Light micrograph showing a histiocytic sarcoma of the small bowel that caused intestinal obstruction. The small intestinal villi are distorted by sheets of pleomorphic tumour cells with abundant eosinophilic cytoplasm. Histiocytic sarcoma is a malignant tumour with histiocytic differentiation. Lymph nodes are the most frequently involved site. Extranodal sites include skin, liver, spleen, bone, and gastrointestinal tract. Some patients develop systemic disease with multi-organ involvement, giving rise to the previously used term malignant histiocytosis.