Doctors who are torn over how aggressively to treat a cancer patient, not knowing whether a tumor has fully regressed or is coming back, might someday be able to find out just by testing the patient's blood. In a study forthcoming his week in Science Translational Medicine, John Hopkins researchers say they have tested a way to spot the "fingerprint" of cancer--the changes to the DNA inside cells that make up cancerous tumors. Jeffery Schloss of the National Human Genome Research Institute, who wasn't involved in the study,