Wednesday March 31 2010
Around 1.5 million women are screened for breast cancer in the UK each year
“Breast cancer screening saves the lives of two women for every one given unnecessary treatment,” reported The Daily Telegraph. It said that researchers have found that the benefits of the screening programme far outweigh any harm it causes, such as unneccessary treatment for cancers that would otherwise have remained symptomless (known as overdiagnosis).
This study was of two large, different populations, with data from 20 years of the UK Breast Screening Programme and a Swedish trial. The researchers estimate that for every 1,000 women aged 50–69 who were screened for breast cancer in the UK, 5.7 breast cancer deaths were prevented and 2.3 overdiagnoses were made. This suggests that for every woman of this age screened for breast cancer who has unnecessary further investigation or treatment, about two lives will be saved.
There has been a long-running debate about the benefits of screening. The Telegraph reported that Professor Duffy, the lead author, said that his study was more robust than others as it looked at twice the length of follow-up data and captured the longer-term benefits of screening. He said, “If you screen today you are not saving lives tomorrow, you are saving lives years from now.”
This is a complex issue, and these results may be contested by researchers on the other side of the screening debate. The view of the NHS and the World Health Organization (WHO) is that the benefits of breast screening outweigh the harms and that breast cancer screening continues to save many lives each year. “Quality screening mammography done every two years in women 50-69 years of age should reduce their risk for death from breast cancer by about 35%”, said the WHO in 2002.
Where did the story come from?
This research was carried out by Professor Stephen W. Duffy and colleagues from Queen Mary University of London, Central Hospital in Falun, Sweden, University Hospital in Linköping, Sweden, National Taiwan University and the American Cancer Society. No sources of funding were reported. The study was published in the peer-reviewed Journal of Medical Screening.
What kind of research was this?
The aim of this research was to estimate how many deaths were prevented by breast cancer screening and to compare this to the number of tumours that were overdiagnosed (cancers that would never have been diagnosed in a woman’s lifetime had screening not taken place).
The researchers looked at women aged 50–69, using data from the Swedish Two-County randomised controlled trial and the UK Breast Screening Programme.
What did the research involve?
The Swedish Two-County trial is reportedly the first published randomised trial of breast cancer screening. Women between the ages of 40 and 74 were enrolled between 1977 and 1981. Some of these women were invited for screening while others were not. The trial randomised 55,985 women to mammography screening (on average every 24 months for women aged 40-49 and every 33 months for women aged above 50) and 77,080 women to not be invited for screening.
Women spent an average of seven years in the screening programme, during which time they received an initial “prevalence” screen to identify existing cases of breast cancer and an average of two further “incidence” screens to identify new cases of breast cancer. Data on deaths were collected up to 1998, providing 21.5 years of follow-up. After seven years, the women in the control group who had not been offered screening during the study were then offered it.
The UK Breast Screening Programme was established between 1989 and 1993 and screens women at three-yearly intervals. Initially, only women aged 50-64 were invited to attend, but from 2002 to 2004, the age range was extended up to 70 years. The range is currently being expanded to include all women aged 47–73. For the purposes of this study, breast cancer incidence data were available for the period between 1974 and 2003, and data on mortality from breast cancer up to 2004.
The researchers examined only the 50–69 year age group in the Swedish Two-County trial so that the population data they looked at were similar to those from the UK screening programme. To calculate the number of women needed to be screened to prevent one breast cancer death over follow-up, the researchers looked at the difference in breast cancer deaths between the screened group and the non-screened group, and divided this figure by the number of women screened.
For the UK programme, they looked at breast cancer mortality in the 50–69 age group pre-1989 (before screening was introduced) and post-1995 (after the screening programme was established). The number of breast cancer deaths prevented was calculated as the difference between observed deaths in women aged 50–69 in these time periods and expected deaths based on mortality changes in women in the non-screened age groups (younger than 50, or 70 or older).
Overdiagnosis was assessed in the Swedish Two-County trial using a complex mathematical formula that took into account the prevalence of breast cancer when the first screen of the trial took place, and the prevalence in the control group after completion of the trial at seven years, when these women were also invited for screening. It also accounted for trends in time and age, and the incidence of cancer detected in the incidence screens during the trial.
The researchers calculated overdiagnoses in the UK programme by looking at the incidence trends of breast cancer by age group between 1974 and 1988. From these figures, they projected the expected incidence of breast cancer between 1989 and 2003, and compared this with actual observed incidence.
What were the basic results?
Screening in the Swedish Two-County trial significantly reduced the rate of breast cancer deaths. This equated to 8.8 deaths prevented per 1,000 women screened over a 20-year period, starting at age 50. Women in the screening group had a 38% reduced risk of dying from breast cancer compared to those in the non-screened group (relative risk [RR] 0.62, 95% confidence interval [CI] 0.51 to 0.75).
The UK Breast Screening Programme was similarly associated with a reduced rate of breast cancer deaths, with 5.7 deaths prevented per 1,000 women screened over a 20-year period. The age group invited to attend screening had a 28% reduced risk of dying from breast cancer compared with those not invited for screening (RR 0.72, 95% CI 0.70 to 0.74).
The number of overdiagnoses was 4.3 per 1,000 women screened for 20 years in the Swedish trial, and 2.3 per 1,000 women screened for 20 years in the UK programme.
How did the researchers interpret the results?
The researchers concluded that these figures demonstrate that the benefits of breast cancer screening in women aged 50–69 outweigh the risks of overdiagnosis, with 2–2½ lives saved for every case overdiagnosed.
This study used data from two large, different populations. Its findings suggest that for every woman of this age screened for breast cancer who receives unnecessary further investigation or treatment, about two lives will be saved.
The researchers note that the findings of their study contrast with those of a recent study, which estimated that overdiagnoses outnumbered breast cancer deaths prevented by screening by 10 to 1. They suggest that these differences were due to a number of reasons:
- The current study looked at breast cancer deaths for every 1,000 women screened, rather than for every 1,000 women invited for screening. The results, therefore, show what benefits are achieved in women of the target age who are actually screened.
- These researchers looked at screening over a 20-year period, rather than the 10-year period that a recent study analysed. They maintain that using data from a longer period allows longer-term benefits to be captured. The lead researcher, Professor Duffy, said in the Telegraph: “If you screen today you are not saving lives tomorrow, you are saving lives years from now – and even then there is always a delay in cancer registrations.”
There has been a long-running debate about the benefits of screening, and other studies have not found the balance of benefits and harms to be so clear cut. The benefits include early diagnosis and reducing breast cancer deaths, which are weighed against the anxiety involved in attending screening and awaiting results, and the risk of false positive results and overdiagnosis leading to unnecessary treatment.
This is a complex issue, and these results may be contested by researchers on the other side of the screening debate. The view of the NHS and the World Health Organization (WHO) is that the benefits of breast screening outweigh the harms and that breast cancer screening continues to save many lives each year. “Quality screening mammography done every two years in women 50-69 years of age should reduce their risk of death from breast cancer by about 35%”, said the WHO in 2002.
Women considering or undergoing breast screening can discuss any concerns with their GP or specialist, who can give specific advice on the options, techniques and therapies for diagnosing and treating breast cancer.