B-cell lymphoma of probable thymic origin: case report.

A case of B-cell lymphoma of probable thymic origin is reported. A 34-year-old woman was found to have an anterior mediastinal tumor in November 1986. The surface lymph nodes were not palpable. A total resection of the tumor mass was performed. The tumor invaded the right pleura, the right lung and the pericardium. Histologically, normal thymus was found at the margin of the tumor tissue. The neoplasm was predominantly composed of large lymphoid cells, separated by rather thick fibrous bands of nodular fashion in some areas. Immunohistochemical staining demonstrated monoclonal cytoplasmic IgG and kappa chains in a small portion of the neoplastic cells in fixed tissue. The cells showed positive staining with cluster of differentiation (CD) 20 (B1) but negative staining with antibodies reactive with T-cells in unfixed tissue. "Malignant lymphoma, diffuse, large cell type (B)" was the diagnosis. The arrangements of immunoglobulin (Ig) and T cell receptor (TCR) beta genes were studied. Clonal rearrangement bands of IgH and Ig kappa genes were observed in the same sizes in both the tumor and the peripheral blood before chemotherapy. The patient received chemotherapy until September 1987, and is in complete remission at present (January 1988). The peripheral blood showed germ line patterns of IgH and Ig kappa genes in complete remission. No rearrangement bands of TCR beta genes were detected throughout. The B-cell lineage was proved both from gene arrangement analysis and with immunohistochemistry.