Understanding your Somatostatin Receptor (SSTR) PET Results

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Background

In my online patient group, there is constant discussion about the meaning of both pictures and words on scan reports.  The one that seems to cause the most confusion is PET scans, mainly somatostatin receptor (SSTR) PETs such as Ga68 and Cu64 variants. Worth adding that it’s the addition of a nuclear tracer that makes PETs seem different. Generally speaking, the PET hardware is essentially the same.  Most have a built-in CT scan, much less frequently an MRI scan.

Confusion is often triggered by healthcare system processes where the patient receives the report before the appointment to discuss the results with the referring physician.  Cue anxiety because the average patient reader does not understand and certain words cause them to worry, often unnecessary worry.  The patient then becomes impatient and will take to patient online groups to try to decode the report.  Given this is such a specialist subject far beyond the understanding of most patients, this can often lead to even worse confusion and anxiety.  One positive though, it can help the patient formulate questions for the expert who will discuss the scan report with them at some point.

Some healthcare systems are bound by law to produce these on patient accessible online information so this problem in patient groups is here to stay.  In patient groups, a heavy dose of moderation is often required when these are posted.

In this blog post (which I will update as time goes on), I wanted to initially focus on a number of frequently confusing areas, and these will be sectioned below. These include:

1. CT vs PET

2. Uptake pitfalls

3. SUV readings

The aim of this post is not a technical briefing on scans, something the author thinks is not an exact science. This post is just to help you formulate questions for your specialists or perhaps even put your mind at rest until you meet your doctor.

1. CT vs PET

Scientists and physicians will often describe scan types in a grouping called conventional imaging (CI) for CT, MRI etc.  PETs are sometimes called ‘functional imaging’ (not to be confused with functional tumours, different contexts).  Sometimes a PET is also known as molecular imaging, and this is the wording now being pushed as the textbook nomenclature to use.  Most PET/CT will use a PET tracer but not a CT tracer as the latter provides the anatomical view to help the PET display better.  There is the potential to use both PET tracer and CT tracer but it’s not routine. 

As above, a PET scan normally has a built-in CT, but much less frequently a built-in MRI is used.  The efficacy of this approach is well documented as combining (or fusing) the anatomical view of the CT with the molecular view of the PET provides a richer picture.

To put that into context, the 3 imaging pictures below from left to right, the left is PET only, right is CT only.  The centre picture is the fused CT and PET picture and allows the radiologist and other specialists to pin down locations of uptake more accurately than on PET only. This is one of the reasons why you are told to lie still on a PET/CT. 


One of the reasons I include this section is firstly a lack of understanding of the purpose of the CT (or MRI) element of PET scans. I have heard patients say their tumours could not be found on CT but were found on PET but in those circumstances, while that can happen with benign issues, tumours being obscured, or tumours lacking somatostatin receptors, it is always useful to double-check the PET/CT output.  Even without a CT tracer being used with my own SSTR PET/CT, I asked my own Oncologist to show me something from my recent PET/CT on CT only and it was there.  

2. Uptake Pitfalls (Not everything that lights up on a scan is cancer or NET (chant it 3 times!)

This phenomenon is well-known and well-documented.  There’s a perception among patients that anything with uptake is abnormal. However, this is not always true and can cause unnecessary alarm and concern. I’m going to refer to the output from someone who I believe is a leading authority on this subject. 

One of the challenges with modern scanning equipment and techniques is the ability to correctly interpret the results – in my opinion, this is almost as important as the efficiency of the machines and radionuclides. This requirement has been acknowledged in many articles and I particularly like this technical paper from a very experienced nuclear medicine physician Professor Michael Hofman from the Centre for Cancer Imaging at the Peter MacCallum Cancer in Melbourne. I had a chat with Professor Hofman who added that this is a very sensitive scan, so often picks up “new” disease, which isn’t really new, just never identifiable on standard imaging. However, there’s an excellent section on pitfalls in interpretation and I’m quoting an abstract below.  Although this document is mainly talking about Ga68 PET/CT for NETs, the majority of these pitfalls will generally apply to most SSTR PETs.

