CA 19 -9 levels

My sister who is 59 was told in Aug 2009 that she had stage 4 Pancreatic Cancer. Her CA19-9 level was 2,900. She stated chemo in Oct and the level dropped after 4 chemo to 1,000. These were high doses of chemo which caused her platelets to drop to 17 which caused her to stop chemo for about a month until they could get her blood levels up.
The doctor has dropped the amount of chemo and she only gets it once a week for 2 weeks and then she is off for 2 weeks. Now her CA-19-9 level is back up to 2,300. She had a CAT scan in late Dec which show no difference from her scan in Aug when they found the cancer. Just to let you know NO test CAT – or PET shows ANY cancer in the Pancreases there are 3 spots on her spine that is all they have found since Aug. The reason she was diagnostic with stage 4 PanCan was from a spot on her T12 which was causing her pain problems. Now for my question “If there is no change in her CAT scan why are the CA 19 -9 levels rising?