Histiocytic Sarcoma Originating in the Lung in a 16-Year-Old Male

We report a 16-year-old male with histiocytic sarcoma (HS) originating in the lung. Partial resection of the lung was performed for a 3-cm mass with a clear boundary detected in the right inferior pulmonary lobe on a health checkup. Histologically, the tumor infiltrated into the surrounding tissue, and was comprised of spindle cells, mainly, and foam cells accompanied by mild nuclear atypia. The tumor cells were immunohistochemically positive for CD68 and CD163, indicating histiocytic lineage and the MIB-1-positive rate was low. Spindle cell morphology of HS is quite rare and only 3 cases of pulmonary HS have previously been reported.