Leiomyosarcoma, light micrograph. Leiomyosarcoma is a malignant tumour of smooth muscle origin. It usually develops in the deep soft tissues of the extremities and the retroperitoneum. It is relatively uncommon and makes up between 10% and 20% of all soft tissue sarcomas. It is a tumour of adults and affects women more often than men. Sometimes, they arise in great vessels such as pulmonary artery or inferior vena cava. Clinically, they present as painless firm mass. Microscopically, they consist of intersecting fascicles of spindle cells with abundant eosinophilic cytoplasm. They have blunt-ended (cigar shaped) hyperchromatic nuclei. This image shows several mitotic figures. Magnification: x400.