The best place to start would be to look at the Society of Nuclear Medicine procedural guidelines (SNM), since it would provide a grand overview of what this type imaging is all about.
We need to make the distinction between pentetreotide versus octreotide when discussing the imaging aspects, however. In nuclear medicine the In-111 pentetreotide is the compound that we work with, because it has a DTPA chelator attached to it to allow it to bind to the indium-111 isotope. Essentially pentetreotide (In-111-DTPA-D-Phe) is a conjugated form of a somatostatin analog. Octreotide on the other hand is just a peptide (an octapeptide to be exact) that mimics the natural somatostatin that is found within our bodies. Basically, octreotide is a synthetic version of the natural ones within our bodies.
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| Fig. 1 This is the pentetreotide peptide. For those who would want to know what it looks like. |
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| Fig. 2 This is the structure for Octreotide. I will say this: I am not a radiochemist, or a radiopharmacist and I did not take organic nor inorganic chemistry. However, I made sure our radiopharmacist was okay with these images before posting. |
Here are some images that we have acquired recently. It will help us with the discussion.
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| Fig. 3 A 24 hour In-111 pentetreotide scan of a patient with gastrinoma. Notice a focal uptake in the midline of the body, along with a faint uptake in the liver. You can see it better in the whole body images on the right. |
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| Fig. 4 Another 24 hour whole body imaging. Note the focal uptake in the midline of the body. This patient was being worked up for insulinoma. |
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| Fig. 5 As above, imaging was performed at 24 hours. However note the uptake in the thyroid. |
Majority of the patients that come to see us receive an In-111 pentreotide injection for some type of neuroendocrine imaging. Most of them involve the gastroenteropancreatic (GEP) tumours such as the insulinomas, gastrinomas, VIPomas (vasoactive intestinal polypetide secreting tumour) and other times we get the occasional request for carcinoids.
Patient preparation is fairly straight forward, but one thing that is interesting is the decision to infuse glucose in patients in those suspected of insulinoma. This is to prevent the potential problem of hypoglycemia during the injection. It is not common practise and it is the decision of the radiologist to incorporate this in their standard of practise with these cases. We do not do it at our site. The one thing that we do screen for are medications, such as the use of Octreotide as a therapeutic agent. Since Octreotide is the synthetic form of somatostatin within our bodies, it's more potent and effective in suppressing gastrin, cholecystokinin, glucagon, TSH, pancreatic polypeptide, secretin, growth hormone, TSH and vasoactive intestinal peptide. Careful consideration of the time when the patient had taken the Octreotide is important as we want to ensure good uptake of the In-111 pentetreotide. More patient preparation information can be reviewed in the SNM guidelines, or in the European Association of Nuclear Medicine guidelines as well.
The imaging of these patients are fairly straight forward as well. Whole body scanning is generally done at 24 hours, but before when we had purchased our SPECT/CT, the standard of practise was to perform whole body imaging as well as, a standard SPECT at 48 hours. The SPECT/CT was a bit of a game changer for us, since our radiologists were comfortable with the images obtained at 24 hours that included the SPECT/CT.
Not all these tumours are created equally. In order to be visualized they require a number of somatostatin receptors on the tumours themselves. There are 6 known receptors: SSTR1, SSTR2a, SSTR2b, SSTR3, SSTR4, SSTR5. Note there are also subtypes as well. With the "lock and key" method of visualization, the sensitivity of the tumours will vary. For example, there is better sensitivity in gastroenteropancreatic neuroendocrine tumors (download the PDF, it's free), except for insulinomas as the result of the lower incidence of somatostatin receptors, especially those of the SSTR2. The sensitivity is approximately 25-60% in comparison to the other tumours in this category. The best practise for our department is that our radiologists screens the requisitions before we begin the bookings and the ordering of indium-111 for compounding.
I want to touch upon one thing however and it is the biodistribution of the In-111 pentreotide. Generally the normal uptake of In-111 pentreotide occur in the pituitary glands, the thyroid, liver, spleen, kidney, bladder and sometimes gallbladder. Bowel is also seen but more so after 24 hours post injection. If you look at Figure 5 in comparison to the other images, there is the obvious uptake in the thyroid bed. I personally have not seen this very often, so my initial inclination was "abnormality" since medullary thyroid cancers and pituitary adenomas express high amounts of somatostatin receptors. Upon reviewing the finalized report, it did not mention the uptake in the thyroid bed, but rather indicating no evidence of any octreotide avid pancreatic tumours or metastases; the biodistribution was normal. Further review into the patient history, just recently, had the patient to undergo an urgent thyroid ultrasound and aspiration for pre-op staging. This patient had a previous biopsy of the left thyroid as well, but it had unsatisfactory cellularity in the sample for any type of determination. The CT work up report for this patient also noted a left thyroid nodule.
I'm not aware of any recent results in regards to the identification of the nodule, but this is something that I will be following up for my own education. This may be a potential occurrence of an incidental finding for thyroid carcinoma.... who knows?
