Gastrointestinal stromal tumours (GIST) are the most common mesenchymal tumours of the gastrointestinal tract. They account for ~5% of all sarcomas. They respond remarkably well to chemotherapy.
GISTs usually occur after the age of 40, with most seen in older patients 1, except when associated with tumour syndromes when they may present earlier and are often multiple (see below). It is equal in both male and female with possible slight male predilection.
Clinical presentation is variable and reflects the variability of radiographic appearances, location (see below) and biological behaviour. Many tumours are incidentally identified on imaging for other indications but mostly present with dysphagia. Some tumours, on the other hand, are aggressive and present with metastases or symptoms relating to local disease 1.
Ulceration is seen in 50% of tumours although it is commoner in large tumours 1. Haemorrhage may occur, with presentation relating to gastrointestinal bleeding.
GISTs are believed to arise from the interstitial cells of Cajal 2-3, with 95% staining positive for CD117 (c-KIT) and 70% for CD34 2. The former is a tyrosine kinase growth factor receptor and the target of ST-571 (imatinib/Gleevec/GlivecTM) 1.
As the tumours are intramural but submucosal, the overlying mucosa often appears intact on pathological and imaging assessment.
Grading GISTs requires assessment of both tumour size and mitotic index 3. Smaller lesions have less aggressive biological behaviour, as do stomach GISTs when compared to tumours elsewhere along the gastrointestinal tract 3.
They are rounded with frequent haemorrhage. Larger tumours may also demonstrate necrosis and cystic change 1-2. Size is variable, ranging from 1 to 30 cm 1.
Histology demonstrates a relatively cellular tumour composed of spindle cells (70-80%) and plump epithelioid cells (20-30%) 1-2. They appear to arise from the muscularis propria layer.
The vast majority of GISTs are sporadic. However, they occasionally occur in the setting of a recognised syndrome, including 2:
Location and classification
Common sites of involvement include 8:
GISTs occur not only anywhere along the gastrointestinal tract, but also in the mesentery, omentum and retroperitoneum, these are then called extra-gastrointestinal GISTs 1,4. Metastatic lesions may also be seen in cases of malignant extra-gastrointestinal GISTs 7.
Specific appearances will vary according to location (see above), but in general, these tumours appear as rounded soft tissue masses, arising from the wall of a hollow viscus (most commonly the stomach) with an endoluminal or exophytic growth. Mucosal ulceration is present in 50% of cases 1 with large necrotic cavities communicating with the lumen also seen.
When large, soft tissue density displacing bowel loops may be seen, which is, of course, non-specific.
On upper abdominal studies, filling defect projecting from the wall of the stomach may be seen, with overlying ulceration or cavitation. The tumour margins are normally seen as smooth and may form right or obtuse angles with the adjacent mucosa due to its intramural origin.
Appearances vary with size and location. Typically the mass is of soft tissue density with central areas of lower density when necrosis is present (usually in larger tumours) that occasionally appear as fluid-fluid levels.
As the tumours are often exophytic, it can be difficult to delineate them on CT if the stomach is distended with barium, though the non-enhancing central necrotic area may be helpful. A deep crescent-shaped ulceration demonstrating an internal air-fluid level may be referred to as the Torricelli-Bernoulli sign 9.
Enhancement is typically peripheral (due to central necrosis) 1. Calcification is uncommon (3%) 1.
Metastases (distant, peritoneal, omental) or direct invasion into adjacent organs may be seen in more aggressive lesions. Lymph node enlargement is not a feature 1.
The presence of necrosis, haemorrhagic and cystic change make appearances variable:
- low signal intensity solid component
- enhancement is usually present, and predominantly peripheral in larger lesions
- T2: high signal intensity solid component
Treatment and prognosis
Surgical en-bloc resection is the primary mode of therapy for GISTs 4. Up to 50% of all GISTs will have evidence of metastatic disease at the time of presentation 3, which significantly impacts prognosis.
Adjuvant chemotherapy with ST-571 (imatinib) is effective in the majority of cases and has had a dramatic impact on prognosis even with only one year of therapy, reducing recurrence at one year from 17% to 3% 4. Longer treatment regimes (2-3 years of imatinib) are currently under investigation 4. Other second-line agents (e.g. sunitinib) are also being studied and used for patients with imatinib-resistant tumours.
Historically, before ST-571 (imatinib) up to 85% of tumours would locally recur or develop subsequent distal metastases despite treatment 3,4 and had proven to be resistant to standard chemotherapy 4.
FDG-PET may be used in monitoring treatment response and is considered superior to CT in monitoring treatment response in the initial phases of treatment10.
General imaging differential considerations include:
- most common in the oesophagus, accounting for 75% of mesenchymal tumours 1
- rare in the remainder of the GI tract
- gastrointestinal leiomyosarcoma: rare
gastrointestinal lymphoma / gastric lymphoma
- lymphadenopathy uncommon for GIST
- more extensive mural thickening
- there is often associated aneurysmal dilatation
- gastrointestinal schwannoma
- gastrointestinal carcinoid
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