Clinical Value of Various Histological Factors in Cutaneous and Subcutaneous Mast Cell Tumors in 197 Dogs

ABSTRACT Background Many histological tests have been correlated with outcome in mast cell tumors (MCTs)in dogs, but their statistical independence is uncertain. Objective To investigate the clinical value of histological factors in the prognostication of dogs with MCTs. Animals One hundred and ninety‐seven dogs with 199 histologically diagnosed cutaneous (n = 153) and subcutaneous (n = 43) MCTs treated surgically in primary care practice. All had a commercial prognostic panel performed (Patnaik and Kiupel grade, mitotic count, Ki67, AgNOR, KiAg, c‐kit mutation in exons 8 and 11 and KIT localization). Methods Retrospective cohort study identifying dogs from searching a commercial laboratory's records (January 2017–August 2020). Follow‐up was collected from clinical records. Outcome measures included MCT specific survival (MSS) and recurrence. Results Multivariable Cox proportional hazard regression identified only mitotic count > 5 (HR 10.2; 95% CI 3.2–32.8; p < 0.001) predicted poorer MSS across all MCTs. In Patnaik grade I or II and Kiupel low‐grade cutaneous MCTs, only c‐kit mutation in exon 11 (HR 20.8; 95% CI 1.80–224.8; p = 0.015) predicted MSS. A c‐kit mutation in exon 11 (HR 10.0; 95% CI 3.0–32.9; p < 0.001), age, and histological tumor free margins < 2 mm independently predicted cutaneous and subcutaneous MCT recurrence. In Patnaik grade I or II, and Kiupel low‐grade cutaneous MCTs, c‐kit mutation in exon 11 (HR 23.20; 95% CI 2.3–231.3; p = 0.007) and AgNOR (HR 13.73; 95% CI 1.6–115.6; p = 0.016) predicted MCT recurrence. Conclusion and Clinical Importance This study suggests a comparatively greater role of c‐kit mutations in exon 11 and AgNOR in the prognostication of MCTs, while Ki67 appears less important.