Abstract
Although CD99 was originally described as a specific and diagnostically useful marker for Ewing sarcoma/primitive neuroectodermal tumor (PNET), the list of tumors known to express CD99 has expanded recently and includes leukemia, lymphoma, synovial sarcoma, and ependymoma. We describe a case of CD99+ primary cutaneous anaplastic large cell lymphoma (C-ALCL) presenting as a subcutaneous tumor on the forearm. Because histopathologically, the tumor was composed of small round cells that were positive for CD99, peripheral PNET was suspected and the patient was referred to our department. Immunohistochemistry allowed for a diagnosis of C-ALCL. A definitive diagnosis of small round cell tumors such as PNET, Ewing sarcoma and lymphoma may be problematic in some cases. With CD99 expression, a diagnosis of pPNET may be incorrectly made in our case. Immunohistochemistry and an understanding of histopathologic features of small round cell tumors are important for a differential diagnosis.[Skin Cancer (Japan) 2013 ; 28 : 259-263]
