Follicular colonization of nodal marginal-zone B-cell lymphoma resembling follicular lymphoma: report of 6 cases.

The formation of neoplastic B-cell follicles is accepted as a diagnostic criterion of follicular lymphoma. However, extranodal marginal-zone B-cell lymphomas (MZBLs) of mucosa-associated lymphoid tissue (MALT) type also sometimes contain numerous lymphoid follicles and may even have a predominantly follicular growth pattern. However, morphologic, immunohistochemical, and genotypic findings suggest that lymphoid follicles in extranodal MZBLs are neoplastic follicles formed as the result of colonization of previously reactive follicles by tumor cells (centrocyte-like cells). We present here 6 cases of nodal MZBL demonstrating a follicular growth pattern. Immunohistochemical study demonstrated that the tumor cells were CD10-, CD20+, CD79a+,CD138-, Bcl-2+, Bcl-6- and IRF4+. Residual nonneoplastic follicular center cells were CD10+, CD20+, CD79a+, Bcl-2-, and Bcl-6+. CD21/CD23 immunostain demonstrated a disrupted follicular dendritic cell pattern characteristic of follicular colonization in extranodal MZBL of MALT type. Taken in conjunction with the morphologic findings, nodal MZBL may also show a follicular growth pattern similar to extranodal MZBL of MALT type. The marginal-zone nature is most recognizable on immunohistochemistry, although the histologic appearance alone may cause some diagnostic problems. It is important for pathologists to consider this type of lesion in diagnostic practice.