@gamaryanne - Good advice that CA19-9 is not the only determinant of disease. However, in my case it's been the only determinant for twice cancelled Whipple surgery. Imagining results were good.
Regarding switch from Folfirinox to Gem/Abrax. During my 4th cycle of Folfirinox I had a severe reaction to Oxaliplatin. I was switched to Gem/Abrax for a few cycles, followed by three weeks of radiation. About six weeks later my Whipple was cancelled due to a sharp rise in CA19-9 value. A month later, with similar high CA19-9 levels and stable imaging results, I opted to postpone surgery again and return to chemo for two months before a third try for Whipple surgery. My current chemo treatment is 4 cycles of my original Folfirinox minus Oxaliplatin, I speculate that six months ago, had I continued with the modified Folfirinox rather switching to Gem/Abrax, my CA19.9 levels would have continued to fall and my Whipple would have happened in January as originally scheduled. However, teasing out cause and effect in our individual treatments is tough if not impossible to do.
Regarding chemo side effects. For me, side effects from Folfirinox were significant and sometimes severe, but side effects from Gem/Abrax were minor.
Last thought for what it's worth. @markymarkfl has suggested more than once to several people that moving near a cancer center of excellence for the duration of one's treatment is very important. In retrospect, if I had done so I'd probably be in a better situation than I am now.... though I'm not yet in a bad one.
@amchurch - Best wishes to you and your brother.