Mammograms may pick up breast cancers which would have disappeared

Nov 25 2008

According to researchers in Norway mammograms may detect some cancers that would have otherwise regressed.

There has been a significant increase in breast cancer rates in some parts of Norway since women there began undergoing mammography every two years and the rates among regularly screened women have remained higher than rates among women of the same age who were screened only once after six years.

The researchers from the Norwegian Institute of Public Health, Oslo, suggest that some of the cancers detected by mammography may have spontaneously regressed had they not been discovered and treated.

According to background information throughout Europe, the start of screening mammography programs has been associated with increased incidence of breast cancer and the researchers say if all of these newly detected cancers were destined to progress and become clinically evident as women age, a fall in incidence among older women should soon follow.

The researchers say the fact that this decrease is not evident raises the question of what is the natural history of these additional screen-detected cancers.

Dr. Per-Henrik Zahl and colleagues examined breast cancer rates among 119,472 women age 50 to 64 who were all invited to participate in three rounds of screening mammograms between 1996 and 2001 as part of the Norwegian Breast Cancer Screening Program.

For their report they compared these to rates among a control group of 109,784 women age 50 to 64 in 1992, who would have been invited for screening if the program had existed at that time.

Cancers were tracked for six years using a national registry, and at the end of that time all participants were invited to undergo a one-time screening to assess breast cancer prevalence.

The researchers found as anticipated, that breast cancer rates were higher among screened women than among the control group before the final prevalence screening - they say even after prevalence screening in controls, however, the cumulative incidence of invasive breast cancer remained 22% higher in the screened group.

Of every 100,000 screened women, 1,909 had breast cancer during the six-year period, compared with 1,564 of every 100,000 in the control group and screened women were more likely to have breast cancer at every age.

The researchers say because the cumulative incidence among controls never reached that of the screened group, it appears that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of six years which raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress.

They say though many doctors may be skeptical of the idea, the excess incidence associated with repeated mammography demands that spontaneous regression be considered carefully - spontaneous regression of invasive breast cancer has been reported in 32 reported cases.

The researchers say while this is a relatively small number given such a common disease, the fact that documented observations are rare does not mean that regression rarely occurs and may instead reflect the fact that these cancers are rarely allowed to follow their natural course.

The researchers say their findings do not answer the question of whether mammograms prevent deaths from breast cancer but simply provide new insight on what is arguably the major harm associated with mammographic screening, namely, the detection and treatment of cancers that would otherwise regress.

Many cancer experts are dubious about the findings and advise women to continue to have regular mammograms as the screening technique unquestionably saves lives by finding breast cancer early on when it is most treatable.

Mammography and breast self-examination for tumours remain the standard methods used for early detection of breast cancer, which is the leading cause of cancer deaths among women worldwide.

Each year about 465,000 women die of breast cancer globally and 1.3 million new cases are diagnosed.

The research is published in the current issue of Archives of Internal Medicine, one of the JAMA/Archives journals.