At a time when genetic testing and genetically personalized treatments for cancer are proliferating, buoyed by new resources like President Obama’s $215 million personalized medicine initiative, women with breast cancer are facing a frustrating reality: The genetic data is there, but in many cases, doctors do not know what to do with it.

That was the situation Angie Watts, 44, faced after she walked into a radiation oncologist’s office last June expecting to discuss the radiation therapy she was about to begin after a lumpectomy for breast cancer. Instead, Dr. Timothy M. Zagar of the University of North Carolina looked down at a sheet of test results and delivered some shocking news.

A genetic test showed she had inherited an alteration in a gene needed to repair DNA. Radiation breaks DNA, so the treatment might actually spur the growth of her cancer, he said. He urged her not to take the risk and to have a double mastectomy instead.

“I’m not a betting man,” he said in a recent interview.

Shaken, Ms. Watts called Dr. James P. Evans, a professor of genetics and medicine at North Carolina. He told her the opposite: The mutation she had was not known to be harmful, so he urged her to go ahead with the radiation.