Cytological grading of canine cutaneous mast cell tumours.

A cytological grading for mast cell tumours (MCTs) would be highly desirable, allowing to select the most appropriate therapeutic intervention prior to surgery. This study evaluates the applicability on fine-needle aspirations (FNAs) of the novel Kiupel grading system, based on number of mitoses, multinucleated cells, bizarre nuclei and presence of karyomegaly. Fifty consecutive cases with pre-operative cytological diagnosis were included. In cytological specimens, approximately 1000 cells were evaluated, and the histological grade was assessed on the corresponding resected specimens. On cytology, the above parameters were significantly different between histologically low-grade and high-grade tumours (P < 0.001). The cytograding correctly predicted the histological grade in 47 cases (accuracy, 94%; sensitivity, 84.6%; specificity, 97.3%). Two high-grade MCTs (4%) were not detected on cytology. The cytograding can provide helpful insights to assist clinical decisions in most cases. However, the risk of underestimation in a minority of patients represents a limit to the overall utility of the technique.