Many breast cancers do not need treatment
Reported November 12, 2009
HUNDREDS of women every year are having treatments for breast cancer, including surgery and chemotherapy, that are unnecessary because nearly one-third of cancers detected by screening tests are not dangerous.
Australian researchers have proven for the first time that breast cancer screening is responsible for substantial “overdiagnosis” of the disease — the detection of cancers that are too slow-growing to cause a problem in the woman’s lifetime but are still treated as if they pose an immediate threat.
The University of Sydney experts found the overdiagnosis rate was between 30 and 42 per cent — meaning between 23 and 29 per cent of women aged 50-69 are having cancer treatments they do not need.
While previous studies have estimated overdiagnosis rates as high as 52 per cent from breast cancer screening, the new study is the first to rule out other known causes of breast cancer as being an alternative explanation for the higher detection rate, which has doubled since screening began.
Obesity, use of hormone replacement therapy and childlessness all increase the risk of breast cancer, and have been rising over the same period.
But the researchers, who included world-renowned cancer expert Bruce Armstrong, have shown these other factors cannot explain the higher cancer rate among screened women, which they said could “only be explained by overdiagnosis”.
Figures published by the Australian Institute of Health and Welfare last month show that 6398 women aged 50-69 were diagnosed with breast cancer in 2006. If the higher 29 per cent overdiagnosis figure were correct, it would suggest about 1855 women each year are receiving unnecessary treatments, including mastectomies and radiation.
Co-author Alex Barratt, associate professor of epidemiology at the University of Sydney, said the study “conclusively shows that . . . it’s a phenomenon caused by screening”.
“We need better screening tests that either don’t pick up these very slow-growing cancers in the first place, or triage tests that are able to tell the difference once they are detected.”
Australia’s breast screening program has been under increased pressure from the numbers of younger women seeking scans following the cases of celebrities such as Kylie Minogue.
In September, an evaluation report of the BreastScreen Australia program proposed restricting access to free screening for younger women.
Some cancer experts have reacted cautiously to the findings, warning that the relevance for individual women was limited because doctors could not tell which cancers detected by mammography could be safely ignored.
Cancer Council Australia chief executive Ian Olver said the study’s results were “a guesstimate” and screening had slashed the death rate from breast cancer by about 35 per cent among women aged 50-69 since mammograms for healthy women were introduced in 1990.
Associate Professor Barratt said any woman who detected a symptom such as a lump, breast changes or discharge should seek medical advice straightaway, as the non-dangerous cancers did not produce these signs.