Histologic Grade Does Not Predict Outcome in Dogs with Appendicular Osteosarcoma Receiving the Standard of Care.

Canine appendicular osteosarcoma is an aggressive bone neoplasm that imposes a short survival time. There are several published histologic grading systems for canine osteosarcoma but no universally accepted system. Location within the skeleton and therapy received are both correlated with survival time, but these factors were not always considered when the prognostic value of published grading systems was determined. Our objective was to compare 2 published histologic grading systems in a population of dogs with appendicular osteosarcoma treated with the standard of care for curative intent. Three evaluators graded 85 tumors using 2 histologic grading systems. The relationships between histologic grade as well as individual histologic features and outcome (survival time and disease-free interval) were evaluated using Kaplan-Meier survival functions and a univariate Cox proportional hazards model. Histologic grade, as assigned by any evaluator, did not correlate with outcome. Increased number of mitotic figures per 3 randomly selected 400× microscope fields, as assessed by 1 evaluator, was correlated with both survival time and disease-free interval; this was the only individual histologic feature that was significantly correlated with outcome for any evaluator. These findings cast doubt on the predictive value of routine histologic grading in dogs with appendicular osteosarcoma who receive amputation followed by adjuvant chemotherapy and highlight the need for better tools to predict outcome in canine appendicular osteosarcoma.