“As many of you know, five years ago, I beat breast cancer,” said a shaky Robin Roberts on Good Morning America Monday morning. “Sometimes treatment for cancer can lead to other serious medical issues and that’s what I’m facing right now.”

Clutching George Stephanopolos’s hand on the sofa next to her, Roberts announced that she has myelodysplastic syndrome (MDS), a relatively rare blood disease that Roberts herself said she’d never heard of until she was diagnosed with it. Likely even more unfamiliar for many viewers than the name of her condition was Robert’s startling remark that cancer treatment can result in other serious health problems, including different forms of cancer, several years after the initial cancer is in remission.

But in the medical world, it has been known for decades that cancer treatment carries with it the risk of causing another kind of cancer to develop. “We always think of the drug as a double-edged sword, where there is a benefit from the drug and there is a harm from the drug,” says Otis Brawley, chief medical officer of the American Cancer Society. “It’s actually one of the reasons why I’m one of the folks who’s been very outspoken about being conservative and only using chemotherapy when we absolutely need chemotherapy.”

The link between treatment with chemotherapy and the development of a second kind of cancer stems from the very nature of chemotherapy’s effects. To fight the spread of cancer, chemo targets the DNA of cancer cells. But in the process, it also impacts noncancerous cells, including stem cells, located in the bone marrow, that produce both red and white blood cells.

“If their DNA is damaged at exactly the right place, they start behaving abnormally,” says Brawley. “When I say exactly the right place: you have to have a hit inside one of these genes that either promotes growth or suppresses growth. And that’s how you can get myelodyplesia.”

MDS is a relatively rare condition that can lead to a depletion of red or white blood cells, anemia, heavy bleeding. Brawley estimates there are 10,000 cases per year in the United States; other estimates put that number closer to 20,000. Of those, according to Brawley, roughly one third are what’s known as secondary MDS—that is, MDS caused by a chemical, whether a chemotherapy drug or an environmental toxin, like benzene, found in gasoline.

MDS is by no means the only form of secondary cancer that can spring from cancer treatment. Carcinomas—the most common kind of tumors--can result from radiation treatment of prostate cancer. Certain kinds of chemotherapy, administered to patients with lymphoma and breast cancer, can produce secondary bladder cancer. Brawley himself says that of all secondary cancers, the one he seems to hear most often about is lung cancer in women who have received radiation for breast cancer.

And while secondary cancers, when they appear, often do so roughly five years after chemotherapy, the timeline is shorter when the toxin is radiation. “That’s why I suspect there’s a lot of myelodysplasia going on in Fukishima right now,” Brawley says.

Certain chemotherapy drugs may have greater risk than others, says Dr. Lewis Silverman, the director of Myelodysplastic Syndrome Center at Mount Sinai School of Medicine. But while the risk may lead doctors to adjust treatment, it doesn’t necessarily lead them to abandon treatment altogether. “As an oncologist, if you’re looking at a patient with breast cancer, and you know that without treatment, there’s a 40 percent change that the tumor can recur, and when it recurs, and it has spread, the ability to cure the disease at that time is much more limited,” says Silverman. “In the scheme of things, it’s a small consideration compared to the treatment to cure somebody when they have early-stage disease.”

In Roberts’s case, doctors reportedly will be treating her new cancer with more chemotherapy, as well as with a bone marrow transplant from the anchor’s sister later this year. Dr. Richard Besser, the chief health and medical editor at ABC News, has been advising Roberts and explaining her condition to the public. In someone as young and as otherwise healthy as Roberts is, he says the goal is nothing short of a cure.