Clinicopathological and genetic analyses of thyroid large B-cell lymphoma in a Japanese population.

Primary thyroid lymphoma is a rare type of cancer. Most cases involve large B-cell lymphomas (LBCLs), which largely show good prognoses. However, the reasons for this have not been understood. To identify the factors influencing the favorable clinical outcomes of thyroid LBCLs, clinicopathological and genetic analyses of 21 cases of thyroid LBCLs were performed, including immunohistochemistry, fluorescence in situ hybridization (FISH), and analysis for MYD88 mutations based on the World Health Organization Classification of Tumors, 5th Edition. The median age of the patients was 70 years (range, 54-80 years). Fifteen patients (71%) had limited-stage disease. The 5-year overall survival rate was 83% (95% confidence interval: 56%-94%). No instances of central nervous system (CNS) recurrence was observed. The series included 15 cases with diffuse LBCL not otherwise specified (DLBCLnos) and 6 cases with transformation of indolent BCLs (T-IBCLs). Immunohistochemistry subdivided DLBCLs into 12 germinal center B-cell (GCB) and 9 non-GCB subtypes. FISH analysis revealed split signals of MYC in 2/17 cases, MALT1 in 0/15 cases, and BCL6 in 3/15 cases. No MYD88 mutations were detected in any of the cases (0/21). The factors contributing to the favorable clinical course in thyroid LBCLs were a higher proportion of GCB phenotypes and the lack of MYD88 mutations in DLBCLnos and T-IBCLs. Even MYC-R cases showed better prognosis. Further studies involving a large series of LBCLs in extranodal organs are needed to expand on the findings of this study.