Clinical features and outcomes of Merkel cell carcinoma in 20 cats

The biological behaviour and prognostic factors of Merkel cell carcinoma (MCC) in 20 cats were studied. The tumours were surgically removed and histopathologically examined. The animals were 8 to 20 years old (median age: 14 years), and the tumours were predominantly located in the neck and head. Follow‐up data were available in 17 cases, and 12 cats died within a year of surgery. The overall median survival time after resection was 243 days (range 16‐360 days). Recurrence occurred in 11 cases, although 6 of them (55%) were found to be margin‐negative. Possible metastasis occurred after the surgery in 10 cases, although 6 of them (60%) were found to be margin‐negative. The histopathological features of MCC included tumour necrosis in 16 cases (80%), vascular invasion in 6 cases (38%) and high mitotic counts (median: 28.5 per high‐power field). Irregular acanthosis was noted adjacent to the tumours in 9 cases (60%). Immunohistochemically, the tumour cells were positive for cytokeratin (CK) 20 and p63 in all cases, synaptophysin in 19 (95%) cases, and CK18 in 16 cases (80%). The study shows that feline MCC is associated with a poor prognosis and exhibited a strong tendency towards local recurrence, regional lymph node metastasis and distant spread.