Immunohistochemistry for CD117 (KIT) is effective in distinguishing cutaneous adnexal tumors with apocrine/eccrine or sebaceous differentiation from other epithelial tumors of the skin.

BACKGROUND: The aim of this study was to evaluate the effectiveness of CD117 immunostaining for diagnosing cutaneous adnexal tumors, which has not been previously established.
METHODS: CD117 immunostaining was performed on representative sections of 87 clinicopathologically confirmed cutaneous apocrine/eccrine tumors, 13 sebaceous tumors, 52 follicular tumors, 55 keratinocytic tumors, 24 metastatic tumors of the skin of known origin and 1 mammary Paget disease. Staining of 5-50% of cells was considered focally positive, and staining of 51-100% of cells was considered diffusely positive. Staining intensity was semi-quantitatively graded as weak, moderate or strong.
RESULTS: A total of 82 (94.3%) tumors with apocrine/eccrine differentiation were CD117-positive, whereas one mucinous carcinoma, one apocrine carcinoma, and three primary cutaneous extramammary Paget diseases were CD117-negative. For most positive cases, staining was diffuse (64/82 cases) and moderate to strong in intensity (74/82). Twelve (92.3%) sebaceous tumors were CD117-positive, showing mainly focal (9/12) and weak (7/12) positivity. Of the 52 follicular tumors, all 4 cases of pilomatricoma and other 5 tumors were positive for CD117. Of the 55 keratinocytic tumors, only 2 cases were CD117-positive. The 24 metastatic tumors and mammary Paget disease were CD117-negative.
CONCLUSIONS: CD117 may be useful for the differential diagnosis of cutaneous adnexal tumors.