This document discusses the case of Elli, a 6-year old Rottweiler presenting with lameness in the left hindlimb for several months. On examination, Elli had a firm swelling on her distal left tibia and was lame on that leg. Radiographs and tests found no abnormalities except for the swelling. The main differential diagnosis was osteosarcoma, which is the most common bone tumor in dogs. Elli underwent amputation of the affected leg and is being treated with chemotherapy using carboplatin to treat the osteosarcoma and prevent metastases. Osteosarcoma has a poor long-term prognosis, but treatment can potentially extend life for 11 months on average.