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

ObjectivesTarsal tumors are rare, but previous reports suggest a predilection for round cell tumors (RCTs) and soft tissue sarcomas (STSs) in this region. This study aimed to determine the proportion of RCTs among feline tarsal neoplasms, refine classification through histologic revision and immunohistochemistry (IHC), assess potential risk factors, and evaluate clinical outcomes based on tumor histotypes.MethodsA retrospective analysis of feline tarsal neoplasms diagnosed between 2010 and 2024 was conducted. Signalment, history, treatment, and outcomes were collected for RCTs and STSs. All RCTs underwent histologic review and IHC (CD3, CD20, CD79a, MUM-1, CD18, IBA-1, E-CAD). A diagnostic algorithm was applied to support the diagnostic process.ResultsThirty-four cases were included: 18 RCTs and 16 STSs. In 39% of RCTs, the initial histotype was undetermined. Following IHC and application of the diagnostic algorithm, 50% of cases were reclassified: seven plasma cell tumors, four progressive histiocytosis, three lymphomas, two histiocytic sarcomas and two undifferentiated RCTs. Male sex, older age and prior tarsal trauma were significantly associated with RCTs (P=0.042, P=0.048, and P=0.009, respectively). Clinical signs and metastases at diagnosis were more frequent in RCTs (P=0.019 and P=0.001, respectively). RCT treatment included chemotherapy (n=7), surgery (n=5), surgery and chemotherapy (n=2), prednisolone (n=1), or none (n=1); two cases lacked treatment data. All STSs were managed surgically without chemotherapy. Time to progression and median survival were significantly shorter for RCTs compared to STSs (139 vs 854 days; 173 vs not reached, respectively; P<0.001).Conclusions and relevanceThis study confirms that feline tarsal RCTs are a heterogeneous group of tumors with a poor prognosis. Risk factors may include male sex, older age, and previous tarsal trauma. A standardized IHC panel combined with a diagnostic algorithm improved histotyping accuracy and should be adopted in clinical practice.