Follicular lymphoma in situ with coincident low-grade B cell lymphoma: A case series

Follicular lymphoma in situ is a rare, recently described entity. It is defined by the accumulation of t(14;18)+, BCL2 strongly positive clonal B cells restricted to germinal centers in morphologically normal lymph nodes, without alteration of follicle morphology, size or distribution, or perifollicular spread. The presence of a follicular lymphoma in situ has three main clinical ramifications, including the potential coexistence of a simultaneous disseminated follicular lymphoma, risk of subsequent overt follicular lymphoma, and association with a second subtype of lymphoma. We report cases of follicular lymphoma in situ in conjunction with lymphoplasmacytic lymphoma (two cases) and mantle cell lymphoma (one case). Each case was evaluated, and diagnoses confirmed using a combination of histologic, immunohistochemical, fluorescence in situ hybridization (FISH), and molecular techniques. One case of lymphoplasmacytic lymphoma was proven positive for myeloid differentiation primary response 88 gene (MYD88) mutation. Our series expands the current literature of this rare phenomenon. We discuss possible etiologies and clinical impact of follicular lymphoma in situ.