Spindle cell lipoma in the dog: histological diagnostic features.

Introduction: Canine spindle cell lipoma (SCL) is an uncommon benign spindle cell tumour that might be misdiagnosed as soft tissue sarcoma (STS) because of the high cellularity and paucity of mature adipose tissue. The aim of this report was to define the typical histopathological features of canine SCL. Materials and Methods: Dog signalment, tumour size and site were retrieved from medical records. Haematoxylin and eosin, Masson’s trichrome, alcian blue and PAS staining and immunohistochemistry for vimentin, smooth muscle actin (SMA), S100, factor VIII-related antigen and MDM2 were performed. Mitotic count (MC) and MIB-1 based labelling index (LI) were assessed. Results: Five canine subcutaneous tumours were retrieved. Tumours contained bland spindle cells with sparse eosinophilic cytoplasm, an oval nucleus and an inconspicuous nucleolus, intermixed with ropey collagen and myxoid matrix stained by alcian blue. Three cases contained scattered mature adipocytes (<10%). MC was <1 in four cases and 1 in one case. LI range was 0.9-3.4. All cases expressed vimentin and were S100, SMA, factor VIII-related antigen and MDM2 negative, ruling out peripheral nerve, muscular/myofibroblastic and endothelial cell origin. The low proliferation rate, the absence of lipoblasts and the lack of MDM2 expression ruled out fibrosarcoma, myxoid liposarcoma and dedifferentiated liposarcoma, respectively. A fibroma was ruled out based on the high cellularity. Conclusions: Histopathological features identified in this caseload (i.e. bland spindle cells, rare adipocytes, ropey collagen and myxoid matrix) paralleled descriptions of SCL in man. The identification of these morphological hallmarks is necessary to diagnose SCL and to avoid a diagnosis of STS characterized by a poorer prognosis.