Gastrointestinal Stromal Tumour is a relatively 'new' diagnosis, often confused with leiomyosarcomas in the past. Most mesenchymal tumours of the gastrointestinal system are GISTs and prior to the year 2000 and the 3rd edition of the WHO classification of gastrointestinal tumours they were frequently diagnosed as leiomyoma/leiomyosarcoma. This makes historical treatments somewhat difficult to apply to the modern setting.
GISTs are most frequent in the stomach (60%) followed by the small intestine (30%); the remainder occur in oesophagus, large bowel, omentum or mesentery. In the stomach, GISTs make up about 2% of all malignancies (compared with adenocarcinoma > 85%, lymphoma ~ 10%, and carcinoid < 1%). There are minimal differences between genders and the average age at diagnosis is 60. They are more common in those of African descent.
Aetiology and Pathogenesis
Most cases are sporadic with no known aetiology. Familial cases arise in patients with germline mutations of cKIT or PDGFRA.
Cases are often clinically silent until they are massive (eg. > 20 cm) where they present with symptoms relating to mass effect. Other cases may be found incidentally. The behaviour of GIST depends on the size and grade of the tumour; those under 5 cm are typically cured by surgical excision whereas larger tumours often recur with liver metastases; lung metastases are rare in GIST (but common for leiomyosarcoma of the gastrointestinal tract).
Patients typically present with mass effect (bloating, fullness, early satiety). If the tumour erodes the gastric mucosa patients may present with haematemesis or melaena. In the oesophagus or bowel patients may develop bowel obstruction secondary to the tumour.
Imaging may show a massive lesion or alternatively nodules within the wall of the stomach, bowel, omentum or mesentery.
Tumour/Normal Tissue Features
Macroscopically the tumours are usually tan. Malignant tumours may have areas of haemorrhage, cystic change or necrosis.
Microscopically the tumours may show spindle cell, epitheliod, or mixed architecture. The characteristic finding in > 95% of GISTs is the presence of cKIT staining on immunohistochemistry.