Clinicopathological relevance of tumour grading in canine osteosarcoma.

Tumour grading assesses biological aggressiveness and is of prognostic signi¢cance in many malignancies. The clinicopathological features of 140 primary canine osteosarcomas and their metastases were analysed, and the
interrelations between them and an established grading system and its constituent parameters (mitotic index, necrosis, pleomorphism) were examined. Of these tumours, 35%were grade III (high-grade), 37%grade II and
28% grade I. Primary tumours that had metastasized were of signi¢cantly higher grade than non-metastatic osteosarcomas. Osteosarcomas belonging to the osteoblastic minimally productive subtype, but not chondroblastic
or telangiectatic subtypes, di¡ered from ¢broblastic osteosarcomas in being associatedwith a signi¢cantly higher number of high-grade cases. Dogs younger than 4 years of age had osteosarcomas with higher grade, score and
mitotic index than did older animals. Appendicular di¡ered fromaxial tumours in having a highermitotic index; distal di¡ered from proximal tumours in being of higher grade; cranial tumours di¡ered from tumours in most other sites in being of lower grade and lower mitotic index. Rib osteosarcomas showed a particularly high degree of necrosis. The mitotic index varied widely between tumour locations. Pleomorphism did not have prognostic merit when examined separately, as most osteosarcomas were highly pleomorphic.