Malignant proliferating trichilemmal tumors with CD34 expression.

Malignant proliferating trichilemmal tumors (MPTT) are rare neoplasm arising from outer root sheath of hair follicle, the diagnosis of which is base essentially on histological features resulting in occasionally misdiagnosis of squamous cell carcinoma. In difficult cases, however evaluation of additional parameters may be needed to differentiate benign proliferating trichilemmal tumor from MPTT or differentiate PTT and MPTT from squamous cell carcinoma. We report two cases of MPTT on which, in addition to histologic features, we have determined p53 immunohistochemical expression pattern, proliferative fraction, and CD34 expression. For comparison, concurrent proliferating trichilemmal tumors (PTT) and trichilemmal cysts (TC) as well as well-differentiated squamous cell carcinoma (SCC) were studied. The two MPTTs show expression of p53 with increased proliferative index as well as all three SCC. The PTTs and TCs stained negative and few basal cells for p53 and Ki-67, respectively. MPTTs exhibit CD34 immunoreactivity, indicating trichilemmal differentiation. The contrast p53 and Ki-67 expression pattern in MPTT and PTT may be helpful in the diagnosis of MPTT. Expression of CD34 may be an additional feature to distinguish MPTT from SCC.