Background: Extranodal natural killer/T-cell lymphoma (ENKTL) nasal type is an extranodal lymphoma, with most cases arising from the prototypic nasal/upper aerodigestive tract and the cell of origin (COO) is frequently of NK cells. Effusion involvement by ENKTL is rare. The cytomorphological and immunophenotypical features and COO of effusion ENKTL are elusive. Design: We report four new cases of effusion ENKTL, reviewing the cytomorphological features and investigating their COO by immunohistochemistry for T-cell receptor (TCR) protein expression and clonality assay for TCR gene rearrangement. We also reviewed 12 additional cases of effusion ENKTL from the literature. Results: All our four cases showed anaplastic tumor cells with fine to coarse azurophilic granules, with the primary tumor organs from the prototypic sites (2), small intestine (1), or indeterminate (1). The COO was either T-cell (3) or NK-cell (1). Compiling our four cases with that from the literature, all 16 effusion ENKTL cases were Epstein-Barr virus-associated and the majority (83%; 10/12) originated from the nonprototypic sites. Five of six (83%) effusion ENKTL cases were of T-cell lineage. Prognosis was poor with a median survival time of only 2.4 months. Conclusions: Our study revealed that ENKTL in the effusion might be characterized by anaplastic large cell morphology, cytoplasmic azurophilic granules, and a higher proportion of T-cell lineage as compared to de novo tumors arising from the prototypic sites. ENKTL rarely presented in effusion, which is associated with dismal outcome. Large international studies are warranted to validate these findings.
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