T-prolymphocytic leukemia with three distinct immunophenotypic subsets.

T-prolymphocytic leukemia (T-PLL) is a rare, aggressive mature T-cell neoplasm usually affecting older adults, often presenting with marked leukocytosis and splenomegaly. It is characteristically associated with recurrent chromosome 14 rearrangements that drive overexpression of TCL1 family oncogenes. Neoplastic cells generally show a mature T-cell immunophenotype with preserved pan-T cell markers, most commonly CD4⁺ with or without CD8 expression. Although T-PLL can harbor substantial genetic complexity that may lead to subclonal evolution, overt phenotypic diversification into distinctly separable abnormal T-cell populations within a single case of T-PLL is rarely documented, particularly among those characterized by multiparametric flow cytometry. This brief report describes a patient with T-PLL in whom three distinct abnormal T-cell subsets were delineated by flow cytometric analysis using TRBC1. This case illustrates striking intratumoral heterogeneity in a neoplasm that is typically regarded as phenotypically uniform and demonstrates the utility of TRBC1 analysis in refining diagnostic assessment.