Ga-68 PSMA-PET scan result vs Biopsy

[timotur]

Just received results of a Ga-68 PSMA PET scan with a stage of T2u(RM) N1(PS) M0, translated meaning stage 2, organ confined, unifocal, right-medial with LN involvement in the presacral LN.

Contrasted with a CDUS-biopsy of T3b + ISV extracap extension, GL 7 (3+4), 55% both lobes.

With the PSMA scan, the presacral LN was barely detectable at SUVmax of 3.3, which is right at the cutoff, so it may or not be a false positive. (SUVmax, statistical uptake value-- a statistical measure to determine significance based on the number of pixels and shape of the image).

What's interesting to me is that the PSMA scan did not pick up the CDUS biopsy-indicated SV involvement, and only indicated only a unifocal tumor versus a bi-lobe tumor.

Later this week, a mpMRI will be done, maybe shed more light on this.

[Tall_Allen]

It's not established when PSMA begins to appear in prostate cancer cells. Its use in staging is also far from established. The biopsy is more definitive than any imaging. I think you have to treat as high risk with N1. Why are you continuing to have a scan rather than treatment?

[timotur in reply to Tall_Allen]

It's all part of staging at UCLA Brachy department. Lots of lag time getting all the prerequisite tests and appointments with Onc (Chang), MO (Scholz), MRI, and PET. Things should speed up after the mpMRI done this week.

I'm reading that PSMA scan is established and very useful tool in staging combined with mpMRI, but no comparisons I've seen with biopsy results, in other words, has anyone see a divergence in pre-tx staging between a guided-biopsy and a PSMA PET scan?

translational-medicine.biom...

researchgate.net/publicatio...

ncbi.nlm.nih.gov/pmc/articl...

urotoday.com/recent-abstrac...

[Tall_Allen in reply to timotur]

i would never characterize PSMA as "established" or "very useful" for staging of T3 cancer. Quite the opposite. Mpmri seems to be good for staging T3b but not T3a. The question is - what is the point of all these scans anyway? It sounds like you will be getting whole pelvic external beam with a brachy boost to the prostate anyway. I'm sure your doctors are grateful for increasing their billing.

[timotur in reply to Tall_Allen]

Yes, UCLA tx plan is for BB + EBRT + 18mosHT. As you know, a T3b biopsy doesn't say anything about bone mets, which is the point of the PSMA scan. If there are bone mets in pre-tx, I assume it changes the MO's choice on HT tx plan (length, dose, drug), but not change the planned BB + EBRT. I asked Dr. Chang if there is a cutoff on number of bone mets to go/no-go with BB + EBRT, he said no. (I had read somewhere the general cutoff is three bone mets to go directly to systemic tx and forego gland tx.) I assume mpMRI will confirm the CDUS-biopsy id of the +SV, but my question is, why didn't PSMA pick up the SV GL-7 piece or the two-lobe involvement, and whether anyone has seen this before in tx-planning stage? At PSA 29, a PSMA scan should be sensitive enough to pick up everything the biopsy did if everything I read aligns correctly.

[Tall_Allen in reply to timotur]

You are right that the cut-off is 3 bone mets. Possibly Dr Chang has not yet seen the newest STAMPEDE trial. Maybe send him this link:

thelancet.com/action/showPd...

But it's moot, since no bone mets were detected.

PSMA scans are probably very good at picking up metastases, but the point I was trying to make was that no one knows how good they are at picking up cancer in the pre-metastatic stage. It should only be used to detect distant mets (N1 or M1) but not to find cancer in and around the capsule. UCLA, NIH and a few others are trying to find out more about its ability to detect cancer in the high risk prostate.

[Blackpatch in reply to timotur]

Hello Timotur

I was pT3b with SVI on both sides and 2.5mm ECE, but the pre-surgery PSMA didn’t pick up either the ECE or SCVI - admittedly, my PSA was only around 15 cf your 29, but I guess the point is that PSMA PET isn’t all that great for fine detail close to the prostate. It may well be that the presence of so much PSMA in the prostate reduces the effective dynamic range in the proximal area, but I’m just guessing.

Stuart

[timotur in reply to Blackpatch]

Stuart, ok, thanks for responding. I agree with your thought that the intensity of the PSMA at the gland reduces the dynamic range around the prostate. As a former comm engineer, it’s similar to the dynamic range of a receiver that is reduced in the presence of strong adjacent signals and weak signal detection is compromised. The range of PSMA expression was SUVmax of 21 at the gland, and 3.4 at the mesorectal lymph node, quite a difference.

[tennis4life]

Can you tell me what hospital did your PSMA PET?

[timotur in reply to tennis4life]

Yes, it was at UCLA, costing about $2450 out of pocket. The CT portion of the exam may be reimbursable, checking on that.