Chronic active Epstein-Barr virus infection as the basis of neoplasm development. Transformation of chronic lymphocytic leukaemia to NK-cell lymphoma/leukaemia was observed by flow cytometry - a diagnostic report.

The case report presents a patient with chronic lymphocytic leukaemia that was diagnosed in 2006 in Rotterdam, the Netherlands. In September 2010, the patient was admitted to the Department of Haematology in Poland due to progression of the underlying disease. The clinical problem during treatment was the suspicion of Richter's transformation into another, more aggressive non-Hodgkin lymphoma. The diagnosis was based on the peripheral blood immunophenotype. The patient was diagnosed with an immunoglobulin deficiency. Unfortunately, repeated examinations did not confirm the transformation hypothesis, despite the increasing symptoms. The patient was treated with various therapeutic regimens until May 2021, when an increased number of NK cells was diagnosed in the peripheral blood. NK-cell lymphoproliferative disease was finally diagnosed de novo. Nevertheless, it was found that the patient had active Epstein-Barr virus (EBV) and Cytomegalovirus (CMV) infection. The suspected NK-cell lymphoma/leukaemia was most likely a complication of the active EBV infection and severe immunodeficiency state.