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根据材料请回答Screen Test Every year millions of women are screened with X-rays to pick up signs of breast canc-er.If this happens early enough, the disease can often be treated successfully.According to a survey published last year, 21 countries have screening programmes.Nine of them, including Australia, Canada, the USA and Spain, screen women under 50. But the medical benefit of screening these younger women are controversial, partly because the tradition brings a small risk of inducing cancer.Also, younger women must be given higher doses of X-rays because their breast tissue is denser. Researchers at the Polytechnic University(理工大学)of Valencia analyzed the, effect of screening more than 160,000 women at 11 local clinics.After estimating the women's cu-mulative dose of radiation, they used two models to calculate the number of extra cancers this would cause. The mathematical model recommended by Britain' s National Radiological Protection Board (NRPB) predicted that the screening programme would cause 36 cancers per 100,000 women, 18 of them fatal.The model preferred by the UN Scientific Committee on the Effects of Atomic Radiation led to a lower figure of 20 cancers. The researchers argue that the level of radiation-induced cancers is "not very signifi- cant" compared to the far greater number of cancers that are discovered and treated.The Valencia programme, they say, detects between 300 and 450 cases of breast cancer in every 100,000 women screened. But they point out that the risk of women contracting cancer from radiation could be reduced by between 40 and 80 percent if screening began at 50 instead of 45, because they would be exposed to less radiation.The results of their study, they suggest, could help"optimize the technique" for breast cancer screening. "There is a trade-off between the diagnostic benefits of breast screening and its risks, "admits Michael Clark of the NRPB.But he warns that the study should be interpreted with caution."On the basis of the current data, for every 10 cancers successfully detected and prevented there is a risk of causing one later in life.That's why radiation exposure should be minimized in any screening programme."
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