A unique case of follicular large B-cell lymphoma arising in the background of thymic follicular hyperplasia

Follicular large B-cell lymphoma (FLBCL) is an aggressive variant of follicular lymphoma, rarely reported at extranodal sites. We describe, to our knowledge, the first case of FLBCL arising in a background of thymic follicular hyperplasia (TFH). A 76-year-old man undergoing evaluation for cardiac surgery was incidentally found to have a mediastinal mass, which revealed thymic tissue with prominent TFH on resection. Within this TFH background, follicles with sheets of centroblasts and brisk mitotic activity and a “starry sky” pattern were identified. Immunohistochemistry confirmed germinal center B-cell phenotype (CD20, CD10, BCL6, weak BCL2) with a Ki-67 > 90 %. Molecular studies demonstrated a clonal IGK rearrangement, IGH::MYC fusion, and DDX3X mutation, while hyperplastic areas lacked clonal rearrangements. This case highlights the thymus as a rare site of B-cell lymphomagenesis and underscores the importance of recognizing FLBCL in unusual locations.