Grail deal....BD1 is going to fly, page-34

I am applying practical thinking here and this answer is not based on any research.

Yes, cancerous cells (along with non-cancerous but also abnormal/defective cells) are being produced in the body all the time. The mechanism for producing abnormal cells is basically mutation (imperfect DNA mapping - which is both the mechanism underpinning the ability of organisms to adapt and develop more successful strains, but also the primary cause of cancer).

Cancer in the body is caused by...

1. Failure of the immune system to eliminate mutant cells that happen to grow and replicate aggressively (often a genetic weakness, or caused by an external agent - a carcinogenic substance, or caused by exposure to ionizing radiation)
2. Mutations that are beyond the ability of the immune system to detect them. The immune system isn't perfect.

Some people have a genetic disposition to certain types of cancer - but, never develop the cancer because, purely by chance, the specific required cell mutation never happens. Luck of the draw is a big factor in who develops cancer - cell mutation is highly variable.

Areas of the body that have high cell replacement rates (particularly epithelial cells like skin and the lining of the digestive tract) are particularly prone to producing mutant cells as mutation is higher simply because cells are being replicated at much higher rates. The body identifies the abnormal cells quickly and the immune system eliminates them. They don't last long, so under normal circumstances there are not many present in the body at any single point in time. Hence, I think their effect on levels of biomarkers would be well below the test sensitivity levels.