Atypical metastatic osteosarcoma pattern in a dog: Lymphatic, neurovascular and subcutaneous metastasis

Abstract A 6‐year‐old, spayed, female labrador retriever was presented with left distal radial swelling. Imaging, cytology and biopsy identified an aggressive radial lesion with metastasis to the left axillary lymph node, and a diagnosis of presumptive osteosarcoma was made. Initial treatment included hypofractionated radiation therapy, axillary lymphadenectomy, carboplatin chemotherapy and GD2/GD3 immunotherapy. Disease progression was 55 days after treatment initiation, with extensive lymphatic and soft tissue metastasis involving the superficial cervical lymph node, brachial plexus and regional musculature. A left forequarter amputation was performed for palliation, and histopathology confirmed metastatic osteosarcoma with lymphatic and neural invasion. Postoperatively, RNA immunotherapy was initiated but discontinued due to rapid local recurrence. The dog was euthanased due to quality‐of‐life concerns. Overall survival time was 121 days from treatment initiation. This case highlights the aggressive biological behaviour of osteosarcoma with lymphatic dissemination, and the challenges in managing advanced cases despite multimodal therapies and novel immunotherapy approaches.