Women have been encouraged to have mammograms, which can detect breast cancer in an early stage when it is most likely to be cured. The (US) National Cancer Institute recommends women age 40 and older have a mammogram every one or two years. Encouraging women to undergo screening mammography is widely felt to be a worthwhile public health program, as it is seen to save lives at relatively low cost.
A new study examined the effect of mammography screening on populations, and concluded, “The total overdiagnosis of breast cancer in publicly available mammography screening programmes (including carcinoma in situ) was 52% (95% confidence interval 46% to 58%).”
“Carcinoma in situ” is a cancer that appears limited. It is usually treated, though, since one cannot predict which ones would progress to become lethal.
Excluding carcinomas in situ, “the overdiagnosis for invasive breast cancer only was 35% (95% confidence interval 29% to 42%),” the investigators concluded.
Background
Screening programs are intended to find disease that is otherwise not apparent. Women are advised to undergo mammography at regular intervals. Mammography is a procedure that takes x-rays of the breasts in order to detect suspicious areas. A suspicious area is usually biopsied. If cancer is detected, treatment is recommended.
The Study’s Premise
Detection of cancers that are harmful will occur even in the absence of mammography screening. They will be diagnosed when the harm they do is discovered. The incidence (number of new cases) in unscreened populations is the true rate of breast cancer. If screening mammography finds a higher rate, the number found more than the “true rate” can be considered overdiagnoses.
The Study
Data were obtained from published sources. Some additional data were obtained from individual sources. Data were available for the United Kingdom; Manitoba, Canada; New South Wales, Australia; Sweden; and parts of Norway. Rates were calculated for before and after screening, and where available, screened and not screened concurrently.
For example, in Manitoba, formal mammography screening programs were started in 1995. Using population data (e.g., there are 2.3 women aged 50-69 for each one aged 70-84), calculations suggest the overdiagnosis rate is 44% (includes in situ cases).
Other Disadvantages of Mammography
- The National Cancer Institute acknowledges that mammography exposes women to radiation. The long term consequences are not fully measured.
- The concept of screening for cancer causes anxiety in women, and finding a suspicious area in a breast is understandably frightening.
- The procedure can be painful; to get a good image, pressure is often put on breast tissue to squeeze it flat.
- A false sense of security from a normal mammogram can lead to ignoring a new abnormality. For example, a new lump or discharge may be ignored because a recent mammogram was interpreted as “normal.”
Implications
The new study in no way suggests that women should not undergo mammography. Women in the proper age groups should get mammography. Discussion about which age groups should be included is legitimate, but this paper does not alter the mainstream belief that mammography saves lives.
What this paper does is encourage more research on how to distinguish which abnormal growths are harmful, and which ones are harmless.
The Study
BMJ 2009;339:b2587 Published 9 July 2009
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