Extraskeletal osteosarcoma in two Guinea pigs (Cavia porcellus).

Osteosarcoma is a malignant osteogenic tumour of mesenchymal origin and is the most commonly reported neoplasm primarily affecting the appendicular skeleton in dogs and cats. When there is no involvement of bones, and the soft tissue is the primary site of the neoplasia, it is known as extraskeletal osteosarcoma (EOS). EOS is well documented in dogs and cats but is rarely reported in non-domestic species. Microscopically, an osteosarcoma is characterized by neoplastic proliferation of mesenchymal cells with production of osteoid matrix or immature bone. The aim of this study is to report two cases of EOS in guinea pigs (Cavia porcellus), as well as to identify and correlate clinical, radiographic, pathological and immunohistochemical findings. In case 1, multiple mesenteric and pancreatic nodules were present. Case 2 had a large subcutaneous mass with pulmonary and renal metastases. In both cases, radiographs revealed soft tissue masses with mineralized areas but no skeletal involvement. Histological examination confirmed malignant mesenchymal proliferation with osteoid deposition. Immunohistochemistry was applied, with the caveat that there are no specific immunohistochemical markers for definitive diagnosis of osteosarcoma. The neoplastic cells in both cases were strongly immunopositive for vimentin and RUNX2. The cases showed variable immunolabelling for S100 and alpha-smooth muscle actin, Ki-67 and p63. These cases enhance the clinicopathological spectrum of EOS in guinea pigs and highlight the importance of combining radiographic, histopathological and immunohistochemical evaluation for accurate diagnosis.