Myeloid Cell Nuclear Differentiation Antigen (MNDA) Positivity in Primary Follicles: Potential Pitfall in the Differential Diagnosis With Marginal Zone Lymphoma.

Myeloid cell nuclear differentiation antigen (MNDA) is an immunohistochemical marker that is used to distinguish marginal zone lymphomas (MZLs) from other small B-cell lymphomas. An index case that showed MNDA staining in primary follicles prompted the current study to evaluate whether MNDA expression is widespread in primary follicles and to address whether it poses a potential diagnostic pitfall. Of the 15 cases with primary follicles identified by a search of the laboratory information system, 7 had positive MNDA staining. In all cases, there was weak nuclear staining similar to what is typical of MNDA staining in MZLs. All cases showed intense nuclear signal in myeloid lineage cells such as neutrophils, which served as positive internal controls. The histologic and cytologic features of primary follicles and MZLs showed overlapping features, particularly in small biopsies. Our results indicate that weak nuclear MNDA staining can act as a potential pitfall in the evaluation of small B-cell lymphomas. Correlation with other immunohistochemical markers that are useful in the workup of small B-cell lymphomas, as well as those that outline immunoarchitectural features of lymphoid follicles, is suggested when both entities are part of the differential diagnosis. Our results underscore the need for caution in the interpretation of weak nuclear MNDA staining in the evaluation of small B-cell lymphomas.