Soft tissue sarcoma. Coloured transmission electron micrograph (TEM) of a section through a malignant (cancerous) tumour that has developed in mesenchymal tissue (muscle, connective tissues and bone). Soft tissue sarcomas, such as leiomyosarcoma, chondrosarcoma, and gastrointestinal stromal tumour (GIST), are very rare, accounting for less than 1 per cent of all cases of cancer. They are most commonly found in patients over 50. Treatment options for soft tissue sarcomas include surgery, radiation therapy, and chemotherapy.