The 2-tier grading system identifies canine cutaneous and/or subcutaneous mast cell tumors with aggressive biological behavior regardless of growth model.

Histologic grading of canine cutaneous mast cell tumors (cMCTs) has prognostic and therapeutic implications, yet validation for subcutaneous MCTs (scMCTs) is lacking. For scMCTs with or without dermal invasion, determining their biological behavior remains poorly standardized and sometimes sparks controversy. This prospective study aimed to assess the prognostic utility of the 2-tier histologic grading system in MCTs with different growth models (GMs) and explore the prognostic impact of the GM itself.