Metastatic Pelvic Osteosarcoma in a Dog

Background: Osteosarcomas are malignant neoplasms of bone tissue, with a high prevalence in dogs, especially in large and giant breeds. More commonly, such alterations affect the appendicular skeleton and, to a lesser extent, the axial skeleton. In order to obtain an accurate diagnosis, it is necessary to combine cytological and histopathological findings with clinical parameters, imaging exams and macroscopic findings. In the present study, we report a rare case of combined-type pelvic osteosarcoma with pulmonary metastasis in a dog.Case: A 5-year-old intact large male dog of mixed breed, was submitted to clinical care because of an increase in volume of the left perineal region. The cytological evaluation, performed without imaging exams, indicated that it was an undifferentiated sarcoma. An incisional biopsy defined the diagnosis as telangiectatic osteosarcoma, and with progressive clinical worsening, the patient died. Necroscopic examination revealed multiple nodules in the lungs and an irregular mass with a hard to friable consistency. The mass was intensely vascularised and extended craniodorsally from the left ischial tuberosity to the base of the renal fossa. Microscopically, the neoplasm was diagnosed as combined osteosarcoma, consisting of the osteoblastic, chondroblastic, and telangiectatic subtypes. Metastases with a predominance of the chondroblastic subtype were observed in the lungs.Discussion: This is the first report of combined-type canine osteosarcoma in the ischium. The case reported here is unusual, as there are few reports of canine osteosarcoma in the pelvic bones, and there is no concrete information regarding its histological appearance. Osteosarcoma is the most common bone neoplasm in dogs, representing up to 80% of the tumours found in such organs. In the present case, the dog was a large young adult with a higher probability of neoplasm development. A cytopathological examination is a diagnostic method with good sensitivity and specificity that can confirm osteosarcomas. However, in this case, the cytological diagnosis, performed without the information from the imaging exam, indicated that it was an undifferentiated sarcoma, given the impossibility of the architectural assessment of the lesion. Biopsy samples sent for histology may not be representative of the entire tumour, leading to misclassification of the histological type. Therefore, the evaluation of fragments from various sites of the lesions is recommended. Regarding