When chemo is first started, the tumor burden is large. That means there is malignancy with a lot of surface area for chemotherapy and radiation to have contact with. And that equates with a lot of available CA19-9 being released into the circulation. A rise is not uncommon for several sessions and is a frequent observation. As the tumor shrinks, the surface area is reduced as there are less cells available to release CA19-9.

ThemCA19-9 is used between scans to get a general picture of what is going on. Early measurements don’t necessarily reflect whether the chemo is effective.And with radiation, it’s effects linger for weeks afterward in causing cell death. This is why oncologists usually don’t order CA19-9 testing during and after radiation treatment for several weeks following.

A CT or MRI is the definitive method oncologists use to determine chemotherapy effectiveness. A current scan is compared to the prior scan by measuring the diameter of the tumor(s).They look for shrinkage, stability or increase in size and make adjustments accordingly.

CA19-9 Treatment and Effectiveness

The link below is to a scientific publication that elaborates further.
https://www.sciencedirect.com/science/article/pii/S2468294221000952