Mast cell tumors are common skin tumors of dogs. These tumors have variable biological behavior, and tumor grading can provide prognostic information and help determine an individual treatment plan.<\/p>\n
This article briefly outlines the histologic grading schemes in use and reviews several recent studies that attempt to apply grading criteria to cytologic samples from primary mast cell tumors. These studies have shown that cytologic grading is promising for predicting patient outcome. Potential limitations are also discussed.<\/p>\n
Take-Home Points<\/strong><\/p>\n Mast cell tumors (MCTs) are common in dogs, making up approximately 20% of canine skin tumors. Their biological behavior is variable, and grading provides significant prognostic information for use in making treatment decisions.1 <\/sup><\/span><\/p>\n Currently, histopathologic evaluation is considered the gold standard for grading MCTs. These grading systems have evolved over the years. Historically, a 3-tier system (Patnaik system) has been used, defining MCTs as grade I, II, or III. Grade I tumors are well differentiated with a low metastatic rate (7% mortality from tumor-related disease) and grade III are poorly differentiated with a high metastatic rate (94% mortality from tumor-related disease).2<\/sup> Patnaik grade II tumors tend to be overrepresented, generally accounting for 42% to 78% of MCTs, but sometimes cited as comprising 91% of MCTs.3-5<\/sup> Reports have shown significant interobserver differences in grading MCTs with the Patnaik system.3,6<\/sup> The biological behavior of grade II tumors is variable, with up to 20% being aggressive,7<\/sup> and together, these factors lead to uncertainty in the appropriate management of many patients using the Patnaik system.<\/span><\/p>\n In 2011, a 2-tier classification system was proposed by Kiupel et al, defining MCTs as either low grade or high grade.6<\/sup> In this system, criteria for a high-grade MCT include the presence of any of the following characteristics: <\/span><\/p>\n Classification is based on the histologic area with the most mitotic figures or nuclear variation. Kiupel et al found that there is good interobserver consistency in categorization with this system and a high correlation between grade and clinical outcome. <\/span><\/p>\n In the 2013 Oncology-Pathology Working Group (OPWG) consensus statement on grading of cutaneous MCTs, the application of both the Patnaik and Kiupel grading schemes was suggested. In 2021, the OPWG suggested incorporation of the 2 grading systems to better predict behavior of some tumors and study survival rate.7<\/sup> MCTs are thus categorized as:<\/span><\/p>\n Cytology is a quick, inexpensive method routinely used to diagnose MCTs. Generally, these tumors exfoliate high numbers of cells that typically contain large numbers of small, round, purple granules, making diagnosis straightforward. <\/span><\/p>\n The cellular features that compose the Kiupel grading system can be evaluated on cytologic preparations. Several recent studies investigated how well cytologic features correlate to histologic grade. Two of these studies directly applied the Kiupel system to cytologic preparations.8,9<\/sup> Histopathology was considered the gold standard for grading, and overall, both studies found good correlation between MCT grades determined cytologically versus histologically. Cytologic grading matched histologic grading in 94% of cases in both studies. Some cytologic limitations were identified, including fewer numbers of cells for evaluation and lower occurrence of mitotic figures on cytologic preparations compared with histologic preparations. <\/span><\/p>\n To overcome these limitations, Camus et al developed a unique cytologic grading system that did not directly apply the histologic grading criteria to cytologic samples.10<\/sup> They statistically evaluated various combinations of cellular features to create a grading scheme that correlated with not only histologic grade but also\u2014and more importantly\u2014patient survival. The cytologic feature that best correlated with 2-year survival was degree of granulation of tumor cells, followed by anisokaryosis, multinucleation, binucleation, and mitotic figures. In this cytologic grading scheme, tumors are classified as high grade if they meet either or both of 2 criteria: <\/span><\/p>\n Granulation of mast cells is categorized as heavily granulated (<\/span>FIGURE 1<\/b><\/span>), mixed granulation (a mixture of heavily granulated and poorly granulated cells), and poorly granulated (FIGURE<\/b><\/span><\/span>\u00a02<\/b><\/span>). Only tumors in which all cells show poor granulation are categorized as high grade based on this criterion.<\/span><\/p>\n\n
Histopathologic Grading of Mast Cell Tumors<\/h2>\n
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Cytologic Grading of Mast Cell Tumors<\/h2>\n
Camus Grading System<\/h3>\n
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![\"\"](\"https:\/\/navc.sitepreview.app\/todaysveterinarypractice.com\/wp-content\/uploads\/sites\/4\/2023\/02\/Meinkoth_TVPMarApr23_GradingMCTs_Fig1A.png\")
Figure 1. Heavily granulated cells from a low-grade mast cell tumor (MCT). (A) Cytoplasmic granules often nearly completely obscure the nucleus and limit evaluation of nuclear detail, particularly if the cells are not well spread out. Sometimes, the nucleus can be seen only as a lighter circle within the darkly stained mass of cytoplasmic granules (arrows). Aqueous Romanowsky stain, 100\u00d7 magnification.<\/span><\/a><\/div><\/div>