Primary gastric follicular lymphoma with parafollicular monocytoid B-cells and lymphoepithelial lesions, mimicking extranodal marginal zone lymphoma of MALT.

The incidence of B-cell non-Hodgkin lymphomas (B-NHL) at nodal and extranodal sites is fairly different. Follicular lymphomas (FL) occur predominantly at nodal sites and rarely in the gastrointestinal tract, while marginal zone lymphomas (MZL) of the mucosa-associated lymphatic tissue (MALT) type predominate in the digestive organs and especially in the stomach. We report a 72-year-old female patient admitted for gastroscopy because of epigastric pain. The antral biopsies showed dense lymphocytic infiltrates, partially forming follicles with widened marginal zones and monocytoid cells. Multiple lymphoepithelial lesions (LEL) were also observed. A MZL of the MALT type was suspected morphologically. Immunohistochemical analysis revealed the lymphatic infiltrates to be CD20, bcl-2, bcl-6 and CD10 positive, and negative for CD43, CD5 and cyclin D1. PCR-based analysis showed a JH/bcl-2 rearrangement, corresponding to the translocation t(14;18). An extranodal FL mimicking MZL was diagnosed. The present case is remarkable, as it demonstrates that the detection of LEL and monocytoid B-cells, although suggestive for MZL, is not entirely specific and can also be observed in FL. Pathologists should be aware of this diagnostic pitfall in classifying gastric B-NHL. In equivocal cases, a careful morphological examination, supported by specific immunohistochemical and molecular findings, should lead to the correct diagnosis.