Mr. Grassley is a 65 year old retired teacher from Ithaca, NY. 10 years ago, he presented with a suspicious digital rectal examination and an elevated PSA level of 9 ng/ml. A biopsy of the prostate revealed no abnormalities. The year, he presented again with a PSA level of 12 ng/ml. Again, a biopsy was negative. Why is the prostate-specific antigen (PSA) not considered an ideal tumor cell marker? Is there a level at which cancer would be indicated?