A 9-year-old male castrated Boxer presented to Cornell’s Oncology Service for evaluation of a mass on the lateral aspect of his right stifle suspected to be a local recurrence of a soft tissue sarcoma (STS). Two other masses from the same area had been removed ten and three months prior via excisional biopsy. Histopathologic analysis of the prior subcutaneous masses revealed results consistent with grade 2/3 and 1/3 STS. Physical examination at Cornell confirmed the presence of masses cranial to and within the surgical scar. An arrhythmia was also discovered. Prioritized differentials included recurrent STS and arrhythmogenic right ventricular cardiomyopathy (ARVC) based on the patient’s breed and clinical history. Full staging tests were performed. Results were negative for metastatic disease. A coxofemoral disarticulation was performed to achieve locoregional control of the disease. Postoperative histopathologic analysis of the amputated limb revealed a grade 2 STS with a mitotic index of 10 and clean surgical margins of ≥ 7.5 cm.