Some cancers express a lot of PSMA while other cancers express very little or no PSMA. So how do we know which scan is more suitable to detect metastasis; Ga68 PSMA or FDG PET?
Could it be that nothing shows up on a PSMA PET but cancer is detected on FDG PET or vice versa?
If this is true, then how do we know which scan is more suitable? I guess it is not wise to have both scans at the same time
There’s a good deal of discussion on this topic. Tall_Allen has posted a link to an overview. He is more eloquent than me, so I’ll let him respond.
User slpvnmd posted this in response to one of my posts
Three links to PET imaging in Prostate Cancer. No real consensus but I firmly believe knowledge always helps. clincancerres.aacrjournals.... ncbi.nlm.nih.gov/pmc/articl... pubs.rsna.org/doi/pdf/10.11... 18F-Fluciclovine is Axumin PET
You need a PSA value well above 10 ng/ml to detect cancer with an FDG PET/CT. This is no problem if you have done all available therapies and nothing works any more. If you are not in this situation you better get a PSMA PET/CT which will usually detect cancer if the PSA value is above 1 or 2 ng/ml. If you suspect PSMA negative tumor, you could get a Choline PET/CT. However, for that your PSA should be above 5 ng/ml. It is different, if you are on Lupron. Then theses scans can detect tumor at lower PSA values.
If you are still early in progression, it is unlikely that anything will show up on an FDG PET. But if you are later in progression, the cancer may be less PSMA avid and may uptake more FDG. They can be done on consecutive days (FDG requires fasting).
If you are early in progression, is the tumor generally PSMA avid?
There seems to be an optimal point - PSMA doesn't rise to the cell surface if too early; cancer cells lose PSMA expression if too late. The Goldilocks moment probably differs across patients. I think Dr.Calais has a clinical trial to see if hormone therapy can be used to increase PSMA expression short-term (as in lab studies).
Thanks Allen. So if cancer cells do not express PSMA, then which scan would be suitable?
It depends how far along you are in progression. FDG, if much farther along. NaF(18) is MUCH better at detecting bone metastases than PSMA. There are many experimental radiotracers.
During my recent visit with Dr. Sartor who is one of the authors of the Vision trial and someone who is obviously very well versed in PSMA scans felt that a PSMA scan was the optimal scan to get. In fact for someone like me who is heavily treated and just starting to see a rise in PSA after several years of cancer dormancy he is likely to recommend me getting one at lower PSA levels than what is typical. Normally he would wait until PSA reaches 0.5, but in my case 0.1 was the number due to the treatments I’ve undergone. His thought was that PSA could be suppressed after years of treatment.
Ed
Thanks Ed. Did you find anything on the PSMA scan at 0.1 psa?
Haven’t had it yet. Latest PSA was 0.05, ya I know that’s really low, but it was undetectable for about 6 years. I like being proactive and Sartor feels the same way. If they can find the source perhaps we can hit it with SBRT, I recently had Provenge treatment so those two would work nicely together.
Hi EdBar, my PSA was 0.12 when the PSMA SCAN detected tumor in surgical bed. On my recent anual CT lung scan a 5mm nodule showed up in right lung, previous smoker and 71. Looks like I'll have a needle biopsy prior to SRT with WPLN. Good luck all.
Screening for Pluvicto
Lu-177 in Austria - Report and underlining high importance of both FDG and PSMA PET/CTs 
Suggestions after Lutetium + Olaparib not working?
Psa low with PSMA uptake in humerus
What is a "normal" max SUV on PSMA-Pet scan?
