The rate of childhood cancer has slowly increased over the last three decades, research has found.
Child cancer is still rare
The International Agency for Research on Cancer, based in France, examined data from 19 European countries.
It found cancer rates increased by around 1% a year for children, and 1.5% a year for adolescents between the 1970s and 1990s.
The research, which stresses cancer before age 20 is still rare, is published in The Lancet.
The scientists say no single factor can be held responsible for the rise, and the underlying causes are likely to be highly complex.
But they suggest lack of exposure to infections and an increase in average birth weight may play a role, as may mixing of different populations.
Some - but not all - of the rise might be explained by better diagnosis of the disease, and better record keeping.
They analysed 113,000 cancers in children, and over 18,000 cancers in adolescents during the 1970s, 1980s and 1990s.
They found the incidence rate by the 1990s was 140 per million for children and 193 per million for adolescents.
The increases were recorded for virtually all tumour types in children.
In adolescents the major changes were seen for:
- Carcinomas that develop in tissues covering or lining organs of the body, such as the skin, the uterus, the lung, or the breast.
- Lymphomas that develop in the lymphatic system, such as Hodgkins disease.
- Soft tissue sarcomas that begin in the muscle, fat, fibrous tissue, blood vessels, or other supporting tissue of the body.
- Germ-cell cancers that develop in the testicles or ovaries.
- Tumours of the central nervous system.
Over the three decades studied survival rates increased substantially, reaching a five-year survival rate of 75% for children in western Europe, and 64% in eastern Europe.
Survival rates for adolescent patients were similar.
Lead researcher Dr Eva Steliarova-Foucher said: "Our results are clear evidence of an increase of cancer incidence in childhood and adolescence during the past decades, and of an acceleration of this trend."
She added: "We can only speculate about the possible factors of this increase, based on results of other research studies.
"But several studies have shown an association of the increased risk of leukaemia with high birth-weight.
"Results concerning the age at bearing children are not consistent among studies, but some have shown the risk increase with young and advanced maternal age and advanced paternal age. Reducing crowded housing postpones early infections, and was shown to be associated with increased leukaemia risk. On the contrary, the attendance to day care apparently reduces this risk. These factors are also indicators of development of socio-economic status, which supports the finding of increasing incidence of leukaemia with increasing socio-economic status.
Vaccinations can have double effect on the incidence. Studies of vaccination against tuberculosis (BCG) have shown reduced risk of childhood leukaemia, but also an excess of some lymphomas, occurring later in life.
Professor John Toy, medical director at the charity Cancer Research UK, said: "Childhood cancer is uncommon but when it does occur it is devastating for all those involved.
"While it is good news that survival is dramatically improving the increase in incidence rates reported in this study are a cause for concern.
"The study shines light on some possible causes for the increase, such as population mixing and socioeconomic status.
"It is essential that researchers investigate these leads in order to develop future prevention strategies.
"It is likely that a complex combination of environmental and genetic factors are involved."
Also writing in The Lancet, Dr Catherine Cole, of the Princess Margaret Hospital for Children in Perth, Australia, said most children with cancer lived in developing countries.
And while 80% of children with cancer survived in the west, most in developing countries die from lack of medical care.
"The challenge now is to ensure equity of access to cancer care for all children.
"Centres of excellence should be developed in low-income countries to train staff."