Cytology and Differential Diagnosis of Canine Spindle Cell Lipoma

ABSTRACTBenign lipomatous tumours in dogs included the very common lipoma and its rare variants, such as spindle cell lipoma. Canine spindle cell lipomas have been seldom described, and their histopathological clues included the concomitant presence of mature adipose tissue and a population of spindle cells immersed in a fibromyxoid matrix. The cytological features of this lipocytic tumour have never been described in veterinary medicine. Herein, a cytological description of a spindle cell lipoma in a 3‐year‐old dog, for which 2 years follow‐up data were available is presented. Cytological smears were characterized by a mixture of mature adipocytes and pleomorphic spindle cells, including multinucleated cells in association with blood vessels, thick collagen fibres and small amount of mucinous matrix. On histopathology, the tumour exhibited some papillary growth and was composed mostly by mature adipose tissue enriched in blood vessels and a smaller number of spindle cells immersed in a matrix with ropey collagen and myxoid substance. Spindle cells were positive to vimentin and negative for muscle, neural and endothelial markers. The cytological findings in the present case paralleled descriptions for human spindle cell and pleomorphic lipomas, which are related variants. The abundance of the mature adipose tissue component and unremarkable follow‐up with no recurrence or metastases documented were compatible with a benign tumour. This case highlighted that a prompt diagnosis of sarcoma should be avoided when a cytological sample from a subcutaneous mass in a dog is characterized by a mixture of mature adipocytes and spindle cells in association with collagen bundles.