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Why would PSMA-PET scan fail to detect known primary GG2 tumor?

Posted by mulberry @mulberry, Jan 29 11:55pm

Background: I had a recent TRUS biopsy resulting in 4 out of 12 cores positive, one 7(3+4) and 3 were 6(3+3) for a GG2 Favorable Intermediate Risk. Doc ordered GPS genetic test on the biopsy tissue which scored 47 (meaning a 58% likelihood of adverse pathology which means if I had RP, the Gleason Grade would be raised >GG2. The doc said he would not recommend Active Surveillance after seeing the GPS score so I should begin learning about treatment options.

To determine extent of disease spread, last week, I had a PSM-PET scan (F-18 Pylarify isotope).

Incredibly, the results state "No focal radiotracer uptake within Prostate"! Apparently the scan failed to "light up" at the exact location where there is confirmed GG2 cancer so I ask how could it detect small, distant cancer cells or show extracapsular extension?

I immediately messaged my busy doctor to ask if I was missing something here. After a week I (nervously) await a reply.

Has anyone else run into this or can offer some explanation?

[The results did report two locations with radiotracer uptake (radioaccumulation). One was stated as "normal radiotracer accumulation of the spleen, pancreas and the adrenal glands" and the other was "mild diffuse radiotracer accumulation within ... apex of the right hemithorax... unchanged from previous CT scan in June 2016". Neither of these appear worrisome but showed that there were radioisotopes present in my system.]

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jsh327 | @jsh327 | Jan 30 7:56am

PSMA PET scans are not 100% accurate. Sensitivity is dependent upon many factors, including tumor size. Many studies including the following: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8832487/. Your doctor should be able to explain why this is possible. It is one of the reasons that prostate cancer often does not show up with low PSA values (< 0.4) after the removal of a prostate.

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mulberry | @mulberry | Feb 5 5:17pm
In reply to @jsh327 "PSMA PET scans are not 100% accurate. Sensitivity is dependent upon many factors, including tumor size...." + (show)
@jsh327

PSMA PET scans are not 100% accurate. Sensitivity is dependent upon many factors, including tumor size. Many studies including the following: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8832487/. Your doctor should be able to explain why this is possible. It is one of the reasons that prostate cancer often does not show up with low PSA values (< 0.4) after the removal of a prostate.

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Sorry for taking so long to get back to you, jsh327. Thank you very much for your reply and link.

As you may have sensed, I was quite worried about my scan when I posted.

Since my posting, I have heard back from my Urologist. I also had a visit with my Radiation Oncologist, he showed me the scan images and how the bladder lights up in a way that can obscure small areas in the prostate. He also checked with their Nuclear medicine/radiology specialist. Thankfully they have assured me that there is no evidence of metastatic disease.

One thing I learned is the concept of micro and macro sites and, if I understood correctly, the cells which can be seen under the pathologist's microscope may not be sufficient to light up on a PSMA-PET scan. So, one concern I had was that I might have one of the more aggressive cancers that don't show up in PSA or PSMA. But the doctors explained that the biopsy tissue would show that "highly differentiated" tissue.

Thanks again!

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