Differentiation of feline cutaneous round cell tumours using immunohistochemistry

Fifty feline cutaneous round cell tumours, comprising a range of histological diagnoses, were exposed to a large panel of immunohistochemical markers for T and B lymphocytes, plasma cells, histiocytes, melanocytes and mast cells with the aim of providing species-specific information on labelling patterns. There were few instances in which an individual marker was consistently expressed in all tumours of a particular histological type. Not all mast cell tumours expressed KIT, and KIT was detected in one melanocytic neoplasm. A high proportion of plasma cell tumours co-expressed CD3 and MUM-1, necessitating the use of additional immunohistochemical markers or features to differentiate between plasma cell tumours and T-cell lymphomas. There was also overlapping marker expression by B-cell lymphomas and plasma cell tumours. E-cadherin labelling was detected in some histiocytic and melanocytic tumours, while interpretation of ionized calcium-binding adaptor molecule 1 labelling was challenging, with many tumours containing significant numbers of tumour-associated macrophages or dendritic cells alongside the neoplastic cell population. Despite the extensive panel of immunohistochemical markers, a proportion of the cases could not be definitively diagnosed; however, the use of this panel allowed for a more definitive diagnosis in most of these cases.