Clonality assessment of cutaneous B-cell lymphoid proliferations: a comparison of flow cytometry immunophenotyping, molecular studies, and immunohistochemistry/in situ hybridization and review of the literature.

Cutaneous lymphoid infiltrates are diagnostically challenging. Although ancillary techniques to assess clonality can help distinguish between reactive lymphoid hyperplasia and lymphoma, one of the most widely used techniques in hematopathology, flow cytometry immunophenotyping (FCI), has not been routinely applied to skin specimens. We performed FCI on 73 skin specimens from 67 patients clinically suspected of having a cutaneous B-cell lymphoma (CBCL) and compared the results with those obtained from immunoglobulin heavy chain (IGH) gene molecular studies (58 cases, primarily by polymerase chain reaction) and either immunohistochemistry (IHC) or in situ hybridization to evaluate for light chain restriction (22 and 2 cases, respectively). Sufficient quantity of CD45 (leukocyte common antigen)-positive cells and staining quality were achieved in 88% of cases by FCI, and clonality was detected in 68% of CBCLs versus molecular studies showing sufficient DNA quality in 74% and only 39% clonality detection, and interpretable/contributory IHC results in 84% of cases with 55% clonality detection. Clonality was documented more frequently in secondary rather than primary CBCLs by all 3 techniques. Therefore, FCI is feasible and appears to be more reliable than molecular studies or IHC/in situ hybridization for detecting clonality in CBCLs and can provide additional prognostically and therapeutically relevant information. The exception is cases with plasmacytic differentiation such as marginal zone lymphoma for which IHC might be a superior tool. We have also shown that a large subset of primary cutaneous follicle center lymphomas express CD10 and/or BCL2 by FCI. Recent advances in FCI beg the question of applicability to cutaneous T-cell and NK-cell lymphomas.