Peripheral T cell lymphomas with follicular T helper phenotype: a new basket or a distinct entity? Revising Karl Lennert's personal archive

Aims: To revise 25 cases selected from Karl Lennert's personal archive (21) and Bologna and Frankfurt Registries (four) because of cytological similarities. Methods and results: All cases were provided with paraffin blocks and studied by immunohistochemistry and molecular techniques. While phenotyping was very informative, among molecular studies only EBER in situ-hybridization (ISH) was successful. Twenty-two cases were concluded as peripheral T cell lymphomas (PTCL). Of these, six were reclassified as angioimmunoblastic T cell lymphoma (AITL), 13 as PTCL, not otherwise specified (NOS), including four follicular variants and one tumour with T-zone pattern, and three as borderline tumours between AITL and PTCL/NOS. All these cases consisted homogeneously of small/medium-sized elements with mild nuclear atypia and an evident rim of clear/pale cytoplasm. On immunohistochemistry, they regularly expressed three to six follicular helper T cell (FTH)-associated markers. EBER-ISH revealed scattered EBV-infected B cells in all tumours except those with 'follicular' growth pattern. The content of follicular dendritic cells and high-endothelial venules varied significantly depending on the histotype. Conclusions: This study shows that: (i) historical material can be still employed usefully, and (ii) the FTH-phenotype corresponds to a broad spectrum of PTCLs that might form a new category to be validated in future molecular and clinicopathological analyses.