CD99 expression in dermatofibrosarcoma protuberans and dermatofibroma.

Background: Differentiating between dermatofibrosarcoma protuberans (DFSP) and hypercellular dermatofibroma (DF) can sometimes be challenging, and a panel of immunostains is often employed. Expression of conventional markers oftentimes overlaps. We evaluated CD99 expression in DFSP and DF and its utility in distinction between these 2 entities.

Methods: CD99 immunostaining was performed on 34 DFSPs and 24 hypercellular DFs. The intensity of staining was graded as "weak," "moderate," or "strong," and the proportion of positive cells was graded as follows: "scattered" when individual cells comprised <5% of the total cellularity of the lesion; "focal" with >5% but <25% of the cells; or "diffusely distributed" with staining of >25% of lesional spindle cells.

Results: Overall, DFSPs showed positive CD99 staining in 21 (61.76%) cases. Moderate and weak patterns of staining were the most frequent, seen in 13 (38.2%) and 7 (20.6%) cases, respectively. CD99 staining in DFSPs was predominantly scattered or patchy (4 and 11 lesions respectively) with less than 25% of cells expressing CD99. In comparison, all 24 DF cases showed strong CD99 positivity in >25% of the spindle cell component (P = 0.0003). The most striking difference related to the distribution of staining. In DFSP, tumor cells in the superficial dermis, when present, were always CD99 negative. In contrast, DF cells in the superficial dermis always demonstrated strong CD99 positivity.

Conclusions: DF strongly expresses CD99 in a diffuse pattern that may serve as evidence in distinction from DFSP. As the differences in staining were most pronounced in the superficial portions of the tumor, CD99 staining may be well suited to superficial biopsy specimens, where distinction in hematoxylin and eosin sections may be most problematic.