Hairy cell leukaemia, light micrograph. Histiocytic sarcoma is a malignant tumour with histiocytic differentiation. It shows a diffuse growth pattern with partial or total effacement of the native architecture by a proliferation of high-grade malignant cells resembling histiocytes. The degree of pleomorphism is variable and mitotic activity is increased. The background stroma contains small lymphocytes, plasma cells, eosinophils, and benign histiocytes.