Histologic Characteristics and KIT Staining Patterns of Equine Cutaneous Mast Cell Tumors

Abstract

Mast cell tumors are uncommon in horses and typically have a benign clinical course, but there are occasional reports of more aggressive behavior. The objective of this study was to review histologic features and KIT expression patterns of 72 previously diagnosed equine cutaneous mast cell tumors to determine if either is associated with clinical outcomes. Biopsy specimens were reviewed using histologic criteria derived from grading schemes, and KIT antibody expression patterns used in canine tumors and surveys were sent to referring veterinarians for follow-up clinical data. Arabians were overrepresented relative to the reference population. Most tumors were well differentiated with low mitotic rates (96%), and aberrant KIT staining patterns, as described in dogs, were uncommonly identified (12%). Associated clinical disease was uncommon and no tumors exhibited malignant behavior. Overall, KIT staining pattern and histologic features were not associated with poor clinical outcome or abnormal tumor behavior.

Equine Cutaneous Mast Cell Tumours Exhibit Variable Differentiation, Proliferation Activity and KIT Expression

“…2D, F). The average age of affected horses was 12 years, similar to that recorded in a recent 228 study (Clarke et al, 2014); however, the generally wide age spread confirms that CMCT can 229 develop at any age in horses. Like others before, we also found male horses to be more 230 Thoroughbred (cross) horses, but without statistical risk association.…”

“…Based on the criteria that we assessed, WD-and PD-244 CMCT could easily be discerned. Interestingly, a recent study on a larger equine CMCT 245 cohort conducted in the USA did not find evidence of such a diversity, but reported all 72 246 tumours in their study to be composed of well differentiated mast cells, with only "minimal" 247 anisocytosis, anisokaryosis and mitotic figures (Clarke et al, 2014). Several factors may be 248 responsible for the differences between the results of the two studies.…”

“…Abnormal 89 cytoplasmic KIT expression in neoplastic cells has been reported in a MCT on the tongue of a 90 mare, without evidence of tumour recurrence or metastatic disease within 8 months after 91 surgery (Seeliger et al, 2007). Also, a recent study did not find any correlation between KIT 92 expression and morphological parameters of malignancy, clinical outcome, or aggressive 93 local behaviour; this was interpreted as evidence of the generally benign behaviour and high 94 degree of differentiation of equine CMCT (Clarke et al, 2014). However, in contrast to this 95 recent publication, in our diagnostic case material, we observed a proportion of equine 96…”

“…Firstly, Clarke and co-249 authors applied different inclusion criteria; while a large proportion of lesions (44%) in their 250 study contained only an "estimated percentage of mast cells in the lesion" of up to 25%, we 251 have excluded processes in which mast cells represented a minority (<50% of the section 252 area) to avoid inclusion of potentially non-neoplastic mast cell rich processes (Scott & Miller, 253 2011). Unfortunately, the lack of photographic documentation of the previous publication 254 does not allow a direct comparison (Clarke et al, 2014). Secondly, it is possible that different 255 morphological grading systems were applied.…”

“…Secondly, it is possible that different 255 morphological grading systems were applied. However, since specific information on scores 256 or measurements used for the qualitative and/or (semi)quantitative assessments is not 257 provided in the published paper (Clarke et al, 2014), a direct comparison is, again, not 258 possible. In any case, this recent study reported only 3 cases (4%) with more than 2 mitotic 259 figures per 10 HPF (Clarke et al, 2014), a cut off MI that would in our study identify 13 260 CMCT (29%), the majority of which (10/13) belong to the tumours classified as PD-CMCT.…”