Having cancer can make one very anxious, I worry about every change in my body. Did the oncologist suggest the six monthly blood test?
I am still undergoing active treatment, however when I asked the oncologist what follow up would be needed after active treatment she did not mention any blood tests.
I am no expert but my understanding is that cysts are fluid sacs, not cancer. Hopefully the reading is due to something insignificant, it is good that you are having the endoscopy/colonoscopy.
Good luck with everything.