Screening for Breast Cancer With Mammography

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Screening for breast cancer with mammography

http://www.cochrane.org/reviews/en/ab001877.html

Screening uses a test to check people who have no symptoms of a particular

disease, to identify people who might have that disease and to allow it to be

treated at an early stage when a cure is more likely. Mammography uses X-ray to

try to find early breast cancers before a lump can be felt. Many countries have

introduced mammography screening for women aged 50 to 69. The review includes

seven trials involving a total of half a million women. The review found that

mammography screening for breast cancer likely reduces breast cancer mortality,

but the magnitude of the effect is uncertain and screening will also result in

some women getting a cancer diagnosis even though their cancer would not have

led to death or sickness. Currently, it is not possible to tell which women

these are, and they are therefore likely to have breasts and lumps removed and

to receive radiotherapy unnecessarily.

Based on all trials, the reduction in breast cancer mortality is 20%, but as the

effect is lower in the highest quality trials, a more reasonable estimate is a

15% relative risk reduction. Based on the risk level of women in these trials,

the absolute risk reduction was 0.05%. Screening also leads to overdiagnosis and

overtreatment, with an estimated 30% increase, or an absolute risk increase of

0.5%. This means that for every 2000 women invited for screening throughout 10

years, one will have her life prolonged. In addition, 10 healthy women, who

would not have been diagnosed if there had not been screening, will be diagnosed

as breast cancer patients and will be treated unnecessarily. It is thus not

clear whether screening does more good than harm.

Abstract

Background

A variety of estimates of the benefits and harms of mammographic screening for

breast cancer have been published and national policies vary.

Objectives

To assess the effect of screening for breast cancer with mammography on

mortality and morbidity.

Search strategy

We searched PubMed (June 2005).

Selection criteria

Randomised trials comparing mammographic screening with no mammographic

screening.

Main results

Seven completed and eligible trials involving half a million women were

identified. We excluded a biased trial from analysis. Two trials with adequate

randomisation did not show a significant reduction in breast cancer mortality,

relative risk (RR) 0.93 (95% confidence interval 0.80 to 1.09) at 13 years; four

trials with suboptimal randomisation showed a significant reduction in breast

cancer mortality, RR 0.75 (0.67 to 0.83) (P = 0.02 for difference between the

two estimates). RR for all six trials combined was 0.80 (0.73 to 0.88).

The two trials with adequate randomisation did not find an effect of screening

on cancer mortality, including breast cancer, RR 1.02 (0.95 to 1.10) after 10

years, or on all-cause mortality, RR 1.00 (0.96 to 1.04) after 13 years. We

found that breast cancer mortality was an unreliable outcome that was biased in

favour of screening, mainly because of differential misclassification of cause

of death.

Numbers of lumpectomies and mastectomies were significantly larger in the

screened groups, RR 1.31 (1.22 to 1.42) for the two adequately randomised

trials; the use of radiotherapy was similarly increased.

Authors' conclusions

Screening likely reduces breast cancer mortality. Based on all trials, the

reduction is 20%, but as the effect is lower in the highest quality trials, a

more reasonable estimate is a 15% relative risk reduction. Based on the risk

level of women in these trials, the absolute risk reduction was 0.05%. Screening

also leads to overdiagnosis and overtreatment, with an estimated 30% increase,

or an absolute risk increase of 0.5%.

This means that for every 2000 women invited for screening throughout 10 years,

one will have her life prolonged. In addition, 10 healthy women, who would not

have been diagnosed if there had not been screening, will be diagnosed as breast

cancer patients and will be treated unnecessarily. It is thus not clear whether

screening does more good than harm. Women invited to screening should be fully

informed of both benefits and harms.

From Center for Medical Consumers

http://www.medicalconsumers.org/

Cancers That Do Not Kill: Prevalent and Usually Treated Aggressively

Autopsy studies show the prevalence of ductal carcinoma in situ, a tiny

non-invasive cancer within the milk duct, is 6-16%. Prior to the introduction of

mammography screening, this diagnosis represented less than 5% of all new cases

of breast cancer, now it is 20%. Since most cases of DCIS are treated with

either breast removal or radiation, it is not known how many would have

regressed or remained dormant without treatment. However, 78 women whose

biopsied tissue was mistakenly diagnosed as benign in the pre-mammography era

provided an opportunity for researchers. They did a followup study and found

that only 20-25% of these untreated women went on to develop invasive cancer ten

years after the biopsy. Some breast cancer researchers believe that the DCIS

diagnosed today with improved imaging techniques is even more likely to be

inconsequential than these 78 cases indicate.

Evidence that some invasive breast cancers found " early " on a screening

mammogram do not always progress to be life-threatening comes from the National

Breast Screening Study of Canada. Over 50,000 women in their 40s were randomly

assigned to have mammograms or not. 82 more breast cancers were detected in the

women given mammograms. (592 invasive and 71 non-invasive breast cancers in the

mammography group, compared to 552 invasive and 29 noninvasive breast cancers in

the control group.) One would expect a higher survival in the mammography group

with its higher rate of cancer detection. But, in fact, the breast cancer death

rate in both groups was exactly the same at 16 years.

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