Abstract
Background:
Canine mast cell tumors (MCT) in the skin are classified into cutaneous (cMCT) and subcutaneous (scMCT) types, which exhibit different clinical behaviors. Although these types have been classified only by histology, preoperative differentiation is important for proper surgical planning.

Aim:
To examine the accuracy of differentiating these types based on the gross features before surgery.

Methods:
Gross photographic and histologic features of 52 MCTs (2014–2022) were retrospectively compared between cMCTs and scMCTs. Based on these results, we grossly classified additional 25 MCTs (2007-2013) into two forms using photographic observations. These observations were then compared with the results of histological classification performed by a blinded pathologist.

Results:
The most notable difference between the two forms was hair loss on the tumor surface. Hair loss was prominent in all 36 cMCTs but minimal or absent in all 16 scMCTs. Histologically, only the cMCT showed prominent follicular reduction due to MCT infiltration. Using the hair loss feature, we classified an additional 25 MCTs: 15 cMCTs, 7 scMCTs, and 3 unclassifiable cases with overlapping features. Agreement with histological classification was 80% (12/15) for cMCT and 100% (7/7) for scMCT. Among the unclassifiable cases, one was cMCT and two were scMCT. Large tumors (3.5-10.5 cm) were found in two of the three unclassifiable cases and in all three cases without agreement.

Conclusion:
Hair loss on the tumor surface is a distinct feature of cMCT that enables accurate visual differentiation from scMCT, except for some large MCTs. This may assist in surgical planning, specifically for sc-MCT.

Key words: Dogs, Gross features, Mast cell tumor, Skin