Background: A Colorado State University (CSU) dog was diagnosed with a poorly differentiated alkaline phosphatase (ALP) positive tumor that was most consistent with a mast cell tumor (MCT) with hematogenous involvement. Immature hematopoietic cells are ALP+ but reactivity in MCT is unknown. Tryptase immunohistochemistry labels MCT granules with limited data on immunocytochemical (ICC) labeling or its relation to grade.
Objective: Evaluate the association of ALP reactivity and tryptase ICC labeling with cytologic grade of canine MCT.
Animals: 38 dogs diagnosed with skin-associated MCT at CSU between December 2021 and November 2023.
Methods: The CSU Clinical Pathology archives were searched for canine skin-associated MCT cytologic specimens with sufficient archived Wright-Giemsa-stained slides. Staining for ALP reactivity was performed on destained slides, as previously published. Tryptase ICC was performed using the Leica Bond-III system. Grades were assigned by two ACVP-boarded pathologists separately using a published two-tier grading system and then discordant cases were resolved. Cohen’s κ and Fisher Exact statistical tests were used.
Results: Independent cytologic grades agreed for 36/38 cases, Cohen’s κ (0.86, 0.68-1.00) was excellent but 9/28 patients with cytologic low grade MCTs had nodal metastasis. ALP reactivity was not different between grades (p=1.00); 2/28 low grade and 0/10 high grade MCTs were ALP+. Tryptase labeling was not associated with grade (p=0.29); 20/20 low grade and 7/8 high grade MCTs were tryptase+. The ALP+ and tryptase- cases had nodal metastasis.
Conclusions and Clinical Importance: Canine MCT is most often ALP-/Tryptase+. An association with cytologic grade was not found.
