Clinicopathological and molecular characteristics of paediatric-type follicular lymphoma: an analysis of 39 cases.

Our aim was to investigate the clinicopathological characteristics of paediatric-type follicular lymphoma (PTFL). The clinicopathological data were collected from 39 patients with PTFL diagnosed at the Department of Pathology, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine, China, from January 2017 to September 2024, including the morphology, immunohistochemical (IHC) analysis, and Ig gene rearrangements detected by polymerase chain reaction (PCR). Fluorescence in situ hybridisation (FISH) was performed to detect breakages in BCL2, BCL6, MYC, and IRF4 genes. In situ hybridisation (ISH) was used to detect Epstein-Barr virus (EBV)-encoded small RNAs (EBERs). Follow-up data were obtained from all patients through telephone interviews and by checking the electronic medical record system. The study cohort comprised 39 PTFL patients, 34 males and five females, with a median age of 15 years (range 7-41 years). The primary clinical presentation was lymph node enlargement in the head and neck regions, without systemic symptoms such as fever, night sweats, or weight loss. All patients were staged as I or II. Of these, 34 patients were followed up, whilst five were lost to follow-up. The follow-up period extended from 3 to 89 months, with no recorded cases of disease progression or mortality. Histologically, the architecture of the lymph nodes was markedly altered by large, irregular follicular nodules, which were either confluent or back-to-back, and contained medium-sized, relatively uniform blastoid cells. Some cases exhibited a mix of cell types characteristic of high-grade classic follicular lymphoma. All cases exhibited prominent 'starry-sky' pattern in neoplastic germinal centres. Mantle zones were often thin or absent. Neoplastic follicles were immunoreactive for CD20, CD79α, BCL6, and CD10, but not for CD3 and CD5. BCL2 was not expressed except for a few atypical cells in Cases 15 and 27 showing weak BCL2 expression. MUM1 was negative, except in Cases 5 and 27, where tumour cells were MUM1 positive. C-MYC expression varied in 24 cases, and the Ki-67 proliferation index was elevated (40-90%). CD21 and CD23 highlighted the follicular dendritic cells, confirming the confinement of tumour cells to the follicles. Immunoglobulin D (IgD) staining revealed attenuated or discontinuous mantle zones. EBV ISH was positive in only one of 37 cases. PCR detected Ig gene rearrangements in 35 of 36 cases. FISH analyses revealed no translocations or rearrangements in BCL2, BCL6, IRF4, or MYC. Collectively, our findings provide a deeper understanding of the clinical, morphological, IHC, and molecular characteristics of PTFL, which portends a favourable prognosis, setting it apart from conventional follicular lymphoma.