"Just having chronic organ failure can also affect your immune system to make you susceptible to certain kinds of tumors," he added.
Another question is what causes the cancers that are not virus-related, such as cancers of the kidney, skin and liver.
"We are going to have to do more work to sort out all of the possible explanations," Engels said. "There are a number of factors that could explain some of these cancers."
Paul McSorley was told about the potential for cancer before he got a kidney transplant in June. He said the doctors were very direct and asked how it made him feel.
"I wasn't crazy about it," said the 55-year-old who lives in Fallston. "But it's better than being married to a dialysis machine. And the other trade-off, of course, is a shorter life or imminent death."
McSorley, who needed a transplant after complications from diabetes and hypertension destroyed his kidneys, said his quality of life improved with the transplant. So far he is cancer-free.
Protasio became gravely ill last year with abdominal pain that made it hard for her to get out of bed. Testing revealed liver cancer, which doctors tried to remove surgically. But when they opened her up they discovered that the tumor was too big. A transplant was the only thing that could keep her alive.
While the transplant saved her from cancer, she knows she could get cancer again because of it.
But Protasio said she quizzed her doctors about taking drugs that could cause cancer. And she welcomes research that might help lower the risks.
The NCI research also found a decreased risk for some cancers, such as cervical cancer. The researchers' theory is that it may reflect increased screening, and therefore earlier treatment, for this particular cancer.
The risk of breast cancer is also lower among transplant patients than in the general population. This is also probably because of better screening, said David Klassen, chairman of the medical advisory board for the National Kidney Foundation of Maryland, who has seen the study.
The study did not look at skin cancers because they weren't included in the state databases.
There is a significant increase in risk for skin cancers among organ transplant patients, said Klassen, who is also director of the kidney and pancreas transplant program at the University of Maryland Medical Center. Patients are put on a sunblock regimen and given regular screenings after a transplant, Klassen said.
Klassen said the study helped reinforce much of what was known about transplants and cancer, but could still be helpful. "It might point research to other cancer therapies."
Said Segev: "It means we have better information to care for people post-transplant."