This is a report on 8 mediastinal tumours that occurred in young adults (19-43 years, mean: 29.4); predominantly in females (6/8). Initial symptoms consisted of thoracic pain and venectasia and in only one case in B symptoms. After surgical tumour reduction, radiation and/or chemotherapy, local recurrence was observed in each case under clinical care; abdominal spread is presently suspected in 3 patients; 3 died 11, 13 and 22 months after diagnosis. None developed leukaemia. The tumours are B-cell neoplasms with a characteristic immunophenotype: leucocyte common antigen+, common acute lymphoblastic leukaemia antigen-, B 1-antigen+, surface and cytoplasmic immunoglobulin-. Flow cytometry revealed DNA-diploidy in 7 cases and a moderately (3.2-3.8%) to extremely high (8.0-20.6%) S-phase component. The proliferation associated antigen Ki67 was detectable in 10-60% of the tumour cell nuclei, thus stressing the considerable or rapid growth. Histopathology is characterized by a diffuse growth pattern and a clearness and abundance of cytoplasm of the pleomorphic tumour cells, which vary in size and nuclear morphology from patient to patient. Apoptoses are more numerous than mitoses. Fibrosis and focal necrosis are common, sclerosis is present in 3 cases. We suggest that primary mediastinal lymphoma of B cell type is a novel B-lymphoma variant.

This article reports eight primary mediastinal tumors occurring in young adults (19 to 43 years, mean 29.4 years), predominantly female (six of eight) adults. Most patients responded badly to aggressive therapy. Progression is presently noted in one patient; five patients died 10, 11, 13, 18, and 22 months after diagnosis. No patient developed leukemia. The tumors were highly proliferative, had a diffuse growth pattern, and comprised clear cells of variable size. They could not be classified histologically, but could, however, be immunohistologically characterized as B cell lymphomas. In all cases, the immunophenotype was LC+, cALLa-, CD19+, CD20+, CD21-, Ig (surface/cytoplasm)-, and PC-1+. In addition, the neoplastic cells exhibited variable defects in the expression of HLA-A,B,C and HLA-DR and inconstant expression of other B cell-restricted/associated antigens. This combination of immunophenotypical and clinical features suggests that the mediastinal clear cell lymphoma (MCCL) is a previously undescribed type of B cell lymphoma corresponding to the terminal steps of B cell differentiation.