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.
