@pdacbrca2 ,

CA19-9 is a helpful indicator but can't be taken as the only indicator. Fortunately it's an inexpensive test that can be performed often to provide insight into your cancer response.

In my case, Stage-IV treatment on cisplatin + gemcitabine + abraxane brought CA19-9 down by about 95% for a while (from around 677 to 34 on one test) with only a slight reduction in tumor sizes on MRI.

My non-medically trained understanding is that the cancer cells can be driven into a "senescent" state for a while; not dying, but not reproducing or spreading either. This is considered "stable disease" and is not a bad place to be. 🙂

Of course "imaging is king" of the non-invasive testing methods, but is also only a single set of eyes that can't see everything. If you had any kind of biopsy taken, you might be able to have the tissue sent to Natera for creation of their "Signatera" ctDNA (circulating tumor DNA) test that can be done repeatedly as you undergo treatment. It provides a quantitative measure in "Mean Tumor Molecules per unit of blood" that is a more direct measure of cancer activity in your bloodstream.

Note however, that this is also only a measure of cancer cell DNA in your bloodstream, and it may not be present in detectable quantities there if your tumors are not shedding it there (possible depending on the vasculature or the senescent state).

At lower tumor burden levels, experience in my case has been that CA19-9 provides a more fine-grained check than Signatera, but this is not the case for everyone. Signatera has failed (below threshold) to detect ctDNA when CA19-9 was rising. But when CA19-9 rises enough, Signatera usually agrees (detects molecules above threshold) and provides more specific, confirmatory evidence of cancer activity because the test is tuned to look specifically for ctDNA matching that from my tumor tissue sample. It's just another set of eyes looking from a different angle.

While a CA19-9 of 1000 level is concerningly high and may rule you out from surgery (as your Stage-IV status probably already has), the drop from 45000 is awesome and suggests your current treatment is providing pretty good control. There is still room to add Abraxane to your Cis+Gem regimen if your oncologist thinks it's appropriate.

Your BRCA2-positive status also suggests you might be a candidate for (maintenance?) treatment with a PARP inhibitor (probably as a monotherapy but possibly also in conjunction with something else). This could be a valuable option if you develop neuropathy, anemia, resistance to the current regimen or any other problems with it. One piece of research to stay on top of: I've lost the links, but I read once that it _seems_ PARP inhibitors might lose their effectiveness around the same time you develop resistance to the platinum drugs, so you (and your oncologist) might want to time a switch to occur before that happens.

I wish you all the best and hope you'll share whatever else you learn back here. 🙂