Canine Mast Cell Tumor Update

Posted by: Paula M. Krimer, DVM, DVSc, DACVP

Canine mast cell tumors have been graded in North America predominantly by the Patnaik system since the system was proposed in 1984.1 The goal of a grading system is to provide an accurate prognosis and guidance for proper therapeutic intervention. This system classifies tumors into grade I (well-differentiated), II (intermediate), or III (poorly differentiated) based on criteria outlined in Table 1. However, most mast cell tumors fall into the grade II category, and the prognosis and course of treatment often remain uncertain. Some tumors do not meet the criteria for a grade III tumor due to low mitotic rate, yet are still biologically aggressive. In addition, there is some variation in how individual pathologists use and apply this grading system.

Recently, a working group composed of oncologists and pathologists conducted a multi-institutional study to address these issues in the article: Proposal of a 2-tier histological grading system for canine cutaneous mast cell tumors to more accurately predict biological behavior.2 In this study, 95 cutaneous mast cell tumors were evaluated by 28 experienced surgical veterinary pathologists using the Patnaik system. There was only 75% concordance for the diagnosis of grade III mast cell tumors and only 64% for grade I and grade II mast cell tumors, confirming the previously-documented difficulty in using this widely-adopted system. The study group proposed an alternative grading scheme that had only two categories: high grade or low grade tumors. High grade tumors fulfill any one of the following criteria: at least 7 mitotic figures/10 high powered fields (HPF), at least 3 multinucleated (3 or more nuclei) cells/10 HPF, at least 3 bizarre nuclei/10HPF, or karyomegaly (at least 10% of cells have nuclei that vary by at least 2-fold).

When this novel 2-tier grading system was applied by 6 pathologists to the same 95 cases, there was a 96.8% consistency in classification of tumors as high or low grade between pathologists. Multivariate analysis of survival using the new system showed that high-grade tumors were associated with significantly shorter survival time and shorter times to metastasis or additional tumor development (Figures 1 and 2).2 Dogs with high grade tumors had a median survival time of less than 4 months while it was more than 2 years for those with low grade mast cell tumors. Overall, it appears to be a better predictor of survival than the Patnaik system. This grading system has not yet gained wide acceptance in the veterinary pathologist community, partially because it is so new. However, it is important that clinicians be aware of the new scheme if they see it on histological reports, and to monitor for future mast cell tumor studies that use this system.

Interestingly, the novel grading system relies solely on cytological criteria in the histological sections. If there is a sufficient sample size, it may be theoretically possible to grade mast cell tumors on cytology rather than histology. This would be useful to clinicians, as they could give clients a more accurate prognosis and estimate cost of treatment before performing an invasive biopsy. To this end, a study has been initiated to correlate the findings in matched pairs of cytological and histological specimens of the same mast cell tumor. Clinicians that submit a sufficiently cellular mast cell tumor on cytology may be contacted to request submission of any biopsy specimen to Dr. Krimer. Alternatively, samples can also be included if the veterinarian submits unstained cytological specimens along with the biopsy of any mast cell tumors diagnosed in-house.

If you have an in-house or reported cytological diagnosis of a canine mast cell tumor and are willing to contribute your case, please contact Dr. Krimer for additional information about this study at 706.542.5568.