How is Gastrinoma diagnosed?
On suspicion of a gastrinoma, tests are used to:
- confirm elevated gastrin and acid levels biochemically
- localise the tumour
Confirming elevated gastrin and acid
Gastrin and acid
As explained in 'What is a gastrinoma?' both gastrin and acid are raised in gastrinomas. Blood tests show a gastrin level of greater than 1000 pg/ml. Invasive tests on the stomach allow the measurement of Basal Acid Output (BAO, the normal minute to minute amount of acid being produced) and the Maximal Acid Output (MAO, the largest amount of acid produced by the stomach). In gastrinoma, the BAO is generally more than 60% of the MAO, or greater than 10 mmol/hour, demonstrating a significant excess of acid production.
Provocative test with secretin
Secretin causes a rapid and sustained elevation of both acid and gastrin from gastrinomas due to the lack of negative feedback exhibited by such tumours. Unaffected individuals, with intact negative feedback, respond with a rise in acid levels only.
Localisation of the tumour (Gastrinoma)
CT, MRI and ultrasound
CT and MRI can be used to localise tumours of size greater than 1 cm diameter. For those smaller than this, endoscopic ultrasound can be used. This is where the ultrasound probe is guided down the oesophagus (the gullet) to scan the potential sites of the tumour from inside with less interference from other tissues that would be in the path of the ultrasound beam were the scan done from outside the body.
Angiography
This highlights areas of increased vasculature that could be a tumour.
Arterial stimulation with venous sampling
Selective injection of secretin (which would stimulate gastrin and acid production in gastrinomas) into arteries supplying only specific structures can be used to investigate whether the tumour is located in a tissue supplied by that artery. Selective intra-arterial calcium injection and hepatic venous sampling has been used to successfully localise gastrinomas. Calcium gluconate is directely injected into the arteries supplying the pancreas and liver after standard selective angiography. Gastrin levels are then measured from samples taken, before the calcium gluconate is injected. The calcium gluconate causes a diagnostic rise (at least 2-fold) in the gastrin levels and localises the gastrinoma to a specific vascular territory.
Proton pump inhibitors (PPIs) and H2-receptor antagonists are routinely stopped prior to the test to allow the fasting gastrin levels to return to normal. A recent study has shown that localistion is still possible even when the patients remain on their PPIs or H2-receptor antagonists. More research is needed to give a definitive answer.
Octreotide Scintigraphy
Very occasionally the presence of very small tumours will require the use of other investigation, such as somatostatin receptor scintigraphy. A radioactive-labelled analogue of somatostatin, indium-111 pentetreotide, is injected. This binds specifically to the somatostatin receptors of the tumour cells. X-rays then show up the area where the radiolabelled molecule is, therefore indicating where the tumour is.
Trans hepatic portal venous sampling
As with most endocrine tumour conditions, this can be employed but its invasive nature makes it less desirable.