Surviving cancer in childhood can lead to health problems later in life, according to a study.
Radiotherapy can damage healthy tissues
Researchers found nearly half go on to develop at least one fairly significant health problem linked to their cancer, or its treatment.
A team from UT Southwestern Medical Center at Dallas examined the medical records of nearly 10,000 childhood cancer survivors.
Their work is published in the Journal of the American Medical Association.
The researchers also found that girls were 40% more likely than boys to suffer long-term problems.
Long-term effects of therapy can include secondary cancers, heart and lung disease, infertility, obesity, hypertension and disruption of the hormone system.
Almost 20% of survivors showed signs of psychological distress.
The researchers hope their work will lead to improved monitoring of cancer survivors as they grow older.
Survival rates for childhood cancer are now approaching 80% - meaning that many more people go on to adulthood.
Researcher Professor Kevin Oeffinger said: "Virtually all organ systems can be affected by radiation, chemotherapy or surgery, leading to a wide array of potential effects later in life.
"This study tells us that children who survive cancer need to be periodically evaluated for the rest of their lives.
"We hope physicians become more aware of the risks and are cognizant of the problems survivors face."
The study focused on cancer survivors who were treated between 1970 and 1986 and were 21 years of age or younger at the time of their diagnosis.
Professor George Buchanan, who also worked on the study, said many of the long-term effects from modern cancer therapy are subtler than side effects seen years ago.
"It used to be in the old days that physicians were simply gratified to see their young cancer patients survive, but as the patients started living longer we began to see side effects from the new therapies that were helping them live.
"And we've only had modern cancer therapy for the last 30 years, meaning we're still learning about these side effects every day."
Dr Elaine Vickers, of Cancer Research UK, told BBC News Online: "Survival rates for childhood cancer have risen rapidly in the last 30-40 years as specific treatments have been developed for children with the disease.
"It is estimated that there are at least 25,000 people alive in Britain today who had cancer as a child.
"It is essential to understand the long-term effects of being treated for cancer, in order to inform the care of children with cancer and ensure that they receive appropriate physical and psychological support.
"The results of this study are important and highlight the unfortunate 'downside' to the vast improvements made in childhood cancer survival. It will be interesting to see how Cancer Research UK's own ongoing studies compare."
The charity Challenging Cancer and Leukaemia in Childhood is also funding resaerch into the area.
Services director Susan George said: "The long-term health of childhood cancer survivors is an issue that is becoming increasingly apparent."
A separate study, released at the European Cancer Conference in Copenhagen on Wednesday, found that only 18% of children who survive childhood cancer have no long-term health problems as adults.
Researchers from the University of Birmingham studied over 5,000 children who were diagnosed with cancer between 1960 and 1999.
Lead researcher Professor Jill Mann said: "Survival from childhood cancers has improved dramatically, but the survivors may have complications from the disease or the treatment."