Fibroblast-activation protein: valuable marker of cutaneous epithelial malignancy

Fibroblast-activation protein (FAP) is a key protein that is characteristically expressed by carcinoma-associated fibroblasts (CAFs). It has been shown to be expressed in CAFs of 90 % of internal epithelial cancers as well as cutaneous epithelial malignancies. We have recently shown that this marker is useful in differentiating between morpheaform/infiltrative BCC from desmoplastic trichoepithelioma (TE). Given this, we sought to assess FAP expression in both benign and malignant cutaneous epithelial entities. Immunohistochemical FAP staining was performed on BCC (n = 26), SCC (n = 26), porocarcinoma (n = 10), metastatic adenocarcinoma (n = 12), keratoacanthoma (KA) (n = 16), TE (n = 14), pseudoepitheliomatous hyperplasia (n = 15), poroma (n = 15), syringoma (n = 10), and chondroid syringoma (n = 6). Control group consisted of scars (n = 10). FAP expression was observed in all scars and all malignant entities, but not in any of the benign cases. Interestingly, ten KA cases exhibited positivity, whereas six were negative. In summary, FAP is a reliable marker of cutaneous epithelial malignancy.