Histological grading of canine mast cell tumor: is two better than three?

Mast cell tumor (MCT) is a common canine cutaneous neoplasm with a variable biologic behavior. Recently, a 2-tier histologic grading system has been proposed by Kiupel et al. to reduce interobserver variations and eliminate the prognostic uncertainty of Patnaik system. The aim of this study is to compare the ability of these two grading systems to predict outcome in a cohort of dogs with cutaneous MCT. One hundred and thirty-seven surgically-resected cutaneous MCTs were examined. For each case the histological grade was assessed according to both systems. The association between grade and survival was evaluated. According to the Patnaik system, 18 MCTs (7.6%) were classified as grade I, 83 (60.6%) as grade II and 36 (26.3%) as grade III. Grade III was associated with poorer prognosis (P < 0.001), but no significant difference between grades I and II was detected. All grade I MCTs were low-grade in Kiupel system, and all grade III were high-grade. Among grade II, 71 (85.6%) were low-grade and 12 (14.4%) were high-grade, with a 1-year survival probability of 94% and 46%, respectively (P < 0.001). The 2-tier system had high prognostic value in predicting tumor mortality and was able to correctly predict the negative prognosis of some grade II MCTs. However, high-grade MCTs classified as grade III had a significantly shorter survival than those classified as grade II (108 and 698 days, respectively, P = 0.027). The simultaneous use of both grading systems might therefore provide a more accurate prognosis than each system individually.