Use of a 2-tier histologic grading system for canine cutaneous mast cell tumors on cytology specimens

Mast cell tumors (MCT) represent the most common malignant skin tumor in the dog. Diagnosis of an MCT can be achieved through cytologic examination of a fine-needle aspirate. However, the grade of the tumor is an important prognostic marker and currently requires histologic assessment. Recently a 2-tier histologic grading system based on nuclear features including number of mitoses, multinucleated cells, bizarre nuclei, and karyomegaly was proposed.

The aim of this study was to assess if the cytomorphologic criteria proposed in the 2-tier histologic grading system are applicable to cytology specimens.

A total of 141 MCT specimens reported as grade I, II, or III according to the Patnaik system with both histologic specimens and fine-needle aspirates available were histologically and cytologically reevaluated in a retrospective study.

According to the 2-tier grading system, 38 cases were diagnosed histologically as high-grade and 103 as low-grade MCT. Cytologic grading resulted in 36 high-grade and 105 low-grade tumors. Agreement between histologic and cytologic grading based on the 2-tier grading system was achieved in 133 cases (sensitivity 86.8%, specificity 97.1%, kappa value 0.853), but 5 high-grade tumors on histology were classified as low-grade on cytology.

Cytologic grading of MCT in the dog is helpful for initial assessment. However, the reliability of cytology using the 2-tier grading system is considered inadequate at this point. Prospective studies including clinical outcome should be pursued to further determine diagnostic accuracy of cytologic mast cell grading.