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Expression of epidermal growth factor receptor and human epidermal growth factor receptor 2 in periocular squamous cell carcinomas of horses.

OBJECTIVE
To determine whether epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 (HER2) are expressed in periocular squamous cell carcinomas (SCCs) of horses.


SAMPLE
Biopsy specimens of SCCs from 46 horses.


PROCEDURES
Pathology records were searched retrospectively for biopsy specimens of periocular SCCs obtained from horses. Slides of the specimens were reviewed histologically to confirm the SCC diagnosis and stained for EGFR and HER2 by immunohistochemical methods. For both EGFR and HER2, the immunohistochemical staining intensity and percentage of stain-positive cells on the slides were determined.


RESULTS
43 of 46 (93%) SCCs were immunoreactive for EGFR. The median score for EGFR staining intensity was 4 (range, 2 to 12), and the median number of mitotic figures was 8 mitotic figures/10 hpfs (range, 0 to 34 mitotic figures/10 hpfs). Mitotic index was not correlated with the percentage of EGFR stain-positive cells or staining intensity. Of the 43 EGFR-immunoreactive SCCs, 38 had stain present primarily in the cytoplasm and 5 had stain equally distributed between the cytoplasm and cell membranes. Thirty-five of 46 (76%) SCCs were immunoreactive for HER2. Mitotic index was not correlated with the percentage of HER2 stain-positive cells or staining intensity. Of the 35 HER2-immunoreactive SCCs, the stain was present primarily in the cytoplasm and 7 had stain equally distributed between the cytoplasm and cell membranes.


CONCLUSIONS AND CLINICAL RELEVANCE
Results indicated that most periocular SCCs of horses expressed EGFR and HER2, which suggested that equine periocular SCCs might respond to treatment with EGFR inhibitors.