Paraffin sections of the specimen show accumulation of large atypical mononuclear cells around small calibre vascular channels with invasion of the walls of some vessels noted. The atypical cells have large round, oval and angulated hyperchromatic and vesicular nuclei and a narrow rim of pale cytoplasm. An occasional mitotic figure is noted.
These atypical cells show strong surface immunostaining for the B lymphocyte markers, CD20 and CD79a. A moderate number of small mature CD3+ T lymphocytes are also present.