RESUMEN
BACKGROUND: Fibroblasts and/or collagen fibrils have not been included in previous cytologic grading schemes of canine mast cell tumors (MCTs), and their association with biological behavior is broadly debated. OBJECTIVES: This study aimed to evaluate the cytologic findings of canine MCT, with emphasis on the microenvironment, and propose a novel cytologic grading system correlated with mortality and histologic grade. MATERIAL AND METHODS: Cytology smears of canine cutaneous MCTs were retrospectively reviewed and compared with their histopathologic counterparts using Cohen´s Kappa test. One-year survival rates were also compared with the cytologic and histopathologic variables using Pearson´s correlation test. RESULTS: From 92 first-occurrence canine cutaneous MCTs, the five features most associated with mortality were selected for a new grading system. The five features were cytoplasmic granulation, fibroblast and/or collagen fibril concentrations, and the presence of mitotic figures, multinucleation, and karyomegaly. Among concordant histopathologic and cytologic cases (ie, the same grades using both systems), mortality rates were 2.6% (1/38) for low-grade and 71.4% (10/14) for high-grade cases (P < 0.001, chi-square). For false-negative and false-positive results, mortality rates were 33% (1/3) and 45% (5/11), respectively (P = 0.707). CONCLUSIONS: Unlike the Camus cytologic grading system, the present amendment excluded binucleation and included fibroblasts and/ or collagen fibrils, which in higher concentrations were associated with increased survival and a low histopathologic grade. Cytologic grading with the inclusion of fibroblast and collagen fibril concentrations correlated with survival, as did the Camus cytologic and Kiupel histopathologic grades; however, further studies are needed to confirm the prognostic value of this novel cytologic grading scheme.