Mammography has been used as a preparatory method for breast cancer screening, considering
different protocols for specific age groups, which suggest the periodicity of its
undertaking. The exam is considered necessary, both by physicians and the public,
in population strategies to reduce mortality caused by breast cancer. However, studies
have questioned the benefit of the periodic mammographic screening. In Canada, an
analysis of women between 40 and 59 years old submitted to an annual mammography test
for 5 consecutive years showed that there was no significant reduction in mortality
rates associated with breast cancer.[1] In the United Kingdom, annual mammographic screening in women between 39 and 41
years old for 7 years resulted in a nonsignificant advantage.[2] These studies indicate that mammographic screening does not seem to offer benefits
to prevent breast cancer in low- or medium-risk women.
From an epidemiological perspective, screening assumes the use of mammography in healthy
women to detect suspected cancer, which will subsequently be submitted to specific
diagnostic tests to confirm the disease and guide the treatment. A better mammographic
screening approach would effectively reduce death rates in women who undergo this
type of exam. Any case of cancer detected through screening should be treated and
followed-up over a long period of time for professionals to verify its impact on mortality
rates. When mortality rates associated with breast cancer are compared, it is necessary
to separate the cases which were detected, treated and followed-up in screening programs
from those in which the diagnosis was performed without screening mammography, according
to the disease stage indicated by the diagnosis.
In this scenario, which confronts recent studies and the traditional epidemiological
concept, it is important to stress the fact that the detection of a breast cancer
case through mammographic screening does not guarantee that the death risk by the
disease is reduced. Consequently, part of the controversy results from the confusion
about what is known as early detection versus method efficacy. It is believed that
a few biases are responsible for the questioning of mammographic screening, such as
execution time, selection bias, and especially overdiagnosis.[3] The latter consists of diagnosing a disease that will lead to neither symptoms nor
death. The Canadian study previously mentioned detected 26% of overdiagnosis, and
the number can reach up to 50% if cases of ductal carcinoma in situ are included,
as revealed by a 25-year follow-up.[1] In the oncology field, overdiagnosis is defined as the detection of cancer that
would not have evolved to be clinically detectable. The type of cancer most frequently
detected through mammographic screening is ductal carcinoma in situ, and studies have
not shown significant effects of this tumor on women's survival after 20 years of
follow-up post- diagnosis.[4]
Previous investigations demonstrated that the probability of a woman who had a cancer
diagnosis receiving an overdiagnosis is 19%.[5]
[6]
[7] Applying this value to the recent cumulative incidence of breast cancer in the United
Kingdom (invasive and in situ) means that 1 out of 77 women screened from 55 to 70
years would have an overdiagnosed breast cancer. Authors also claim that the consequences
of overdiagnosis are unnecessary surgeries, radiotherapy, and chemotherapy. Nevertheless,
they conclude that it is impossible to distinguish between life-threatening carcinomas
and overdiagnoses initially, and that further studies assessing accurate overdiagnosis
rates are fundamental. There are other issues associated with mammographic screening,
although they are more infrequent. Around 4% of women submitted to the test are called
to repeat the exam and possibly undergo a biopsy.[8] Among them, 20% will have cancer, 70% will need further imaging studies, and 30%
will require biopsy, procedures that cause psychological damage and anxiety.[8]
The main question concerning the implementation of screening programs relates to the
disease mortality though. The first conclusions regarding breast cancer mortality
are based on a Cochrane systematic review.[9] The global relative risk, comparing screened and non-screened women, is 0.80 (confidence
interval of 95%, 0.73–0.89), that is, the reduction in relative risk of breast cancer
mortality in screened women is 20%.[9]
In terms of absolute gain, women screened from 50 to 70 years will experience no benefit
in the first 5 years of screening, but mortality reduction will last for 10 years
after the last exam. Regarding the direct impact on deaths provoked by breast cancer,
1 death will be prevented in a group of 235 screened women.[9]
Despite the data from studies performed in developed countries, reflections are necessary
when the Brazilian reality is evaluated. In our country, 40% of 50- to 69-year-old
women do not undertake mammograms.[10] Consequently, our context still needs improvement in the potential of basic screening
for the disease. Taking into account that the Brazilian female population between
50 and 69 years is around 15 million,[11] and that 60% of these women are effectively screened through mammography, there
would be 40 thousand breast cancer-related deaths prevented.
There is a striking contrast between developed and developing countries when it comes
to unmet needs. In the former, mammographic screening is questioned and efforts are
made toward breast cancer therapy, which is undeniably the main factor associated
with decreased mortality rates. Developing countries still have a great challenge
to increase effective screening rates. The Brazilian reality seems not to allow the
dismissal of mammography in breast cancer screening. However, faced with current evidence,
women must be informed about the benefits and potential risks of overdiagnosis, so
their decision on participating in a screening program is as clarified as possible.
