Secondary <i>BRAF</i> -mutated histiocytic/dendritic cell sarcoma transdifferentiated from follicular lymphoma with prolonged response to BRAF/MEK inhibition and subsequent evolution to high-grade B-cell lymphoma

Transdifferentiation from follicular lymphoma (FL) to histiocytic/dendritic cell sarcoma (HDS) is rare and requires molecular confirmation of shared clonal origin. Targetable mutations such as BRAF V600E may offer therapeutic opportunities in such aggressive neoplasms. We report an exceptional case of untreated localised FL transdifferentiated to an HDS after 18 years. Shared BCL2 rearrangement and mutation profile confirmed a clonal link, while the HDS acquired an additional BRAF V600E mutation. Treatment with BRAF/MEK inhibitors yielded a sustained 18-month clinical response. The disease later relapsed as high-grade B-cell lymphoma with MYC and BCL2 rearrangements (HGBCL- MYC/BCL2 ), still harbouring the BRAF mutation. Complete remission was achieved with Rituximab, Cyclophosphamide, Hydroxydaunorubicin, Oncovin and Prednisone, but the double-hit lymphoma relapsed 14 months later. This case illustrates sequential transformation from FL to BRAF -mutated HDS with excellent response to BRAF/MEK inhibition, followed by evolution into HGBCL- MYC / BCL2 responding transiently to immunochemotherapy, emphasising the value of repeated histological and molecular reassessment in FL evolution.