I would try not to read too much into a CA 19-9 change from 56 to 61 by itself. That's a very small fluctuation, and CA 19-9 doesn't necessarily decline in a perfectly straight line even when treatment is working.

What I would find much more encouraging is what you described on your CT: the liver metastases are no longer visible, the lung metastases are stable or shrinking, and the pancreatic mass has decreased from 6 × 5 cm to 4 × 3 cm. Those are meaningful objective signs that you've responded to treatment.

CA 19-9 can fluctuate for reasons other than tumor growth, including inflammation, changes involving the liver or bile ducts, blood sugar control, and normal biological and laboratory variation. I wouldn't expect CA 19-9 to necessarily give you a warning or gradual slowdown before reaching its lowest point either. Sometimes it plateaus, sometimes it bounces around, and sometimes it rises slightly and then falls again.

I would be more interested in the trend over several measurements than this one result. If 61 became 100, then 200, then continued climbing, that would certainly deserve attention. But 56 to 61, with your CT showing a strong response, would not make me assume the treatment has stopped working.

You might also ask your oncologist whether a circulating tumor DNA blood test such as Guardant360 or Signatera would provide any useful additional information in your particular situation. These tests don't replace CA 19-9 or imaging, but sometimes they can provide another piece of the puzzle.

After 11 infusions and the scan results you've described, there is a lot here to be encouraged about. I would wait for the next few CA 19-9 measurements and, most importantly, continue watching what the scans show.

Wishing you continued shrinkage and hoping that 61 turns out to be nothing more than a little bump in the road.