Focal follicular features in tonsillar diffuse large B-cell lymphomas: Follicular lymphoma with diffuse areas or follicular colonization

Focal follicular features in diffuse large B-cell lymphomas (DLBCLs) are bound to raise the question of follicular lymphoma (FL) with diffuse areas, because the diagnosis of FL is based on the presence of follicular areas, even though focal. We report 7 cases of primary tonsillar DLBCLs with focal follicular features that presented with morphologic, immunohistochemical, and biological features distinct from those of FL. Histologically, these tumors were characterized by involvement of pericryptal follicles with adjacent dominant diffuse areas. Monomorphous large tumor cells were evenly spaced with abundant, often clear cytoplasm, and blastoid nuclei often with a delicate nuclear membrane. Importantly, residual germinal centers (GCs) were present in the form of either an intrafollicular GC remnant or an isolated GC in the midst of diffuse tumor. An extrafollicular and/or parafollicular growth pattern was also observed. Bcl-6 staining revealed a predominantly sporadic occurrence of Bcl-6⁺ cells, comprising <50% of tumor cells, and none displayed diffusely dense collections (>75%) of Bcl-6⁺ tumor cells characteristic of the GC or FL. Staining for CD10 was negative in 6 cases. Five of 7 patients were younger than 60, the median age of other patients with primary tonsillar DLBCL. No extratonsillar involvement was seen at 18 months after diagnosis. After chemotherapy or radiotherapy, complete remission was achieved with ease in all patients, but 2 patients who were treated with chemotherapy alone relapsed at 24 and 30 months. In conclusion, tonsillar DLBCL includes a small (10%) but distinct subgroup that warrants distinction from FL with predominant diffuse areas or de novo DLBCL. It appears that the focal follicular features in tonsillar DLBCL likely represent follicular colonization of marginal zone B-cell lymphoma, probably high-grade, if the possibility of FL is excluded.