d) Smooth Muscle Tumours

Benign leiomyomas are uncommon outside of the uterus. Leiomyosarcoma forms one of the larger groups of malignant soft tissue tumours.


Leiomyoma of the soft tissues is very rare with an equal sex distribution.
Leiomyosarcoma comprises 15-20% of soft tissue sarcoma.


Leiomyoma of the soft tissue can occur in many locations, including the retroperitoneum.
Leiomyosarcoma is relatively less common in the limbs and more common in the retroperitoneum which it shares with liposarcoma.

Aetiology and Pathogenesis

No specific aetiology is known.

Natural History

Due to propensity for arising in the retroperitoneum, close to or in association with the inferior vena cava, leiomyosarcomas may cause obstruction of the IVC with Budd-Chiari (upper IVC), renal failure (middle IVC) or lower limb oedema (lower IVC). More commonly it present with a mass lesion. Local recurrence and distant metastases are common with retroperitoneal sites; superficial sites are often curable due to smaller size at presentation; and those arising in deep tissues are of intermediate behaviour.

Tumour/Normal Tissue Features



Colour is grey, white or tan. Some specimens may have a whorled appearance but this is more consistent with leiomyoma. Haemorrhage or necrosis may be visibly macroscopically in high grade lesions.


The tumour is comprised of spindle cells with anaplastic features. They are often arranged in fascicles which intersect at sharp angles. Pleomorphic features are rare and there appears to be no 'pleomorphic leiomyosarcoma' group.


Immunohistochemistry is important to differentiate leiomyosarcoma from gastrointestinal stromal tumour which shares similar features. Leiomyosarcoma stains negatively for cKIT, but positive for smooth muscle actin and desmin in most cases.


Genetics are not fully understood and a variety of patterns has been observed.