Histological prognostic factors in canine appendicular osteosarcoma: 57 cases (2014–2022)

Appendicular osteosarcoma is the most common primary malignant bone neoplasia in dogs, with a generally guarded prognosis. Multiple clinicopathological factors have been suggested to be associated with a negative prognosis in dogs with osteosarcoma. The objective of this study was to identify histopathological prognostic factors in canine appendicular osteosarcoma. We hypothesize that the presence of microvascular invasion and certain histological subtypes will negatively impact prognosis. Dogs with surgically resected primary appendicular osteosarcoma were enrolled. The following histopathological results were recorded: subtype of osteosarcoma (chondroblastic, osteoblastic, fibroblastic, giant cell, telangiectatic, poorly differentiated, combined), presence of microvascular invasion, lymphatic invasion, and completeness of surgical margin. Survival analysis was performed to evaluate disease outcome associations with histopathological data. Fifty-seven dogs were included. The majority (43) of dogs had osteoblastic osteosarcoma. Only two dogs had chondroblastic osteosarcoma, one dog had giant cell osteosarcoma, and no dog had fibroblastic or poorly differentiated osteosarcoma. Microvascular invasion was evident in 5 dogs. There was no significant difference in progression-free interval (PFS) or overall survival time (OST) between tumor subtypes. There was also no difference in PFS and OST with tumoral microvascular invasion (median 271 days and 168 days, respectively) or without microvascular invasion (median 209 days and 262 days, respectively). Neither microvascular invasion nor histologic subtype were significantly associated with outcome in dogs with appendicular osteosarcoma. Further evaluation of subtype as a prognostic indicator is warranted, given the predominance of a single subtype in this study. The clinical significance of osteosarcoma subtypes as a prognosticator needs further evaluation based on this study given the predominance of a single subtype.