The findings add more weight to theories that successfully treated cancer is "probably the equivalent of a lifelong chronic illness. You shift from one phase to another," says study co-author Dr. Steven Lipshultz, chief of pediatric cardiology at the University of Rochester.
He adds that children who survive cancer should get regular cardiovascular checkups to make sure their hearts are not deteriorating.
According to Lipshultz, researchers have become increasingly concerned about heart disease risks as cancer in children has become less deadly thanks to chemotherapy and radiation.
Consider leukemia, the most common childhood cancer. In 1970, only about 10 percent of children survived the illness. Now, about 90 percent do, Lipshultz says.
But while cancer treatments "are terrific for killing leukemia cells in young children, they're not specific for just leukemia cells," Lipshultz says. "They can damage other cells in the body."
Those cells include those of the heart, which cannot easily regenerate itself, he explains. In some cases, the heart ends up with fewer muscle cells and must work harder.
While doctors have known about heart problems in young cancer survivors for about 20 years, the new studies examine how soon the damage begins. The studies were to be released Monday at the American Society of Clinical Oncology's annual meeting in Chicago.
In one study, researchers examined 176 young adults who had survived various types of cancer -- including leukemia, lymphoma, and Hodgkin's disease, among others -- as children. The researchers compared the subjects to 64 of their healthy brothers and sisters.
The cancer survivors, who were typically 18 to 21 years old and about 15 years from treatment, generally reported feeling healthy. Those who had undergone radiation and chemotherapy had higher heart rates -- a bad sign -- and higher cholesterol levels. All the cancer survivors were more likely than their siblings to show signs of heart disease.
As a group, the cancer survivors got less exercise than their siblings and had higher levels of body fat. Those latter two factors put them at higher risk of high blood pressure.
In the second study, researchers examined 48 adults from the Boston area who were treated for Hodgkin's disease under the age of 25. All had been treated with radiation at some point between 1970 and 1991, and four had also received chemotherapy.
Even though they felt healthy and most were in their 30s, 65 percent of the subjects showed signs of mild to moderate heart valve defects and 58 percent had problems with the electrical systems of their hearts.
"The evidence of cardiovascular disease was there, but these people hadn't noticed it, yet," Lipshultz says in a statement.
The findings will help doctors be more alert about the potential health problems facing survivors of childhood cancer, he says. "Knowing that heart problems occur and place patients at risk allows us to monitor patients who are at risk."
It's still unclear whether adults who undergo cancer treatment face the same future heart disease risks as children, he says.
However, researchers have found that adult cancer survivors do suffer from higher levels of fatigue and "difficulty trying to maintain normal activities," says Dr. Len Lichtenfeld, deputy chief medical officer of the American Society. Women who get treatment for breast cancer may be at special risk from radiation treatment because it targets an area near the heart, he says.
"We're recognizing more and more that having long-term survivorship [of cancer] comes at a price," he says.