US experts aiming to promote better-informed decision making in healthcare have questioned the benefits of mammography for breast cancer screening, saying that to save one life, nearly 3,000 women have to be tested. Women are often told that mammography saves lives, but the question “how often?” is rarely asked, they said.
But UK experts have criticized the findings, saying that figures from the UK, where breast cancer screening follows a different pattern to that of the US, shows that only 250 women have to be tested to save one life.
The US study is the work of John D Keen and James E Keen, of the John H Stroger Jr Hospital of Cook County and the University of Nebraska, respectively, and is to be published in the open access journal BMC Medical Informatics and Decision Making.
Keen and Keen wrote that unrealistic expectations may influence whether a woman decides to take part in breast cancer screening, and that over 90 per cent of women believe that early detection saves lives.
“Our goal was to help women and physicians understand the stated purpose of mammography,” they wrote.
In their study they analyzed the claim that “mammography saves lives” by calculating its lifesaving absolute benefit in reducing breast cancer deaths in women aged from 40 to 65 using data from the Surveillance, Epidemiology and End Results (SEER) Program and other selected sources.
Keen and Keen calculated three measures: the absolute reduction in risk, the number of women who have to be screened routinely to save one life, and the proportion who survive with and without screening.
They also estimated the average benefit of a single mammogram: a concept of “life-saving proportion” that can be used to assess the costs versus the benefits of screening.
The results showed that:
- The life-saving benefit of mammography gradually increases with age in tandem with the screen-free absolute death risk, which is about 1 per cent over 15 years starting at age 55.
- The corresponding risk of developing breast cancer is about 6 per cent.
- For every 1,000 women screened, repeated screening starting at age 50 saves about 1.8 lives over 15 years.
- The average benefit of a single screening mammogram is 0.034 per cent.
- In other words, 2,970 women must be screened once to save one life.
- This means 23 cancers must be found starting at age 50, or 27 cancers at age 40 and 21 cancers at age 65, to save one life.
- The survival percentage in younger women at age 40 is 99.52 per cent without and 99.62 per cent with screening (assuming a base case 20 per cent relative risk reduction).
- This translates to a 0.1 per cent increased chance of survival with screening than without it.
Keen and Keen concluded that:
“The life-saving absolute benefit of screening mammography increases with age as the absolute death risk increases. The number of events needed to save one life varies depending on the prospective screening subset or reference class. Less than 5 per cent of women with screen-detectable cancers have their lives saved.”
They also commented on what do we we mean when we say “a life saved”? There is a perception that this means a woman is cured from cancer because it was found through screening and had she not been screened she would have died. However, they wrote that:
“All women with breast cancer may theoretically benefit from screening mammography through slowing the disease and therefore slightly prolonging their lives”.
“For a woman in the screening subset of mammography-detectable cancers, there is a less than 5 per cent chance that a mammogram will save her life,” they added.
And finally, coming back to the their reason for the study, to help women and doctors make better decisions about screening, they wrote that:
“By comparing mammography’s life-saving absolute benefit with its expected harms, a well-informed woman along with her physician can make a reasonable decision to screen or not to screen for breast cancer.”
There has been a mixed reaction to the controversial study.
In an accompanying commentary, Dr Michael Retsky of Harvard Medical School praised the study, and said it was a step in the right direction. Too often women aged 40 to 49 are invited to have mammograms without being properly informed of the risks, he wrote.
In another commentary, Dr Stephen Duffy of Cancer Research UK criticized the study, saying that direct results from empirical data might be more trustworthy than modelled estimates combining data from disparate sources.
Prof Julietta Patnick, Director of the NHS Cancer Screening Programmes told the Telegraph newspaper that:
“This paper is based on observational data from the US, which are not directly comparable to the UK because we have a very different screening programme here.”
She said that the UK programme is supervised by an independent expert working group. The group “considers new evidence and makes recommendations about the development of the programme when appropriate.”
She said that the group “has calculated that the programme saves 1 life for every 8 cancers detected.”
However, according to the New York Times, the NHS has agreed to rewrite its handouts on breast cancer screening after a group of experts wrote to the Times of London complaining that none of them “comes close to telling the truth”. They said the fliers overstate the benefits and understate the risks of breast cancer screening.
One of the risks appears to be a diagnosis that leads to unnecessary overtreatment for a cancer that was very slow growing and unlikely to have spread in the patient’s lifetime.
Dr. Lisa M. Schwartz, an associate professor at Dartmouth Medical School, who recently wrote a book about how to interpret health statistics and risk, and who also signed the letter to The Times of London, told the New York Times that ultimately women should make their own decision:
“People can make their own decision, and we don’t need to coerce people into doing this,” said Schwartz, explaining that there is always a trade off between benefits and harms and women should know that.
“There’s no question on one count,” said Schwartz, “if you get screened, it’s more likely you’ll have a diagnosis of breast cancer.”
“What is the point: will screening mammography save my life?.”
John D Keen and James E Keen.
Sources: BioMed Central, Telegraph.co.uk, New York Times.
Written by: Catharine Paddock, PhD