EXPRESS: Feline tarsal tumors: histologic spectrum, risk factors, and prognostic insights.

Feline tarsal RCTs have poor prognosis; use standardized IHC panels for accurate diagnosis.

This retrospective study analyzed 34 feline tarsal neoplasms (18 round cell tumors [RCTs] and 16 soft tissue sarcomas [STSs]) diagnosed between 2010-2024. Initial histologic classification proved challenging, with 39% of RCTs initially untyped. Comprehensive immunohistochemistry (CD3, CD20, CD79a, MUM-1, CD18, IBA-1, E-CAD) and diagnostic algorithm application resulted in 50% reclassification, identifying seven plasma cell tumors, four progressive histiocytosis, three lymphomas, two histiocytic sarcomas, and two undifferentiated RCTs. Male sex, advanced age, and prior tarsal trauma were significant RCT risk factors. RCTs presented with more clinical signs and metastases at diagnosis compared to STSs. RCTs received multimodal treatment including chemotherapy, surgery, or combination therapy, while STSs underwent surgical management alone. Notably, RCTs demonstrated significantly worse prognosis with median survival of 173 days versus not reached for STSs, and time to progression of 139 versus 854 days respectively. This study confirms feline tarsal RCTs represent a heterogeneous, aggressive tumor group with poor outcomes, emphasizing the clinical importance of standardized immunohistochemical panels and diagnostic algorithms for accurate classification and prognostication.

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