Diagnostic challenges in a t(14;18)-negative and t(3;14)-negative follicular lymphoma with transformation to diffuse large B-cell lymphoma, along with negative BCR-IG clonal rearrangement, a pathological perspective

Background Follicular Lymphoma (FL) is an indolent B-cell non-Hodgkin's lymphoma that originates in the germinal centers(GC) of lymphoid follicles. The tumor cells are primarily made up of follicular centrocytic (CC) and centroblast cells(CB), and presented witn at least a focal follicular pattern identifiable by morphology. In approximately 85% to 90% of cases, BCL2 rearrangement involving t(14;18)(q32;q21) can be identified. However, the transformation of follicular lymphoma (FL) into diffuse large B-cell lymphoma(DLBCL)-particularly negative for t(14;18), t(3;14), and lacking a clonal BCR-IG rearrangement—was considered rare but noteworthy. Case Presentation A 50-year-old female patient who underwent resection of the thyroid and surrounding lymph nodes due to clinical suspicion of thyroid cancer with regional lymph node involvement. Postoperative pathological examination revealed DLBCL in the thyroid, arising from transformation of high-grade FL, while the lymph nodes also showed evidence of high-grade FL(BCL2 negative). Immunohistochemical analysis demonstrated that both the thyroid and lymph node lesions were negative for germinal center(GC) markers CD10 and BCL6, as well as for BCL2. FISH testing did not identify any break-apart rearrangements in the BCL2 or BCL6 genes, and PCR analysis failed to detect clonal BCR-IG rearrangement. Conclusions Here, We presented a rare case of nodal FL with BCL2-negative expression, accompanied by transformation into DLBL involving the thyroid. While the histomorphology of this case appeared relatively classic, its immunophenotypic and molecular profile presented unique features, making it a particularly interesting example of FL transforming into DLBL.