Differentiating spitzoid melanomas from Spitz nevi through CD99 expression

Background: A true diagnostic marker differentiating Spitz nevi (SN) from spitzoid melanoma (sMM) has been elusive. CD99, a transmembrane glycoprotein, believed to play a role in many neoplastic processes, has yet to be investigated in this regard. Recently, the expression of CD99 has been shown in 60% of primary melanomas. Other studies exploring the expression of CD99 in melanocytic lesions have not been performed. Here, we evaluate the presence of CD99 in these two histologically difficult to differentiate entities. Methods: Cases of sMM were selected based on the presence of key microscopic words: spitz, spindled and/or epitheliod. In addition, all the cases of SN over the past 7 years were retrieved. Each case was stained with anti-human CD99 and analyzed for the presence of CD99 staining. Results: Fifteen of 27 cases (56%) of sMM expressed CD99 compared with only 3 of 58 cases (5%) of SN. Conclusions: This study shows 56% of sMM and only 5% of SN express CD99. Eight of the sMM showed strong diffuse staining, a pattern not seen in any of the SN. This study does not elucidate the role that CD99 plays in these melanocytic processes; however, it does show that CD99 can be a useful tool in distinguishing sMM from SN.