Pseudoclonality in cutaneous pseudolymphomas: a pitfall in interpretation of rearrangement studies

Summary Background Pseudoclonality is a well‐known problem in the interpretation of rearrangement studies of lymph node biopsies. Recently, pseudoclonality has been demonstrated in skin lesions of borreliosis. Studies on pseudoclonality in cutaneous pseudolymphomas are lacking but pseudoclones may pose a risk for overinterpretation of such lesions as cutaneous lymphoma. Objectives To determine the frequency of pseudoclonality in cutaneous pseudolymphomas identified by clinicopathological correlation and follow up. Methods Study of 30 lesions of pseudolymphomatous cutaneous infiltrates (including insect bite reactions, borrelial pseudolymphomas and pseudolymphomatous drug eruptions) by histopathology, immunophenotyping, T‐cell receptor γ rearrangement and IgH rearrangement. Results Seven infiltrates were B‐cell pseudoclonal; four were T‐cell pseudoclonal. Moreover, B‐cell clonality was identified in four cases. Immunophenotyping demonstrated that B‐cell pseudoclonality and B‐cell clonality occurred when infiltrates were moderately dense and included only a minority of B lymphocytes. T‐cell pseudoclonality also occurred mostly in moderately dense infiltrates. Conclusions B‐cell and T‐cell pseudoclones are not uncommonly encountered in moderately dense pseudolymphomatous infiltrates (23% and 13%, respectively). B‐cell clonality is seen occasionally in pseudolymphomatous infiltrates (13%), especially when they are sparse in B lymphocytes. Therefore, rearrangement studies cannot be interpreted without correlation with morphological patterns and immunophenotyping of infiltrates and they need to be confirmed by duplicate or triplicate tests in order to prevent overinterpretation.