“Although GaTate PET/CT is a highly sensitive and specific technique for NETs, the attending physician or radiologist must be aware of various physiologic and other pathologic processes in which cellular expression of SSTR can result in interpretative error. Most of these processes demonstrate low-intensity and/or nonfocal uptake, in contrast with the focal intense abnormality encountered in NETs. Causes of interpretative pitfalls include prominent pancreatic uncinate process activity, inflammation, osteoblastic activity (degenerative bone disease, fracture, vertebral hemangioma), splenunculi or splenosis, and benign meningioma.  Moderately intense uptake is also seen in the liver, salivary glands, and thyroid gland.” and “The highest-intensity physiologic uptake of GaTate is seen in the spleen, followed by the adrenal glands, kidneys, and pituitary gland” 

It follows that failure to interpret nuclear scans alongside the patient’s clinical history and tempered by the known behaviours of Neuroendocrine Neoplasms can sometimes result in two big issues for patients:

1. Unnecessary worry when ‘something’ shows up.

2. Something which leads to irreversible treatment when it is not required.

Short extract of interpretative pitfalls in this table:

You can see from above there is physiological uptake. Normal uptake of PET tracers occurs in many sites of the body and may cause confusion in interpretation, particularly in oncology imaging. Osteoblastic and Inflammatory conditions can also light up given concern.   Finally, other tumours that can light up (many benign) and can be incidentally diagnosed, mainly Meningioma which is known to have somatostatin receptors. 

Finally, just to point out there can also be false positives in conventional imaging. 

3. SUV Readings

This is one of the most confusing issues for patients who continually read SUV numbers on their scan reports.  Worth reminding people that the nuclear radiologist is writing the report for the doctor who ordered the scan, it is that doctor’s job to interpret what it means.  It’s a technical report and as you may have gathered from above, it’s not an exact science. 

A PET scan report will also indicate SUV (standardized uptake value). While it is natural to believe a higher SUV indicates new, growing, or more aggressive malignancy – that is absolutely not the case.  Even when the numbers increase from scan to scan, this does not necessarily mean growth or more aggressive behaviour. 

In SSTR PET, there are two useful applications of SUV figures. 

Firstly, it can help determine if a NET patient is likely to benefit from somatostatin receptor-based therapy.  In the old octreotide scan days, this was done using something called the Krenning score, a semi-quantitative method of assessing the degree of tracer uptake.  Some radiologists suggest it is equally applicable to modern SSTR PETs and do actually use it. 

Relative uptake Krenning score

  • 0: none
  • 1: much lower than liver
  • 2: slightly less than or equal to liver 
  • 3: greater than liver
  • 4: greater than spleen

Secondly, SUV can assist in the identification of tumours more likely to be NET via a suggested threshold figure.  Some studies put that at around 16.  Again, this is not an exact science and is not an absolute measure. Dr Hofman mentions this issue in this seminal paper above “……in contrast with the focal intense abnormality encountered in NETs”. 

Finally, quotes from nuclear radiologists who are “big hitters” on the issue of interpreting SUV numbers.  I’m thankful to Josh Mailman for sparking this exchange on twitter. 

Read more by clicking on the picture

Do these issues also exist with FDG PET?

Essentially yes but there could be some subtle differences.  I refer you to this link where there is a bunch of frequently asked questions (FAQ) on Understanding PETs (specifically in reference to FDG). The conversation is chaired by Arif Sheikh who has a vast experience including with NET.  He is also co-chair of the Neuroendocrine Tumor Outreach Working Group in the Society of Nuclear Medicine and Molecular Imaging (SNMMI).  Click on the picture below to read the Q&A on FDG PET interpretation.   

arif-sheikh-bio.jpg
Click on the picture

If you can see it, you can detect it

Click on the picture to read more

All you need to know about SSTR PET

Click on the picture to read more

Somatostatin Receptors

Click on the picture to read more

Disclaimer

I am not a doctor or any form of medical professional, practitioner or counsellor. None of the information on my website, or linked to my website(s), or conveyed by me on any social media or presentation, should be interpreted as medical advice given or advised by me.  Neither should any post or comment made by a follower or member of my private group be assumed to be medical advice, even if that person is a healthcare professional as they are not members of the private group or followers of my sites in any official capacity.  Please also note that mention of a clinical service, trial/study or therapy does not constitute an endorsement of that service, trial/study or therapy by Ronny Allan, the information is provided for education and awareness purposes and/or related to Ronny Allan’s own patient experience. This element of the disclaimer includes any complementary medicine, non-prescription over the counter drugs and supplements such as vitamins and minerals.

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