Systemic Metastatic Progression of Canine Mammary Comedocarcinoma with Pathological Femoral Fracture: A Case Report

Canine mammary comedocarcinoma is considered an aggressive histological subtype with a high potential for metastatic dissemination. A 13-year-old intact female Jindo-mixed dog underwent mastectomy for a grade II mammary comedocarcinoma, and no metastatic involvement was identified in the regional lymph nodes or thorax at initial staging. On postoperative day (POD) 65, the dog developed an atraumatic pathological fracture of the left femoral diaphysis. Despite two surgical fixation procedures, the fracture progressed to non-union with progressive osteolysis. During subsequent follow-up, a subcutaneous mass and radiographic evidence of pulmonary metastasis were identified, and metastatic involvement of the femur was subsequently supported by histopathological examination. The dog died spontaneously on POD 188 following progressive disease and deterioration in clinical condition. Necropsy revealed extensive metastatic carcinoma involving the femur, medial iliac lymph node, lungs, and surrounding soft tissues. This case describes rapid and extensive multisystemic metastatic progression of canine mammary comedocarcinoma despite initially negative regional lymph node and thoracic staging. Notably, the atraumatic pathological femoral fracture was the first clinically recognized manifestation of metastatic disease. This case highlights the potential for skeletal involvement to cause substantial morbidity during metastatic progression of canine mammary carcinoma and emphasizes the importance of considering skeletal metastasis in dogs with aggressive mammary carcinoma presenting with unexplained pathological fractures or persistent failure of fracture healing.