Gastrointestinal Stromal Tumours
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GISTs are rare sarcomas that arise in the gastrointestinal tract, anywhere from the oesophagus to the anus. A sarcoma is tumour that grows from either muscle, nerves, blood vessels or fat. GISTs are thought to arise from the pacemaker nerve cells of the bowel (Interstitial cells of Cajal). The most common sites are the stomach (60%) and the small intestine (25%). Less than 5% of GISTs arise outside of the gastrointestinal tract.
GISTS are found by investigation in approximately 1 in a 100,000 people a year. However, occult GISTs <1cm in size are observed in up to 20% of individuals in autopsy series. GISTs occur mostly in adults between the age of 65-69 and are equally common among the sexes.
The symptoms of GISTs are non-specific and depend on the size and location of the lesion. Small GISTs <2cm in size are usually asymptomatic and found incidentally during investigation for unrelated disease. The most common presenting symptom is gastrointestinal bleeding, which occurs in 50% of patients. Large tumours may also present as an abdominal mass, or with abdominal discomfort, gastrointestinal obstruction or weight loss.
GISTs are a type of cancer but the majority are very indolent and do not spread to other sites in the body. Their behaviour is determined by their genetic subtype. Approximately 10% of patients present with spread to other sites in the body, known as metastases. The most common sites of metastases are the liver, omentum and peritoneum.
GISTs are detected either incidentally in asymptomatic patients, or during the evaluation of the symptomatic patients. They have characteristic appearances across various imaging techniques. Your doctor will advise which tests are appropriate for you.
Endoscopy – To identify the location of the GIST and assess if there is any ulceration which may lead to bleeding.
CT – To characterize the tumour, assess involvement of surrounding organs and presence of metastases.
EUS +/- Biopsy – Useful for characterization and biopsy for confirmation of the diagnosis.
Localised GISTs <2cm in size (microGISTs or miniGISTs) can be surveyed with yearly Endoscopic Ultrasound, with intervention only recommended if there is a significant change in size. The larger the GIST, the more aggressively it is likely to behave and therefore benefiting from treatment.
Surgery is the primary treatment for localised GISTs >2cm that have not spread to other parts of the body. The operation can usually be performed with a minimally invasive approach such as laparoscopy or robotics. The aim of the surgery is to remove the tumour en-block without a breach of the capsule to reduce the risk of its recurrence.
A patient who presents with spread to other sites in the body (metastases) must not be operated on upfront but should receive Tyrosine Kinase Inhibitor (e.g. Imatinib) therapy as initial treatment. Approximately, 80% of GISTs respond to Tyrosine Kinase Inhibitors. Also, large localized tumours and/or those challenging to resect because of the involvement of adjacent organs may require upfront treatment with Tyrosine Kinase Inhibitors prior to surgery.
Decision making for GIST treatment should be subject to a Multi-disciplinary Team review and advise.
Risk assessment in localized GIST aims to identify tumours that are more likely to recur after curative surgery. The assessment is based on the following factors including: tumour size, tumour location, mitotic index (MI) and tumour rupture before or during surgery. If the risk score is high, patients may be offered Tyrosine Kinase Inhibitor therapy to prevent GIST relapse.