Breast screening - benefits, costs of screening
National Screening Unit
Tuesday, 15 April 2008
Breast screening epitomises benefits and costs of screening
Research around the world indicates that women who undergo regular breast screening by mammography reduce their chance of dying from breast cancer by about a third or more.
Speaking to the National Screening Unit Symposium today, Professor David Roder of the Cancer Council South Australia described the reduction as a very positive finding, but told the delegates the downside is that some women are investigated for ‘lesions’ detected by screening that turn out not to be cancer.
Others, he said, may be treated for small cancers that would not have been life threatening. Not providing treatment for these lesions is not an option because their future behaviour cannot be predicted and not treating them would be unsafe.
Professor Roder said deaths from breast cancer in Australia and many other Western populations had dropped by about 25 percent since 1990.
“There has been debate for some time about whether this decrease is due to better treatment or to screening offering an early diagnosis,” he told the delegates in Wellington. “Research seems to indicate that there is about a 50-50 contribution.”
David Roder led research looking at the risk of dying of breast cancer among South Australian women who had gone through regular breast screening. “We found there was about a 35 percent drop in deaths from breast cancer in the screened group. This finding is reflected in research elsewhere in the world.”
While such a decrease justifies the introduction of universal breast screening, Professor Roder said such success comes at a cost.
“Apart from the financial cost of providing mammograms, each woman would need to have about ten screens during the 50-69 year age range, one every two years, which could be a source of anxiety. In addition, screening mammography may pick up ‘false positives’ – leading to follow-up investigations of cancer-like images that turn out not to be cancer.
“Based on South Australian data, it is expected that around 20 percent of women undergoing screening would be called back during their 20 years of screening to check something picked up during a screen. Of that 20 percent, about 25 percent would ultimately be found to have a cancer. About 20 percent of these cancers would be what is called ‘in situ’ cancers that are yet to invade underlying tissue.”
Such lesions frequently invade, but not always. Some may in fact never grow, or grow so slowly, they would not have been a problem. It has been estimated that between 7 and 8 percent of cancers picked up in screened women may not have progressed, although there is considerable uncertainty,” Professor Roder said.
“So while it’s clear regular universal breast screening has a very positive outcome on death rates for women generally, it also means some women may go through anxiety, and possibly treatment for lesions that may not have progressed.”
There is intense debate about this, with some pathologists and epidemiologists saying these downsides are trivial; others claiming the percentage of women treated for lesions that may not have progressed could be quite high. Professor Roder said research into the behaviour of in situ and other small cancers is very difficult. “What woman with a suspected cancer is going to bide her time, for research purposes, without treatment, possibly for several years, to see ‘what happens’?”
Despite the difficulties, Professor Roder said British and German researchers were investigating means of identifying potentially non-progressive cancers. The purpose, he said, was to better identify if there is a sub-group of cancers which may be safely left.