Histopathological diagnosis and adjuvant therapy with toceranib phosphate in a dog with extraskeletal osteosarcoma: a case report

Abstract

In this paper, we explore diagnosing and treating osteosarcoma in the mammary gland using a combination of tyrosine kinase inhibitors (TKIs) and cyclophosphamide. A case involving a 14-year-old female mixed-breed dog with a 12.4 × 11 cm singular mass in the mammary region is discussed. Initial cytological analysis from a fine-needle aspiration suggested the possibility of mixed-type mammary carcinoma and carcinosarcoma. The mass was surgically excised and sent for histopathological examination, which confirmed it as extraskeletal osteosarcoma of the mammary gland. Following surgery, metronomic chemotherapy with cyclophosphamide and fibrocoxib was implemented, along with targeted therapy using toceranib and cyclophosphamide. Seven months post-chemotherapy, the dog exhibited lameness in the right hind limb and metastasis in the 4th thoracic spine. Within 10 months, the condition progressed to tetraparesis, leading to euthanasia 1 month later. This case highlights the need for differential diagnosis among mammary gland mixed-type carcinoma, carcinosarcoma, osseous metaplasia, and osteosarcoma in the context of osteocytes for accurate prognosis. The dog survived 332 days post-surgery with the chemotherapeutic regimen involving toceranib and cyclophosphamide, indicating the effectiveness of TKI and metronomic chemotherapy in extending survival beyond the median for extraskeletal osteosarcoma. © 2025 The Korean Society of Veterinary Science.