Restricted Expression of the Constant Region 1 of T‐Cell Receptor β by Flow Cytometry Facilitates Detection of T‐Cell Neoplasms With High Specificity but Moderate Predictive Value

Objectives To determine the diagnostic value of disclosing monotypic T‐cell populations by expression analysis of the constant region 1 of T‐cell receptor β (TRBC1) by flow cytometry, for the detection of T‐cell neoplasms, in a routine hematopathology practice setting. Methods A panel of antibodies ECD‐CD16/PECy5.5‐CD4/PECy7‐CD2/APCAF700‐CD7/KO‐CD8/FITC‐TCRγδ/PE‐TRBC1/APC‐CD26/APC‐CD57/APC‐CD30/APCH7‐CD45/BV421‐CD3 was applied. One thousand and twenty‐nine cases investigated during 26 months were reviewed and categorized as monotypic or polytypic depending on the expression of TRBC1 in all identifiable CD3+ T‐cell subsets. Results Although TRBC1 restriction proved to be sensitive (90.7%) with a high negative predictive value (99.1%), in some T‐cell neoplasms, particularly those with significant inflammation, including angioimmunoblastic T‐cell lymphoma, monotypic T‐cells were not detected. The specificity was high (95.6%) albeit with a modest positive predictive value (67.1%), reflecting T‐cell clones of uncertain significance (T‐CUS) without T‐cell neoplasm. Of these clones, 53.6% exhibited a T‐cell large granular lymphocytic leukemia‐like phenotype, most commonly found in the peripheral blood or bone marrow, while others were identified in association with unrelated primary tumors or other comorbidities. Seventy‐nine percent of the T‐CUS cases remained stable over time during 1–77 months of follow‐up. Conclusion Including TRBC1 antibodies in a routine flow cytometry panel facilitates the identification of T‐cell neoplasms. The analysis must be interpreted within its clinical context since T‐cell lymphomas with a small and sometimes surface CD3‐negative neoplastic T cell population, may display normal patterns of TRBC1‐expression with a background of reactive T cells. Conversely, monotypic T‐cells can be found in the absence of T‐cell neoplasm.