[Immune deficiency/dysregulation-associated lymphoma with an immunophenotype resembling EBV-positive nodal T/NK-cell lymphoma during treatment of follicular lymphoma].

In the 5th edition of the WHO classification, Epstein-Barr virus (EBV)-positive nodal T/NK-cell lymphoma (EBV+ nTNKL) is a newly defined, poor-prognosis disease originating from cytotoxic T cells. Here, we report a rare case of lymphoma arising in immune deficiency/dysregulation with an immunophenotype of EBV+ nTNKL that emerged during treatment for follicular lymphoma (FL). A 71-year-old woman was diagnosed with FL grade 3A and received chemotherapy with rituximab. Despite treatment, the disease relapsed repeatedly. During follow-up, she presented with fever and lymphadenopathy. An axillary lymph node biopsy revealed a diffuse proliferation of abnormal lymphocytes expressing CD3, CD4, TIA-1, granzyme B, and EBER, consistent with the EBV+ nTNKL immunophenotype. CHOP therapy was administered; however, disease control proved difficult, and the patient died 5 weeks after diagnosis. This case exemplifies a rare occurrence of EBV-associated lymphoma arising during the course of B-cell lymphoma. Our experience underscores that rapid clinical deterioration (e.g., fever, hepatosplenomegaly, or B symptoms) during the course of FL should prompt consideration of EBV-associated lymphoma in the differential diagnosis, alongside histologic transformation. Further case reports and molecular analyses should help improve diagnostic accuracy and establish treatment strategies for this rare and aggressive disease.