Does Mammography Screening Save Lives?
NBCC believes that there is insufficient evidence to recommend for or against universal screening mammography in any age group of women. Women who have symptoms of breast cancer such as a lump, pain or nipple discharge should seek a diagnostic mammogram. The decision to undergo screening for asymptomatic women must be made on an individual level based on a woman’s personal preferences, family history, and risk factors. Mammography does not prevent or cure breast cancer and has many limitations. Women are told that mammography screening saves lives, but the evidence of a mortality (death rate) reduction from screening is conflicting and continues to be questioned by some scientists, policymakers, and members of the public. Ultimately, resources must be devoted to finding effective prevention and treatments for breast cancer and tools that not only detect breast cancer truly early but distinguish between threatening and non-threatening subtypes.
All breast cancers are not the same. Some patients will have fast-growing, aggressive tumors while others will have slower-growing, less aggressive tumors that are less likely to metastasize and, therefore, have a better prognosis. Screening is more likely to detect the slower-growing, less aggressive tumors because of longer asymptomatic periods. Faster-growing tumors generally have a shorter asymptomatic phase than slower-growing tumors, and so are less likely to be detected. However, faster-growing tumors are also often associated with a poorer prognosis. This “length-time” bias can make screening appear more beneficial than it is. Screening is detecting the tumors with a better prognosis.
“Lead-time” bias can also contribute to a misrepresentation of the benefit of mammography. If a lethal cancer is found earlier through screening, the patient would appear to live longer because of “lead time.” Screening is not helping patients in these situations live longer, it is only helping them find out about their cancers sooner.
The issue in breast cancer to spark the most controversy has by far been the issue of mammography screening. The debate has been over whether the scientific evidence supports widespread screening mammography for healthy women who have no symptoms. This is not about screening women who have a family history of breast cancer, or a prior diagnosis of DCIS, or a genetic mutation that puts them at high risk, but rather screening healthy, low-risk, asymptomatic women. Does it save lives?
Research on Mammography Screening
Well-designed and carried out clinical trials are the best way to determine if any medical intervention actually works. People are randomly assigned to either receive the intervention or not, and the two groups are compared to each other using statistics. In the case of screening for breast cancer, the goal is to decrease the death rate from the disease. The best way to answer the question, “Does Mammography Save Lives?” is to look at breast cancer survival rates from clinical trials with large groups of women, half who received mammography screening and half who didn’t.
Mammography Screening Studies
There are eight published, large, randomized, clinical trials that looked at the benefits of screening mammography for survival. Four of the trials were conducted in Sweden, one was conducted in Canada, two were conducted in the United Kingdom, and one was conducted in the United States. The eight trials are known as:
- The New York trial or HIP trial (1963) – enrolled 60,495 women ages 40-64
- The Malmo trial (1976) – enrolled 42,283 women ages 45-69
- The Two-County trial (1977) – enrolled 133,065 women over age 40
- The Edinburgh trial (1978) – enrolled 44,268 women ages 45-64
- The Canadian trial (parts 1 and 2; 1980) – enrolled 89,835 women ages 40-59
- The Stockholm trial (1981) – enrolled 60,117 women ages 40-64
- The Gothenburg trial (1982) – enrolled 49,924 women ages 39-59
- The Age trial (2006) – enrolled 160,921 women ages 39 – 41
These eight trials included few women over the age of 70 and very few women of color. None of the trials examined women younger than 39 years old, and only one trial — the Age trial — was specifically designed to look at the impact of mammography screening in women during their 40s.
The trials vary in reliability and quality of the results. Some were poorly designed or carried out. Two of the trials are generally considered to be the most reliable – the Malmo and Canadian trials. In both trials, the women who got mammography screening had the same breast cancer mortality or death rate as the women who did not. The Age trial found a reduction in breast cancer mortality from screening during the 40s, but it was a weak finding and not statistically significant. Results from the other five studies were also weak and not statistically significant, though when the results are pooled, there is a 15% relative risk reduction in breast cancer mortality from mammography.
It is important to keep in mind that looking at relative risk reduction ignores the actual or absolute risk of developing and dying from breast cancer. The absolute risk of a woman dying from breast cancer is less than 1% without any screening. Looking at this another way, the chance that an average 50-year-old woman will not die of breast cancer over the next 15 years is 99.12%. Mammography screening increases those odds by 0.17% to 99.29%. (Keen, 2009)
Women tend to overestimate both their risk of developing breast cancer and the actual benefit mammography has in reducing the chance of dying from breast cancer. The absolute benefit of mammography is modest at best, but so is the risk of getting breast cancer in the first place.