft, dirty grey-black, circumscribed and appeared as two distinct areas separated by parenchyma. Cytology revealed oval to polygonal cells containing black granular cytoplasmic pigments, obscuring the nuclear details. Anisocytosis, anisokaryosis, variable nuclear:cytoplasmic ratio, prominent nuclei were seen. Histopathologically, the neoplastic cells were round to spindle to polyhedral in shape with severe/deep and brown black pigmented cytoplasm. On special staining with Fontana, the granules of neoplastic cells stained black and nuclei stained pink. The tissue was then subjected to proliferation marker Ki-67 immunohistochemistry, where more than 75% of neoplastic cells showed strongly positive immunolabelling.