By Caroline Ryan
BBC News Online health staff in Copenhagen
A common infection could cause brain and lymphatic cancers in children, researchers have suggested.
Some cancers are common in children
They say that is the most likely explanation for the clustering of cases that they have seen.
The finding means doctors will be able to now investigate common origins of the different cancers in children.
Experts from Cancer Research UK looked at the incidence of childhood cancer cases across the north west of England over the last 50 years.
They told the European Cancer Conference in Copenhagen that the pattern of cases they saw could not be explained by chance.
Distribution of cases
Researchers looked at around 5,000 cases of all childhood cancers that had occurred in the north west from 1954 and 2001.
They found cases of Hodgkin's Disease, a cancer of the lymphatic system, and a kind of brain tumour called astrocytoma occurred in the same areas and time periods much more frequently than would have been expected.
Clusters were defined as covering children born within a year of each other and living within 5 km of each other when they were born.
The same kind of clustering was seen with Acute Lymphoblastic Leukaemia (ALL) and astrocytomas and with ALL and Non-Hodgkin's Lymphoma.
The figures were adjusted to take account of the differences between built-up and rural areas.
Although most of the clusters were small, typically around three or four cases, the researchers from the Cancer Research UK Paediatric and Familial Cancer Research Group at the Royal Manchester Children's Hospital said the frequency of the clusters was unusual.
The team had previously found separate clusters of childhood brain cancers and ALL.
'Big step forward'
Dr Richard McNally, who led the research, told BBC News Online: "We found something that's not random, that isn't likely to be a chance occurrence.
"It's the first time we've found these clusters so the big step forward is that it points to a common factor between these cancers.
"We would infer that it's to do with something sporadic, some sort of occasional environmental exposure.
"The obvious cause would be infections, which come and go in waves.
"It could be that these cancers result as a rare consequence of exposure to certain infections."