Review of Histological Grading Systems in Veterinary Medicine

Tumor grading is a method to quantify the putative clinical aggressiveness of a neoplasm based on specific histological features.A good grading system should be simple, easy to use, reproducible, and accurately segregate tumors into those with low versushigh risk. The aim of this review is to summarize the histological and, when available, cytological grading systems applied inveterinary pathology, providing information regarding their prognostic impact, reproducibility, usefulness, and shortcomings.Most of the grading schemes used in veterinary medicine are developed for common tumor entities. Grading systems exist forsoft tissue sarcoma, osteosarcoma, multilobular tumor of bone, mast cell tumor, lymphoma, mammary carcinoma, pulmonarycarcinoma, urothelial carcinoma, renal cell carcinoma, prostatic carcinoma, and central nervous system tumors. The prognosticrelevance of many grading schemes has been demonstrated, but for some tumor types the usefulness of grading remains con-troversial. Furthermore, validation studies are available only for a minority of the grading systems. Contrasting data on theprognostic power of some grading systems, lack of detailed instructions in the materials and methods in some studies, and lackof data on reproducibility and validation studies are discussed for the relevant grading systems. Awareness of the limitationsof grading is necessary for pathologists and oncologists to use these systems appropriately and to drive initiatives fortheir improvement