WASHINGTON - Once breast cancer returns to spread beyond the breast, it's no longer curable - but it can be held in check, sometimes for years, depending on how aggressive the tumor proves to be.

Elizabeth Edwards, wife of Democratic Presidential hopeful John Edwards, speaks about her recurrence of cancer during a news conference in Chapel Hill, N.C., Thursday, March 22, 2007. [AP]

Doctors don't yet know exactly how widespread Elizabeth Edwards' returned cancer is, beyond a small but definite spot found in her right rib after she coincidentally broke a rib on the other side. Also being checked out is a suspicious spot in her lung.

"We can suppress it, we can even shrink it, but we can't eradicate it," said Dr. Lisa Carey, Edwards' oncologist.

But the key question when breast cancer turns metastatic - the doctor's ominous term for spreading cancer - is whether it is hormone-sensitive.

No matter where breast cancer turns up in the body, if its growth is fueled by estrogen, women have a better chance of longer survival - and a shot at controlling the cancer with a battery of anti-estrogen drugs instead of needing harsher chemotherapy, at least right away.

Carey didn't reveal whether Edwards' cancer is this better "estrogen receptor-positive" form, saying that she was awaiting further test results before deciding on a treatment.

"It's hopefully and very likely treatable," said Dr. Julia Smith, head of the New York University Cancer Institutes' breast cancer prevention program. But, "it will limit her life. ... The goal now is to give the least amount of therapy and least toxic therapy, and drag it out for as long as possible, so she can have the best quality of life as long as possible."

Almost 213,000 US women will be diagnosed with breast cancer this year, and almost 41,000 will die, according to the American Cancer Society. The vast majority survive with a combination of surgery, chemotherapy and radiation, just as Edwards had in November 2004 - if the disease is caught early.

But about 20 percent of women thought to have been treated successfully at first nonetheless see their cancer return. How soon that happens, where it happens and how soon it's caught helps determines their prognosis.

There are few good estimates of survival time for these patients. But treatment is easiest if the recurrence is limited to the breast area. Survival is much shorter if the cancer spreads aggressively to vital organs like the liver, lungs or brain.

The bone seems to be somewhere in the middle. Indeed, a subset of patients with estrogen receptor-positive tumors that appear in only the bone have a good chance at surviving for 10 years, said NYU's Smith. "Unfortunately, we don't know who that group is in advance," she said.

Hormone status isn't the only clue to how tough to treat a returning cancer will be. Another critical one is whether the tumor churns out a protein called Her2, a sign of particularly aggressive breast cancer, and thus will need Her2-targeting drugs.

"When dealing with advanced breast cancer, you're not dealing with a disease. You're dealing with a wide spectrum of tumor biologies that differ from patient to patient," explained Dr. Kathy Albain, a breast and lung cancer specialist at Loyola University in Chicago. "What they're waiting for now is to learn more about this tumor's biology, to see what messages it's giving, to best target the treatment."

Primary treatment choices aside, most patients with cancer that has spread to the bones also require other drugs called bisphosphonates to stop a vicious cycle: The cancer secretes substances that chew up bone, prompting the bone to try to grow in ways that in turn stimulates the cancer, Albain explained. Bone-protective bisphosphonates interrupt that process.

Edwards' oncologist offered no survival predictions.

"I don't have a crystal ball about how she is going to do. I can tell you that many patients with exactly the circumstances that she has, do very well for a number of years," Carey said. "And the fact that she is a healthy person, and that there isn't a lot of the cancer, and that she doesn't have symptoms, all work in her favor."

Edwards' recurrence happened fairly early, just over two years after her initial diagnosis. But she said she blessed the broken rib that warranted the X-ray this week that in turn uncovered the cancer lurking nearby before it was too widespread.

"I'm very lucky I cracked this rib," she said.