This mediastinal tumor has aggressive features and causes SVC syndrome. At this age group, a germ cell tumor or sarcoma should be considered among the differentials. Lymphoma is felt less likely due to the invasive/compressive behavior over the adjacent mediastinal structures. Thymic carcinoma is a less likely differential for this age group. 

CT-guided core biopsy was performed: 

MICROSCOPIC DESCRIPTION: The sections of core biopsy show connective tissue infiltrated by sheets of discohesive atypical cells with scant cytoplasm and irregular intermediate to large nuclei, accompanied by small lymphocytes. No glandular formation, intracytoplasmic vacuoles or pigment are seen. The cells exhibit nuclear fragility and show crush and streaking artefact.

IMMUNOHISTOCHEMICAL PROFILE:

• CD20 - Positive diffuse staining in large cells
• PAX-5 - Positive in large cells
• CD3 - Negative in large cells, positive in reactive small T cells
• CD5 - Negative in large cells, positive in reactive small T cells
• bcl-2 - Positive
• TdT - Negative
• MUM-1 - Positive in large cells
• bcl-6 - Positive in 30% of large cells
• CD10 - Negative
• Ki67 - Positive in 80-90% of large cells
• cMyc - to follow
• AE1/AE3 - Negative

DIAGNOSIS: Anterior mediastinal mass, core biopsy: Diffuse large B cell lymphoma, activated B cell phenotype.