What's a Met and what's not?

In January I had a GA68 PSMA scan to identify mets after my recurring PSA rose from <.01 to .7 in a fairly short time.

The scan did not identify any distant mets, but did state "increased tracer uptake (SUV max 3.5) is seen in the prostate bed, more extensive on the right". My RO showed me the image and it looked a bit horrifying with very bright highlights in the prostate bed.

Now my question is "is this a met?". According to the Cancer.net site the definition is "Metastasis is the medical term for cancer that spreads to a different part of the body from where it started". Now I accept that the prostate bed is right where the prostate was, but it isn't the prostate (which has gone now) where the cancer started.

And more importantly, should it be treated like a met? According to the Chaarted and Stampede trials it is appropriate to add early chemo to ADT for recurrence especially when there are mets. Is that the case for me? Can't say I am keen to undergo chemo at this time, but if it is really effective I'll do it. A discussion to have with the Onco but would appreciate any opinions in the meantime.

Thanks, Chas

PS I am also intrigued as to why I should have recurrence in the prostate bed after 70 Gys RT within 6 months of the RP.

I think the biological line that must be crossed to qualify as a metastasis is that the cells can survive outside of the prostatic environment. Sometimes, tumors that bulge out from the prostate capsule bring the prostatic environment with them (stage T3). Sometimes, tissue is left behind (positive margin) or they shed true mets that seed and grow in tissues of local organs (Stage T4) or local LNs (stage N1). Sometimes, mets have been found in the prostates of men who had them removed after distant metastases have been found - indicating that they went out and came back. The PET scan only shows the cancer that is big enough to detect, but most metastases are much too small to detect. You can talk to your RO about whether it is safe to give some additional SBRT or brachytherapy to just that spot - maybe that can delay progression, maybe not.

There are some data suggesting that salvage radiation doses may not be high enough:
/pcnrv.blogspot.com/2016/11/dose-escalation-for-salvage-radiation.html

CHAARTED showed there was no benefit to chemo unless there was a "high volume" of metastases. They defined that as the presence of visceral metastases or four or more bone lesions with at least one beyond the vertebral bodies and pelvis.