The core biopsy shows a proliferation of atypical lymphoid cells, infiltrating into the fat and skeletal muscle. It forms diffuse sheets. No follicular structures are seen. The atypical lymphocytes are mostly medium in size, though some large ones are also present.
They have enlarged clefted and hyperchromatic nuclei, small nucleoli and scanty cytoplasm. The background is sclerosed. The atypical lymphoid cells are CD20, PAX-5, CD23, bcl-2, bcl-6 and MUM1 positive. The Ki-67 index is about 50%. They are CD3, CD5, CD10, CD30, Cyclin D1, c-Myc, TdT, ALK-1 and EBER-CISH negative. The features are those of diffuse large B-cell lymphoma with activated B-cell-like phenotype.
Mediastinal mass biopsy: Diffuse large B-cell lymphoma.