Leiomyosarcoma. 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. Grossly, leiomyosarcomas are large bulky tumour, often measuring 10 cm or more, especially when arising in the retroperitoneum. The tumour shown here measured 18 cm in largest dimension. The cut surface may be firm, tan-white, with whorled appearance resembling leiomyomas. Other cases are soft, fleshy masses with foci of haemorrhage and necrosis, as seen in this image.