More on PET Scans, particularly PSMA PET Scans

[Paul Edwards]

Paul Edwards (not a doctor) says:

For some time I’ve been wanting to gather together all the bits and pieces in the forum on PET Scans.  Whatever you do have a look at the amazing video at the end of this post.

 

A positron emission tomography (PET) scan is an imaging test that uses a radioactive substance called a radiotracer to look for disease in the body.

 

Before carrying out a PET scan, a radioactive medicine is produced in a cyclotron (a type of machine). The radioactive medicine is then tagged to a natural chemical.  This natural chemical could be glucose, water, or ammonia.  The tagged natural chemical is known as a radiotracer.  The radiotracer is then inserted into the patient’s body, normally through a canula which has been inserted into the patient’s arm.

When it is inside, the radiotracer will go to areas inside the body that use the natural chemical.  

 

For example, the ¹⁸F-FDG (fluorodeoxyglucose) is a radiotracer that is tagged to glucose. The glucose goes into those parts of the body that use glucose for energy.  Cancers, for example, use glucose differently from normal tissue - so, an FDG PET Scan can show up cancers.

 

The ¹⁸F-FDG PET Scan is probably the commonly used PET Scan in hospitals.  Because prostate cancer is slow-growing, it does not take up glucose as much as other cancers.  For this reason researchers have been looking to develop radiotracers that were more suitable to use for imaging prostate cancer.

 

For several years the ¹¹C-Choline PET scan introduced at the Mayo Clinic has been regarded as leading the way in the imaging of prostate cancer.  The major limitation of ¹¹C-Choline is that it has a 20-minute half-life (The half life is the time required for one half of the atoms of a given amount of a radioactive substance to disintegrate).  This means that ¹¹C-Choline must be used very quickly after it is produced. For this reason, it must be produced on site very close to where it is administered.  Normally radiotracers are produced off site for safety reasons.

 

¹¹C-Choline has shown limited sensitivity in men with very low PSAs.  One study showed a 5% detection rate where PSA levels were less than 1.

 

¹⁸F-Fluorocholine is another radiotracer that has been trialled in Europe and Australia with good results. However, it is less sensitive than ¹¹C-Choline and requires a higher PSA level in order to get an effective image.

 

A PSMA PET scan is one that uses a radiotracer which is targeted to a protein (Prostate-Specific Membrane Antigen) that is found in prostate cancer.

 

There are different types of radiotracers are being developed for PSMA PET Scans.  For example, in the United States Johns Hopkins University has developed a ¹⁸F-DCFBC radiotracer and Memorial Sloan Kettering Cancer Centre has developed a Zr⁸⁹-J591 radiotracer.  In Australia we are now using a Gallium(Ga⁶⁸) radiotracer which was developed in Germany.

 

The Gallium PSMA PET Scan produces a sharply defined image at very low PSA levels. Because the Gallium PSMA PET Scan targets the Prostate-Specific Membrane Antigen protruding from the outer membrane of the cancer cells, the radiotracer “lights up” on the PET images showing clearly metastases to lymph nodes as well as to bone.

 

[in my case with a very low PSA of 0.58, the Gallium PSMA PET Scan detected 4 metastases that were not visible on other scans.]

 

Seeing is believing.  Memorial Sloan Kettering Cancer Centre has produced an excellent video showing the difference that a PSMA PET Scan makes:

 

http://www.mskcc.org/videos/prostate-specific-pet-scans

 

Whilst radiology oncologists are predicting that the PSMA PET Scan will revolutionise the treatment of prostate cancer, that revolution is some time away.  At the moment the technology is still being trialled in Australia.  There is limited availability of these scans: only a few hospitals in Australia are offering them. The scans are expensive and are not covered by Medicare.

[alanbarlee]

Great post Paul.

 

While we're on the subject, ithere are a few more imaging techniques that could be added to the list - some old, some new.

 

Everyone will be familiar with a conventional and widely available CT scan, which typically uses a contrast agent to improve X-ray resolution of organ and bone edges, as well as small items like lymph nodes. It's been around for quite a while, and is still very useful - particularly to geographically locate and size PET images.

