Norwegian research found that breast cancer rates were higher among women who were screened more regularly than a control group.
X-ray showing breast cancer
This suggests that some of the cancers might have spontaneously regressed, the researchers say, and that some women could have been spared painful treatment. There have been 32 documented cases of breast cancer regression, they add.
But cancer charities advise women over 50 to continue coming forward for the breast cancer screening programme.
What is the research?
The findings are from a study of breast cancer frequency among women in four Norwegian counties. A total of 119,472 women aged 50-69 were screened every two years from 1996 to 2001 and compared with a control group of 109,784 women, matched for age, who were screened once in the six-year period.
The researchers found that invasive breast cancer was 22 per cent more common in the frequent screening group - 1,909 cases per 100,000 compared with 1,564 in the control group.
Frequently screened women were found to be more likely to have breast cancer at every age.
What do the researchers say?
Lead researcher Dr Per-Henrik Zahl, from the Norwegian Institute of Public Health in Oslo, said: 'Because the cumulative incidence among controls never reached that of the screened group, it appears that some breast cancers detected by repeated mammography would not persist to be detectable by a single mammogram at the end of six years.
'This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress.'
What do other researchers say?
Dr Kat Arney, science information officer at Cancer Research UK, said: 'There is much we still don't understand about the development and progression of breast cancer.'
But she urged women to go for screening because 96 per cent now survive if diagnosed with breast cancer by screening.
The UK screening programme is successful at picking up breast cancer in its early stages, when treatment is effective, she said.
- Arch Intern Med 2008; 168: 2,302-3
- Some invasive breast cancers may regress without treatment.
- It is impossible to know which cancers will regress.
- GPs should continue to recommend screening for women aged 50 and over.