Diagnostic immunohistochemistry for canine cutaneous round cell tumours-retrospective analysis of 60 cases

Introduction. Canine cutaneous round cell tumours (CCRCTs) include various benign and malignant neoplastic processes. Due to their similar morphology, the diagnosis of CCRCTs based on histopathological examination alone can be challenging, often necessitating ancillary immunohistochemical (IHC) analysis. This study presents a retrospective analysis of CCRCTs. Materials and methods. This study includes 60 cases of CCRCTs, including 55 solitary and 5 multiple tumours, evaluated immunohistochemically using a basic antibody panel (MHCII, CD18, Iba1, CD3, CD79a, CD20 and mast cell tryptase) and, when appropriate, extended antibody panel (vimentin, desmin, alpha-SMA, S-100, melan-A and pan-keratin). Additionally, histochemical stainings (May-Grunwald-Giemsa and methyl green pyronine) were performed. Results. IHC analysis using a basic antibody panel revealed 27 cases of histiocytoma, one case of histiocytic sarcoma, 18 cases of cutaneous lymphoma of either T-cell (CD3+) or B-cell (CD79a+) origin, 5 cases of plasmacytoma, and 4 cases of mast cell tumours. The extended antibody panel revealed 2 cases of alveolar rhabdomyosarcoma, 2 cases of amelanotic melanoma, and one case of glomus tumour. Conclusions. Both canine cutaneous histiocytoma and cutaneous lymphoma should be considered at the beginning of differential diagnosis for CCRCTs. While most poorly differentiated CCRCTs can be diagnosed immunohistochemically using 1-4 basic antibodies, some require a broad antibody panel, including mesenchymal, epithelial, myogenic, and melanocytic markers. The expression of Iba1 is specific for canine cutaneous histiocytic tumours, and more sensitive than CD18. The utility of CD20 in the diagnosis of CCRCTs is limited.