Histologic transformation of follicular lymphoma: pathologists' viewpoint

Outcomes of patients with histologic transformation (HT) of follicular lymphoma (FL) have been believed to be poor. The most common histologic subtype of transformation from FL is diffuse large B-cell lymphoma (DLBCL), which accounts for 90% of the cases, and the remaining 10% of the cases include classic Hodgkin lymphoma, high-grade B-cell lymphoma, plasmablastic lymphoma, B-acute lymphoblastic leukemia/lymphoma, histiocytic/dendritic cell sarcoma, and anaplastic large cell lymphoma-like lymphoma. Because the histologic criteria for the diagnosis of DLBCL transformed from FL are unclear, convenient histopathological criteria for HT are required. One of the proposed criteria of HT from our institute is the presence of diffuse architecture with a proportion of large lymphoma cells of ≥20%, and for challenging cases, Ki-67 index ≥50% is used as a reference. Patients with HT to non-DLBCL have poorer outcomes than those with HT to DLBCL; thus, rapid and accurate histologic diagnosis is desired. In this review, we discussed the recent literatures describing the histopathologic variety and proposal of definition of HT.