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A British investigation into breast-screening which found a concerning rate of over-diagnosis indicates about 180 women will be treated unnecessarily each year in New Zealand, University of Otago cancer-screening authority Associate Prof Brian Cox says.
Unnecessary treatment included mastectomy, radiotherapy, and chemotherapy.
The Government should consider whether this was a good use of resources, he said.
Prof Cox extrapolated from the findings of an independent panel in the United Kingdom, which said about one in four women diagnosed with breast cancer through screening would never have had a problem had it not been identified by screening.
Some disease does not develop into clinical cancer, but screening does not distinguish between what is dangerous and what will not cause a problem.
"This over-diagnosis of sub-clinical disease that would not develop into clinical cancer in a person's natural lifetime results in investigation and treatment, and the side effects of treatment ... "
While the over-diagnosis problem with screening programmes was well known, the rate, about 19%, was higher than previous estimates.
"Resources could be better spent on other aspects of breast cancer care and prevention than screening women under 50," he said.
When calculating New Zealand's figure, he took account of the fact its screening programme admitted women aged from 45-69, whereas Britain's lower age level is 50.
New Zealand was out of step with other countries in its lower age level.
"The evidence regarding the balance of harm versus benefits was not strong enough to reduce the age of screening in the programme [from 50] to 45 years of age, as was done in July 2004."
Over-diagnosis potentially increased with lower entry ages to programmes, because more screening took place.
However, he noted uptake had been lower in the 45-49 age group in New Zealand.
"The breast-screening programme here has had some difficulty getting women 45-49 to participate." Prof Cox said about 60 of the 180 women treated after screening would be diagnosed with invasive breast cancer. The rest would be told they had ductal carcinoma in situ.
"Currently, it is not known how many deaths from breast cancer are prevented by breast-screening in New Zealand, although it has been shown that the reduction in the annual number of deaths from breast cancer from screening women under age 50 years can be expected to be very small."
New Zealand Breast Cancer Foundation medical adviser Dr Belinda Scott said the key issue was promoting medical advances to allow better identification of which women required treatment.
The breast-screening programme saved lives, even if some women did undergo unnecessary treatment, she said.