 

Similarly with the traditional bone scan, which uses technetium to highlight osseous bone metastases - but which is not very sensitive unless the PSA is 20 or more (i.e. it is likely to miss small mets with lower PSA levels).

 

A new imaging option for bone is 18NaF-PET/CT which is a bone-specific version of the PET scans mentioned in the previous post . It uses radioactive sodium fluoride as a tracer, which has an specific affinity for bone, differentiating between normal bone and bone mets.

 

MRI is an established and sensitive non-radiation imaging technique, especially for soft tissue imaging, but its higher cost means that it is not as widely available outside major centres as is CT, nor is it as eligible for Medicare rebates.

 

A new and highly sensitive version of this is 3-tesla parametric MRI, which uses more powerful magnets and high-level computer image enhancement. As with the 68Ga-PSMA PET/CT scan mentioned in the previous post, this is not yet available other than in a small number of centres - although a few of our members have been able to access one or other of these through clinical trials or referrals.  Other than through trials, these are presently patient-paid.

 

Cheers,

 

Alan

[John Murphy]

Hi Paul,
Great to read your last post and also to follow up on the replies.
I would like to add some details and progress of my current situation.
Diagnosed in 2005 with a PSA of 32 and Gleason of 4 plus 3, 7 with mets to 4th rib and other hot spots according to MRIs, bone scans and CT scans. Immediately commenced Lucrin 3 monthly intramuscular injections. PSA dropped to less than 1.
After much research began the Liebovitch protocol ie, Lucron plus 150 mg duasteride (sp?) and Avodart for a period of 13 months.
PSA stayed below 1 for a couple of years after with no treatment. As PSA rose to around 5 recommenced Lucrin until it increased to over 85. Then the shit hit the fan. After a spell in our great little Ararat hospital with a UTI something had to be done. My oncologist at the time said everything was "OK".??? I contacted Dan Moon in Melbourne and after viewing the same scans as my oncologist he immediately booked me into Peter Mac for a big and serious procedure. Long story short I lost my bladder, prostate and a section of my bowel (mainly to achieve clear margins). After a long spell in hospital incurring two bad infections I was finally sent home.
Last month I returned to the Epworth to finally have my iliostomy reversed. Now I have only one bag my urostomy which I am managing very well. During my spell at the Epworth I was set up to begin Sterotactic radiotherapy after having had a PSMA scan. This scan revealed 4 hot spots therefore falling into the oligometastasis category. One was in my prostate bed, one in my right hip, one in my pelvic iliac node and one in my 4th rib. Not that bad as I was expecting more as scans in the past revealed spots all over. So much for all my previous scans!! Now into my first week of radiation with no side effects although expect some tiredness towards the final days.
My radiation is 10 treatments at 50 Gray in total. Treatment per day lasts for about one hour. Not looking forward to the expense but as I am at the safety threshold I think will cost around 2 grand. Can't take it with you.
Saw the radiation oncologist today ( Pat Bowden). Prognosis ? Who knows. But a far better outcome than I would have had 10 years ago. So Paul,Popeye and others keep corresponding.
Cheers,
John Murphy

[DavidAbrahams]

Thanks Paul, very interesting and appreciated. Hope to hear early in the New Year when I can have a Ga68 PSMA PET Scan at the RBWH in Brisbane. All the Best David

[Paul Edwards]

A friend in Sydney has asked me to add this info for anyone in Sydney who may be interested in a PSMA PET Scan.  It is offered by both St. Vincent's Public Hospital and Royal North Shore Public Hospital.  Waiting time for a booking in both cases is about 2-3 weeks and about 2-3 hours should be allowed for the scan. You have to be prepared to lighten your bank balance because the scan is not on the Medical Benefits Scheme.  The cost varies significantly from one institution to another. Peter MacCullum, Melbourne $1200; Wesley Hospital, Brisbane $600; St Vincent's $750 and Royal North Shore $590.