Nodal marginal zone B-cell lymphoma with prominent follicular colonization - difficulties in diagnosis: a study of 15 cases.

AIMS: While colonization of reactive follicles is well described in mucosa-associated lymphoid tissue lymphoma, this is not fully appreciated in nodal marginal zone B-cell lymphoma (NMZL). The aim was to address how to recognise this feature/entity and to discuss diagnostic difficulties faced by histopathologists in dealing with such lesions and their biological implications. METHODS AND
RESULTS: Fifteen NMZLs with prominent follicular colonization are described, 14 of which were referrals from other hospitals. All cases had a follicular pattern and showed prominent 'follicular colonization'. In many follicles the colonization was partial, and follicles also had a reactive germinal centre component. The phenomenon of follicular colonization was highlighted by immunohistochemistry. The benign follicle centre cells expressed CD20, CD10 and Bcl-6 and were negative for Bcl-2 and MUM1. In contrast, the colonizing marginal zone lymphoma cells expressed CD20, Bcl-2 and often MUM1 and were negative for Bcl-6 and CD10. Partially colonized follicles showed a 'moth-eaten' appearance on CD10, Bcl-2, Bcl-6 and MUM1 immunohistochemistry. In none except one was the referring diagnosis NMZL.
CONCLUSION: Recognizing and appreciating follicular colonization in a subset of NMZLs, appropriate use of immunohistochemistry and knowledge of immunohistological features can aid in making the correct diagnosis.