SCIENTISTS have established beyond doubt that cancer can be transmitted from mother to baby in the womb, but Australian women have been told the incidence is ''extraordinarily rare'' and no cause for alarm.

The discovery was made by scientists in Britain when a baby developed cancer a few months after her mother died from leukaemia.

The girl, born to a Japanese woman, 28, developed lymphoma, but a team at the Institute of Cancer Research, a college of the University of London, found that leukaemia cells had crossed the placenta and spread from the mother to her unborn baby, defying accepted theories of biology.

The scientific adviser to Cancer Council Australia, Nik Zeps, said yesterday that there had been anecdotal evidence for more than 100 years that cancer could spread from mother to baby, but this case gave scientists the first hard evidence that the phenomenon existed.

''We need to stress that it is very rare, extraordinarily rare, for a pregnant woman to contract cancer and additionally rare for that to ever be passed on to an infant, but the most interesting part of this case for researchers is how the cells escaped the baby's immune system,'' Dr Zeps said.

Publishing their findings in the Proceedings of the National Academy of Sciences, researchers said about 17 cases of suspected mother-to-child transmission had been noted throughout the world - usually leukaemia or melanoma - although Dr Zeps said he was unaware of any cases in Australia.

If the cells crossed the placental barrier the baby's immune system should have recognised them as foreign invaders and destroyed them.

In the latest case no one knew the mother had cancer during her pregnancy. She had a normal delivery in hospital, giving birth to an apparently healthy baby girl. But a month later the mother was diagnosed with advanced leukaemia and died.

When the baby was 11 months old she was taken to hospital with a swollen right cheek. Tests showed she had a tumour in her jaw and the cancer had spread to her lungs.

Researchers then found the cancer cells of mother and baby carried the identical mutated cancer gene, called BCR-ABL1, but the infant had not inherited this gene. This meant that the cells must have come from the mother.

Scientists then looked for evidence of some form of immunological acceptance or tolerance of the foreign cells by the foetus.

They examined the genes of the cancer cells in the infant and found a deletion mutation - some DNA missing in the region that controls expression of the major histo-compatibility locus. This was significant because HLA molecules primarily distinguish one individual, and his or her cells, from another, so the absence of these on the cancer cells meant the infant's immune system would not have recognised that they were foreign.

''Hopefully, this case will now open up new avenues of research into how targeting immune systems can prevent or treat cancer,'' Dr Zeps said.

with Guardian News & Media