Indolent lymphomas in the pediatric population: follicular lymphoma, IRF4/MUM1+ lymphoma, nodal marginal zone lymphoma and chronic lymphocytic leukemia.

Indolent lymphomas in the pediatric population were discussed during the 2014 European Association for Haematopathology/Society of Hematopathology workshop in Istanbul, Turkey. This session was focused on pediatric-type follicular lymphoma (FL), and its differential diagnosis with the newly recognized entity of IRF4/MUM1+ lymphomas mainly involving Waldeyer's ring. The differential diagnosis between t(14;18) negative FL grade 1/2 and pediatric-type FL in adults was highlighted. The overlapping pathological and clinical features between FL and nodal marginal zone lymphoma (NMZL) in children and young adults were recognized and morphologic and immunophenotypical criteria helpful for the differential diagnosis were presented. Both pediatric-type FL and NMZL are indolent processes that should be distinguished from atypical lymphoid hyperplasia of the tonsils and lymph nodes. The demonstration of a B cell monoclonal population by molecular studies is strongly recommended for the diagnosis. Recognition of these indolent variants to avoid overtreatment was emphasized. Whereas most indolent lymphomas in the pediatric population show characteristic clinical, pathologic, and genetic features that differ from the adult counterpart, other rare indolent lymphoid tumors such as chronic lymphocytic leukemia (CLL) have similar characteristics. In this report, novel findings, areas of special interest, and diagnostic challenges emerging from the cases submitted to the workshop will be discussed.