Screening for breast cancer
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Screening for breast cancer includes activities which test members of asymptomatic populations for breast cancer. Many advanced countries have breast screening programs. The most widely adopted method for breast cancer screening is mammography.
There are few areas in imaging fraught with more controversy than screening for breast cancer. Due to the emotive issues surrounding the diagnosis, the scientific literature on breast screening and its issues reaches the lay press quickly and is sometimes reproduced in non-scientific, often potentially inaccurate terms.
Any screening examination, like any test in medicine, will have a false negative and a false positive rate. Mammography as a screening exam does not find all cancers in all women, and will in some cases be read as positive in women without cancer. Nonetheless, mammography is currently believed to the best universally available tool we have to find breast cancers reliably and reproducibly early in the disease process.
The rationale behind screening
The rationale behind screening for breast cancer is the universally accepted dictum that (all other factors being equal) earlier diagnosis results in increased survival.
Screening for breast cancer is often cost and risk effective, feasible and evidence-based, however, cost, risks and treatments available vary dramatically in different populations around the globe.
There are three steps necessary to prove that screening is effective in breast cancer 7:
1. the test can find cancers when they are smaller than without the test: mammography has succeeded here without doubt. To such an extent that "overdiagnosis" is now a concern.
2. randomized, controlled trials must show a reduction in mortality: there is no argument here in the accepted medical literature. Seven trials have demonstrated this 2,7.
3. when the test is introduced into the general population, the death rate declines: the number of deaths per thousand of the population. Screening mammography has reduced the death rate of women 7,8.
Increased survival rates for breast cancer are due in part to earlier diagnosis but also markedly improved treatments. Not only are more women surviving this disease, but they are also doing it with cosmetically acceptable results. This is partly due to the use of screening mammography. Treated patients may now enjoy a good quality of life without the radical surgical interventions previously used which were often associated with unpleasant sequelae.
The initial literature support for screening came from the Two Counties Trial in Sweden. Follow up for the trial participants continues and the data support the continued use of screening to detect breast cancer at an early stage when the therapy is potentially curative and cosmetically acceptable. The Two Counties trial and the subsequent Health Insurance Plan (HIP) study in New York showed a decrease in deaths from breast cancer in those aged 40-74 years. A direct result of the HIP study was that screening was introduced in the USA, although it is not performed universally on all relevant populations there.
Concepts of risks
As they pertain to screening and breast cancer, there is significant confusion among the general population and many professionals about the concepts of different risks (relative risk, absolute risk and attributable risks).
Most cases of breast cancer are understood to be sporadic. The prevalence of genetic mutations associated with breast cancer such as the BRCA1 and 2 genes varies by population. Although some studies look at the occurrence of breast cancer in people with first-degree family history, this does not mean that all of these cases are genetic.
- age to begin screening:
- as a general rule, biologically, cancers in younger women tend to grow faster and hence metastasize earlier. The lag period in the time before a lesion is picked up is therefore potentially shorter in women under 50 than in postmenopausal women. Some of the cancers seen in screening in those over 50 were actually potentially diagnosable when the patient was under 50 years 9. Nonetheless, on a population basis, in certain contexts, specifically low resource settings, screening populations under 50 is not recommended 14. Recommendations vary country by country and across different professional organizations within the same country.
- age to stop screening
- although it is currently accepted that screening is not cost-effective after a certain age, as life expectancy rises, the precise age for each population is currently a matter of ongoing research.
- unfortunately, there are currently no pathological or imaging features to distinguish the progressive cancers - with lethal potential - from the indolent cases. In general, much overdiagnosis will likely represent cases of low-grade DCIS (ductal carcinoma in situ) and those cases of indolent IDC that on review were actually present on mammograms of years ago but have only now been diagnosed for whatever reason. Some cases of cancers that are well followed without intervention do not seem to change much with time in terms of how they appear on radiological studies available at present. Some breast cancers will not be fatal. Indeed in some autopsy series, up to 2% of female autopsies have breast cancer.