Frequent Expression of CD99 in Anaplastic Large Cell Lymphoma A Clinicopathologic and Immunohistochemical Study of 160 Cases

Originally described as a diagnostically useful marker for Ewing sarcoma, CD99 immunoreactivity has also been documented in a variety of other tumors, including hematopoietic neoplasms. By using conventional paraffin immunoperoxidase staining and tissue microarrays, we retrospectively investigated CD99 expression in a series of 160 anaplastic large cell lvmphoma (ALCL) cases. Of the 160 cases, 103 (64.4%) were positive for CD99. The distribution of CD99 positivity was similar for nodal (661103 [64.1%]), extranodal, (21132 [66%]), and primary cutaneous lesions (16125 [64%]). CD99 expression was present in 96 (64.4%) of 149 of the common type, 4 (80%) of 5 of the small cell variant, and 3 (50%) of 6 of the lymphohistiocytic variant cases. CD99 expression was slightly more frequent in anaplastic large cell lymphoma kinase (ALK) + cases compared with ALK-cases (43154 [80%] vs 44181 [54%]). With 2 exceptions, ALK+ ALCL was seen only in patients younger than 41 years. We conclude that CD99 is frequently expressed in ALCL, with a slightly increased frequency in the younger age ALK+ cases. Nodal and extranodal ALCL should be considered in the differential diagnosis when a CD99+ neoplasm is encountered