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The tumor cells are arranged in diffuse sheets and usually lack fibrous septa and lymphoid infiltrate. Rare cases with lymphoid infiltrate have been reported, but granulomatous response has not been seen. The tumor is composed of a polymorphous population consisting of three cell types: small, lymphocyte like cells with smudged chromatin; intermediate sized cells with round or oval nuclei and granular chromatin; large cells which may be multinucleated. The large size cells have characteristic filamentous (spireme) chromatin. Mitotic activity is frequently increased.