Good post Enos, I have just been fixated on the diagnostic aspect but I see that it does have great value in monitoring. Also, since you would be working with a known set of cancer patients, it could be a lot easier to get approved to monitor cancer than as a general screening test
My approach would still go for approval as a blood chemistry test and let the doctors use it as they want. That way, it could be on the market while trials were run as a cancer monitoring test.