Chemotherapy may be the culprit, but treatments are changing
Survivors of childhood cancer have a higher life-long risk of developing a new form of the disease, a study shows.
The Journal of the National Cancer Institute study blames potent therapies rather than genetics, and is the first to show the risk is so long-term.
The study of 50,000 also found those diagnosed after 1975 appeared to have a slightly higher risk of cancer as treatments became more aggressive.
But they also led to a big improvement in child cancer survival rates.
A team from the Institute of Cancer Epidemiology in Copenhagen studied 47,679 people who were diagnosed with cancer before the age of 20, between 1943 and 2005. They were drawn from the cancer registries of Denmark, Finland, Iceland, Norway and Sweden.
In all, they were three times more likely to develop a new cancer than their contemporaries - and the risk remained even as people approached their seventies.
Among survivors, the generation diagnosed between 1975 to 2005 were more likely to have developed second cancers at comparable ages than either the generation treated between 1960 and 1974, which saw first-generation chemotherapy, and the period before 1960, with no chemotherapy at all.
This increase occurred despite the advances in radiation treatment in which doses were markedly reduced, leading the team to point the finger at chemotherapy - either as an independent factor or one which exacerbates the carcinogenic effects of radiation.
Brain tumours were found to affect survivors more than the general population, due to the susceptibility of the brain to cancer treatments.
"What we need now is two-fold: new treatment ideas to decrease the risk of later effects, and much better surveillance of childhood cancer survivors during adulthood," said Dr Jorgen Olsen, who led the research.
"Cancer treatments don't just increase the risk of other cancers, but can lead to all sorts of other problems - from cardiovascular to reproductive."
Dr James Nicholson, a paediatric oncologist at Addenbrooke's Hospital in Cambridge welcomed the research as one of the most comprehensive studies yet, but stressed change to treatment was already afoot.
"We have known about this for a while, and we are now in a position where we can decrease the intensity of treatment in many cases and still get the same results.
"But a study like this does raise awareness of the problem. If it means alarm bells ring earlier when there are symptoms in people who were treated for cancer as a child that would be a very good thing."
Judith Kingston, a consultant at Barts and the London, said the paper highlighted the importance of following-up survivors to learn about the long-term effects and to investigate how treatments might be modified.
"However for the children with "bad" or high risk tumours, we still need to give intensive chemotherapy and/or radiotherapy to effect a cure and this intensive therapy will come at the cost of potentially increasing that child's risk of developing a second cancer," she said.
"I am afraid it is a problem we have to recognise, be alert to and warn the parents about, but the risk needs to be put into perspective, as it affects a small minority of patients, whilst the majority of children will continue to lead healthy lives after treatment for childhood cancer."
Ed Yong, Cancer Research UK's health information manager, said: "More and more children are surviving an early fight against cancer and this study suggests that they still have a slightly higher risk of different cancers later on in life.
"Even so, when a child is diagnosed with cancer, the priority must be to save life. Thanks to research, over the past few decades we have seen tremendous improvements in the treatment of childhood cancer.
"In the 1960s, only a quarter of children who were diagnosed with cancer survived for more than five years. Now around three quarters survive.