No test is 100% accurate. Elevated CA125 could indicate cancer or it could indicate a number of other benign conditions. The same is true for CT scans. They can detect abnormalities that may be cancer but could also be a lot of other things. Plus frequent CT scan can actually cause cancer or other problems from the high radiation exposure.
So it seems reasonable to monitor your CA125 levels, and should they increase, then follow up with additional diagnostic tools.