Monica Long had expected a routine appointment. But here she was sitting in her new oncologist’s office, and he was delivering deeply disturbing news. Nearly a year earlier, in 2007, a pathologist at a small hospital in Cheboygan, MI, had found the earliest stage of breast cancer from a biopsy. Extensive surgery followed, leaving Ms. Long’s right breast missing a golf-ball-size chunk. Now she was being told the pathologist had made a mistake. Her new doctor was certain she never had the disease, called ductal carcinoma in situ, or D.C.I.S. It had all been unnecessary — the surgery, the radiation, the drugs and, worst of all, the fear.