Objective—To compare the Kiupel (2 categories) and Patnaik (3 categories) histologic grading systems for predicting the presence of metastasis at the time of initial examination in dogs with cutaneous mast cell tumors (MCTs). Design—Retrospective case series. Animals—386 client-owned dogs with cutaneous MCTs. Procedures—Medical records of dogs with newly diagnosed, histologically confirmed cutaneous MCTs that had undergone complete clinical staging were reviewed for clinical and histopathologic data. Results—All Patnaik grade 1 MCTs (n = 52) were classified as Kiupel low-grade MCTs, and all Patnaik grade 3 MCTs (43) were classified as Kiupel high-grade MCTs. Of the 291 Patnaik grade 2 MCTs, 243 (83.5%) were classified as Kiupel low-grade tumors, and 48 (16.5%) were classified as Kiupel high-grade MCTs. Dogs with Patnaik grade 3 MCTs were significantly more likely to have metastases at the time of initial examination than were dogs with grade 1 or 2 MCTs (OR, 5.46), and dogs with Kiupel high-grade MCTs were significantly more likely to have metastases than were dogs with Kiupel low-grade MCTs (OR, 2.54). However, 3 of 52 (5.8%) dogs with Patnaik grade 1 tumors, 48 of 291 (16.5%) dogs with Patnaik grade 2 tumors, and 44 of 295 (14.9%) dogs with Kiupel low-grade tumors had metastatic disease. Conclusions and Clinical Relevance—Findings indicated that in dogs with cutaneous MCTs, prognostication should not rely on histologic grade alone, regardless of grading system used, but should take into account results of clinical staging.

Mast cell tumor (MCT) is a common canine cutaneous neoplasm with variable biological behavior. A 2-tier histologic grading system was recently proposed by Kiupel et al to reduce interobserver variation and eliminate prognostic uncertainty of the Patnaik system. This study compared the ability of these 2 grading systems to predict survival in a cohort of dogs with MCTs. However, surgical margins were unknown, and the risk of developing new/metastatic MCTs was not studied. Histologic grade was assessed according to both systems for 137 surgically resected cutaneous MCTs. The relationship between grade and survival was evaluated. According to the Patnaik system, 18 MCTs (13.1%) were classified as grade I, 83 (60.6%) as grade II, and 36 (26.3%) as grade III. Grade III was associated with a poorer prognosis (P < .001), but no significant difference between grades I and II was detected. Grading according to the Patnaik system was based on consensus grading among 3 pathologists, and interobserver variability was not considered. All grade I MCTs were low grade in the 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 < .001). The 2-tier system had a high prognostic value and was able to correctly predict the negative outcomes of some grade II MCTs. Data also confirm that histologic grading cannot predict biological behavior of each MCT and should be supplemented with molecular methods for more accurate prognostication.

Objective—To assess the usefulness of histologic evaluation of surgical margins to predict local recurrence of cutaneous malignant tumors in dogs and cats treated by means of surgical excision. Design—Prospective case series. Animals—40 dogs and 20 cats. Procedures—60 surgically excised tumors (20 soft tissue sarcomas [STSs], 20 mast cell tumors [MCTs], and 20 carcinomas) were examined histologically. Margins were classified as clean, close, or infiltrated; histologic grade was assessed in STSs and MCTs. Recurrence rates and recurrence-free intervals (RFIs) during a 24-month follow-up period were recorded, and method accuracy was calculated. Results—Surgical margins were clean in 29 of 60 (48%) tumors, close in 11 (18%), and infiltrated in 20 (33%). Tumors recurred in 27 of 60 (45%) animals, with a mean ± SD RFI of 229 ± 173 days. Recurrence rates for animals that had tumors with infiltrated (16/20) or close (8/11) margins were significantly higher than recurrence rate for animals that had tumors with clean margins (3/29). Margin classification was a significant predictor of RFI. Accuracy of the method to predict recurrence was 94% for carcinomas, 87% for STSs, and 76% for MCTs. Conclusions and Clinical Relevance—Histologic assessment of margin status was useful for predicting local recurrence of cutaneous malignant tumors in dogs and cats treated by means of excision alone. Method accuracy varied among tumor types and grades. Recurrence times suggested postsurgical follow-up should continue for ≥ 2 years. Results were similar for animals with infiltrated and close tumor margins, and careful postsurgical management is recommended for both.

Feline cutaneous mast cell tumors (MCTs) have been histologically classified as mastocytic (well differentiated or pleomorphic) and atypical/poorly granulated. Their biologic behavior ranges from benign to malignant, but prognostic factors are not well defined. Histologic classification, number of tumors, mitotic index, cytoplasmic granularity, and infiltration by eosinophils or lymphocytes were evaluated retrospectively in 25 feline cutaneous MCTs. Immunohistochemistry was applied to assess KIT (CD117) pattern and immunoreactivity score, telomerase expression (human telomerase reverse transcriptase), and proliferation index (MIB-1/Ki67 index). Case outcome was obtained via telephone interviews. The tumors comprised 15 mastocytic well-differentiated, 7 mastocytic pleomorphic, and 3 atypical/poorly granulated MCTs. Immunohistochemically, CD117 was expressed in 13 of 25 tumors (52%), and telomerase reverse transcriptase was expressed in 15 of 22 (68%), with no correlation to histologic classification. Mitotic index, KIT immunoreactivity score, and Ki67 index were significantly higher in mastocytic pleomorphic MCTs than in the other 2 categories. Five cats (20%) died of tumor-related causes. Multiplicity of lesions, pleomorphic phenotype, KIT immunoreactivity score, and mitotic and Ki67-indices correlated with an unfavorable outcome. Mitotic index was the strongest predictive variable. These results suggest that histologic classification, CD117/KIT immunohistochemistry, and proliferation indices may help to identify potentially aggressive cases of feline cutaneous MCT. Aberrant KIT protein localization and telomerase immunoreactivity warrant further exploration as potential prognostic markers.