Primary central nervous system extranodal nasal-type natural killer/T-cell lymphoma with CD20 expression.

We report a unique case of primary CNS extranodal natural killer/T-cell lymphoma (PCNS ENKTCL) with CD20 expression and the monoclonal rearrangement of Ig heavey chain (IgH) gene. Resection specimens were evaluated using HE-stained sections, immunohistochemistry, in situ hybridization and PCR. Histopathologic examination, immunohistochemistry and molecular studies showed the intermediate-sized lymphoma cells expressing CD2, CD3ε, granzyme B, TIA-1, CD20 and Epstein-Barr virus-encoded RNA, with germline T-cell receptor gene and the monoclonal rearrangement of IgH gene. Clinicopathologic and radiographic evaluation indicated the lesion was of exclusive left cerebellar localization. Thus, PCNS ENKTCL with the CD20 expression was diagnosed. ENKTCL with both CD20-positive expression and the monoclonal rearrangement of IgH gene may be mistaken for B-cell lymphoma; thus, the comprehensive evaluation of histomorphology, more extensive immunoprofiles and molecular studies is essential to reach the correct diagnosis. The rare PCNS ENKTCL shows a highly aggressive nature and a poorer prognosis.