Cancer-treatment quality has been particularly hard to track because claims information tells insurers relatively little about the patient. The data often don't make clear, for instance, if a breast-cancer patient is in remission or relapse. The differences in the disease's stage are uniquely important for cancer, since tumors change over time, necessitating different treatments.

So for the past three years, UnitedHealthcare has been collecting clinical information directly from oncologists. The company then compared the choices that a doctor made for a particular patient's treatment with claims data and guidelines developed by the National Comprehensive Cancer Network, a consortium of 21 leading treatment centers. In November, the company mailed the individual reports to 1,321 oncologists.

The company especially probed the use of high-price biotech drugs, which it says it found in some cases are being prescribed inappropriately. One NCCN guideline for colon-cancer patients advises that patients get chemotherapy after surgery, but in 31% of cases, the care did not comply with the rule. Instead, in the bulk of those cases, patients were prescribed Avastin, a Roche Holding AG biologic that is unproven in that patient group, says Dr. Newcomer. And among patients with non-small-cell lung cancer, about 24% received Avastin, even though their disease didn't meet the right criteria to get the drug, UnitedHealthcare found.