For cancer survivors, five years is the magic number, when doctors have long declared them "cured".

But a major new study blows that theory out of the water, finding 10 to 15, even more, is not long enough for a breast cancer survivor.

Doctors have long known that in some people, cancer returns.

Experts didn't know why, how many people were at risk, or for how long.

A new study answers some of those questions for breast cancer patients.

Carole Gust thought she would never miss her yearly mammogram, especially after being diagnosed with breast cancer 13 years ago at age 39.

"But yes I did a year ago because I moved into a house being renovated, my son was going off the college. Time goes by quickly," she said.

A new study from MD Anderson in Houston says younger patients, like Gust, should not let down their guard.

Research shows 20 percent of women treated for breast cancer before menopause, relapsed within 15 years.

The women in the study also had advanced or aggressive cancer.

"I think what this study demonstrates to us is that we have to be diligent for the rest of our lives, which is not the same as saying we have to be fearful. But we need to continue to look for small cancers or subtle evidence of recurrent disease, long after we've finished radiation therapy or chemotherapy," said Dr. Mark Fulmer, diagnostic radiologist.

Doctors say active hormones in younger women may play a role in whether cancer returns.

Carole Gust says prayers to remain disease free have been answered for 13 years.

To be sure she has many more healthy years ahead, she won't miss a check-up again.

"You have to be diligent," she said.

Doctors say this is the first study -- so there's no reason, yet, to change treatment based on age.

But some speculate if 20 percent of younger women with aggressive cancer relapse, there may be an increase in mastectomies.

E-mail jstjames@wfaa.com.