text: how can a doctor know if a patient has cancer in an organ?   some cancers, such as prostate cancer, have a marker in bloodwork.  many cancers are not found from simple bloodwork, but from an elevation in certain blood count levels.  that tells the doctor's what organs are working or not working.  this doesn't mean cancer, just that a system is off.  to be sure of cancer, there must be a pet,ct,mri or biopsy of the suspected organ.