Mantle cell lymphoma mimicking chronic lymphocytic leukemia/small lymphocytic lymphoma on flow cytometry

One of the most common applications of clinical flow cytometry is to diagnose chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). When classic features are present and correlation with other diagnostic modalities is possible, the distinction of CLL/SLL from other subtypes of B-cell lymphoma is straightforward. However, features deviating from prototypical phenotypes occur not infrequently and can present a diagnostic dilemma. One potential pitfall that has not been well described is mimicry of CLL/SLL by mantle cell lymphoma. A review of flow cytometric data and corresponding clinical, morphologic, and cytogenetic data for 32 cases of mantle cell lymphoma and 67 cases of CLL/SLL was performed to assess the frequency and characteristics of mantle cell lymphoma with features resembling CLL/SLL (i.e., moderate to strong CD23 expression; dim to negative light chain, CD20, CD22, and/or FMC7 expression; and/or dim CD45 expression). Eleven (34%) mantle cell lymphomas showed one CLL/SLL feature, two (6%) showed two features, one (3%) showed three features, and one (3%) showed five features. Of the markers tested, only dim CD45 expression could reliably exclude all cases of mantle cell lymphoma. Mantle cell lymphoma is capable of mimicking nearly the full spectrum of CLL/SLL features, and evaluation of CD45 intensity can be helpful in challenging cases, observations not previously made. These results suggest that additional markers beyond those traditionally used to diagnose CLL/SLL should be evaluated and results correlated with other diagnostic modalities to ensure accurate classification.