Indolent pediatric lymphomas/lymphoproliferative disorders: a report of the 2024 EA4HP/SH lymphoma workshop

The boundaries between indolent neoplastic and reactive lymphoproliferations were discussed during the 2024 European Association for Haematopathology/Society for Hematopathology workshop in Dubrovnik, Croatia. Session 4 focused on the revision of indolent pediatric lymphoid neoplasms/lymphoproliferations. Forty-one cases were submitted, representing good examples of indolent pediatric lymphomas/lymphoproliferations and their diagnostic challenges. The morphologic spectrum of pediatric-type follicular lymphoma (PTFL) was discussed, especially exploring unifying features of cases with and without marginal zone differentiation (MZD) (the first also referred to as pediatric nodal marginal zone lymphoma). Unusual features like relapses/recurrences; extranodal involvement and high proliferation rate in the marginal zone differentiated component were observed in PTFL with or without MZD. Rare follicle center-derived neoplasms occurring in children other than PTFL where discussed, such as primary testicular follicular lymphoma and follicle center lymphoma of the lower female genital tract. Testicular follicular lymphomas were compared to cases known from literature, defining a better picture of their clinicopathological features. The criteria for the diagnosis of atypical lymphoid hyperplasia were reviewed and nodal and extranodal cases are discussed. Plasmacytic proliferations in the pediatric population are exceedingly rare, but do exist in clonal and non-clonal forms. A heterogeneous group of extranodal lymphomas is included, among them extranodal marginal zone lymphomas (MZL) of the stomach with a large cell transformation and an intramuscular MZL with comparable features of a cutaneous MZL/lymphoproliferation with IgG4 expression. In this report, the different spectrum of indolent pediatric lymphomas compared to the adult counterpart will be highlighted, to avoid misdiagnosis and overtreatment.