FDG elements of PSMA scan

[CrocodileShoes]

I had a FDG scan a few days ago . At the heart of my confusion is the function of FDG in giving a positive outcome to Pluvicto infusions. Is it a good thing to have lots of 'FDG Avid' showing up, or do you want those aspects of the scan to be as hidden as possible (so that the PSMA -Avid cells can be identified and zapped? Here's the report:

FDG Scan – 23/03/2024

INDICATIONS

Metastatic prostate cancer. Being considered for possible PSMA therapy in Melbourne (Australia). Needs FDG PET to assess suitability.

COMPARISON

A prior contrast-enhanced CT performed in Leeds on 13/02/2024 and a subsequent PSMA PET-CT performed in South Tees on 07/03/2024 were reviewed.

TECHNIQUE

Half-body acquisition from skull base to upper thigh. 255 MBq of 18F-FDG injected. Blood glucose = 9.6 mmol/L - the patient is not a known diabetic. Time of flight (TOF) and Q.Clear reconstructions used for PET imaging evaluation.

SUVs derived from TOF recons.

FINDINGS

Appearances are largely unchanged compared to the recent PSA may PET-CT scan. The extensive loco-regional recurrent primary tumour within the pelvis is FDG-avid and as documented in the prior PET-CT report involves the bladder base and encases the sigmoid colon. There is disseminated nodal disease extending from the pelvis through the retroperitoneum into the thorax showing moderate FDG aviditybelow that seen on the PSMA study. The known bilobar liver metastases and widespread bony metastatic disease involving the axial and proximal appendicular skeleton also shows extensive FDG uptake. There are bilateral nephrostomies in-situ with physiological tracer excretion into the nephrostomy bags. The remainder of FDG biodistribution is unremarkable.

On review of the low-dose CT component there is more prominent bibasal atelectasis/consolidation than on the previous PET-CT, gynaecomastia, evidence of previous bowel surgery with a right iliac fossa stoma in-situ with associated wide necked para-stomal hernia, lower anterior abdominal wall incisional hernia, vascular calcification and truncal subcutaneous oedema. There are unchanged degenerative changes within the spine.

IMPRESSION

The known widespread disease recurrence is FDG-positive at the majority of sites seen on the recent PSMA PET-CT scan.

Bibasal consolidation which may reflect intercurrent infection. Elevated blood glucose which may reflect occult diabetes, a formal testing should be considered.

__________________________________________

Any theories?

[GP24]

The avid mets on an FDG PET/CT will usually also be visible on a PSMA PET/CT. But if you have a lot of mets which are avid on an FDG PET/CT but are not visible on a PSMA PET/CT they will not treat you in Melbourne (but at other places, e.g. Sydney). The reason is, the Lutetium-177 can only treat the mets which are visible on a PSMA PET/CT. It will not touch the mets which are not visible on a PSMA PET/CT. If you just have a few of these "invisible" mets, you could treat them with SBRT after the Lutetium-177 therapy. Dr. Emmett will do that for you.

[MateoBeach]

What is important for predicting some benefit from Pluvicto is if the FDG avid lesions that you have are in the same distribution, the same sites, that are PSMA avid. This is called concordance. If this is the case then treating the PSMA avid cancer with Pluvicto will also possibly kill the FDG avid cancer in the same vicinity. If they do not line up it is called discordant and Pluvicto and similar PSMA targeting would be futile. Both scans need to be reviewed carefully by the planned treating physician.

[CrocodileShoes in reply to MateoBeach]

Well, comparing the PSMA report with the FDG report, there seemed to be a lot of 'concordance', so that sounds hopeful.

Thanks for your intervention, Paul, I know that you know these teams better than anyone else on here...

[lcfcpolo in reply to CrocodileShoes]

Hi Crocodile. Where did you go for the FDG PET scan please. Good luck.

[CrocodileShoes in reply to lcfcpolo]

I had it done at Leeds . I understand that they are quite widespread however.

[Tall_Allen]

It is called "discordant." It means you are not a good candidate for Pluvicto. It could make things worse instead of better. Here's why:

prostatecancer.news/2019/12...

[kainasar]

psma - neg / fdg - pos /= pluvicto

Nomogram to predict the presence of PSMA-negative but FDG-positive lesion in castration-resistant prostate cancer: a multicenter cohort study - PubMed

pubmed.ncbi.nlm.nih.gov/381...