Did the Patnaik grade scheme perform as expected in this cohort?

• A. Yes there were no differences in MST or DFI.
• B. Yes, there were significant differences in both MST and DFI.
• C. No differences in MST or DFI.
• D. Not evaluated in this cohort.

Answer: (B)

The main idea is that a useful grading system should separate outcomes by prognosis. The Patnaik scheme aims to categorize canine mast cell tumors into groups with distinct behavior, and MST (median survival time) and DFI (disease-free interval) are the outcomes that reflect overall prognosis and time to relapse. If this cohort shows statistically significant differences in both MST and DFI across the grades, that means higher-grade tumors behave more aggressively and patients have shorter survival and shorter relapse-free periods. In other words, the grading system is effectively discriminating risk in this group, which is why this option is the best choice. If there were no differences or if the outcome wasn’t evaluated, the scheme would not be shown to perform as expected, which is not the case here.