EXPRESS: Poorly differentiated sarcoma of the calcaneus with associated tendinopathy in a young cat

Case summary: A 1.5-year-old cat presented with lameness localized to the right hock. Osteolysis was noted on radiographs of the medial calcaneal tuber, with associated common calcaneal enthesopathy/tendinopathy and fluid distension of the gastrocnemius bursa on ultrasound. The tendinopathy and bony osteolysis was further examined on CT. Conservative management failed to result in improvement. After loss to follow-up, re-evaluation 150 days after initial specialist surgery evaluation, revealed significant progression of the osteolytic lesion on radiographs and CT, clinical deterioration and atypical mesenchymal cells on histopathology of a bone biopsy. Following amputation, histopathologic and immunohistochemical assessment of the bone lesion was consistent with a poorly differentiated sarcoma, with metastasis to the popliteal lymph node. Relevance and novel information: This case report outlines an uncommon presentation of neoplasia of the calcaneus of a young cat. It provides a detailed description of the progression of this condition and the diagnostic approach through imaging and biopsy. The case highlights the importance of vigilant monitoring and further assessment of lameness if the expected response to treatment does not occur. Collectively, the clinical, radiographic, histologic and immunohistochemical (IHC) analysis of this case contribute to our understanding of feline spindle cell neoplasms.