THE IMPORTANCE OF SCREENING FOR BREAST CANCER
This is the first in a series of posts that I hope will answer many of the questions that you might have about breast cancer and the importance of screening and early detection.
THE PROBLEM
In the United States, breast cancer is the leading cause of “non-preventable” cancer death among women. Although more women die from lung cancer each year, a large number of lung cancers could be prevented by not smoking. At this point in time there is no known method of preventing breast cancer.
The American Cancer Society estimated that in 2019, there would be 268,600 new cases of invasive breast cancer in women and approximately 2,670 cases would be diagnosed in men. In addition, they estimated an additional 48,100 cases of Ductal Carcinoma In Situ among women. They estimated that approximately 41,760 women and 500 men would die from breast cancer in 2019 (18). Although far from victory, the number of deaths would have been much higher (approximately 70,000) were it not for screening, early detection and improved treatments that have reduced deaths by over 40% since 1990.
Although having a relative (mother-sister-daughter) who has been diagnosed with breast cancer increases a woman’s risk of developing breast cancer, herself, the vast majority of breast cancers are “sporadic”. We have no idea why an individual woman developed her cancer. Furthermore, we have no idea which women will not develop breast cancer. Although there are factors such as a family history that increase an individual’s risk we have no accurate way of predicting who will develop breast cancer and, almost as important, who will not develop breast cancer.
At this point in time we must assume that all women are at risk of developing breast cancer.
The risk of developing breast cancer also increases with age. In the U.S. a woman at age 40 has approximately 1 chance in 1000 of developing breast cancer in that year. The risk increases steadily so that by age 50 she has 2 chances in 1000 of developing breast cancer in that year. By age 60 it has increased to 3 chances per 1000 and 4 chances per 1000 by age 70.
Future posts will go into greater detail about risks, but we are still unable to accurately determine why one woman develops breast cancer and another doesn’t.
THE IMPORTANCE OF SCREENING FOR BREAST CANCER
In the 1950’s it was thought that breast cancer was systemic – that it had already spread to other parts of the body before it could be found. This stimulated the start of the huge, ongoing effort to find systemic treatments to try to cure metastatic disease (thus far unsuccessful).
The first randomized, controlled trial (RCT) of screening that used both clinical breast examination (CBE) and mammography was performed in the 1960’s within the Health Insurance Plan of New York. The HIP study proved that earlier detection could actually lead to some women being cured from their breast cancers. This is fundamental. It means that breast cancers grow for a period of time in the breast. At some point cancer cells develop the capability of leaving the breast (through the blood or the lymphatics) and can spread to other parts of the body. If these metastatic cells are able to survive and grow in other organs the cancer likely becomes incurable. Breast cancer in the breast can become a problem, but cannot be lethal. It is breast cancer that is metastatic to other vital organs leading to their destruction that is lethal. The goal of screening is to detect as many cancers as possible at a time before they have become “successfully” metastatic – a time during which cure is possible.
RANDOMIZED CONTROLLED TRIALS
As will be explained in later posts, RCT’s are the only way to prove that a test (such as mammography screening) actually saves lives. In the HIP study there were 23% fewer women ages 40-64 who died in the screening arm compared to the control arm. As will be explained in the future, RCT’s eliminate all the biases such as “leadtime bias, length bias sampling, overdiagnosis, etc.” that can compromise other studies. RCT’s provide the proof that screening saves lives.
The HIP trial was followed by a number of other RCT’s. In particular there were trials in Sweden that only employed mammography (no clinical breast examination). The RCT’s proved that early detection using mammography saves lives for women ages 40-74 (the ages of the women who participated in the trials) (19, 20).
SUPPORTING EVIDENCE
Once you prove, by rigorous RCT’s, the efficacy of the test, the next question is what happens when the test (mammography) is introduced into the general population. Does the benefit that was seen in carefully performed trials translate into a real benefit in general populations. These are called “observational studies” and numerous studies have shown a marked reduction in breast cancer deaths among women ages 40 and over who have access to screening and who participate in screening compared to those who don’t (21-37).
Another way to evaluate the importance of screening is called “failure analysis”. In such a study in the Harvard Medical School teaching hospitals we looked at the screening histories of women who actually died from breast cancer. This showed that more than 70% of the women who died from breast cancer, despite having access to modern therapies, were among the 20% of women who were not participating in screening (38). Spencer et al had similar results (39).
In still another way to evaluate the benefit of mammography screening in reducing breast cancer deaths was reported in a recent study of tens of thousands of women in Sweden that showed that the incidence of deaths was 60% lower at 10 years for women who participated in screening compared to those who did not, and 47% lower at 20 years (40) despite all having access to therapy.
These studies all applied to women ages 40 and over.
In the U.S. the death rate from breast cancer had been unchanged for 50 years dating back to 1940. Screening began in the U.S. in the mid 1980’s and soon after, in 1990, as predicted, the death rate began to fall (41). Deaths have continued to fall so that there are now over 40% fewer women dying each year from breast cancer in the U.S. It has been estimated that over 600,000 lives have been saved since 1990 (42).
It is fairly certain that much of the decline in deaths has been due to screening. Over the same time period as the death rate for women was falling, the death rate for men with breast cancer went up in 1990. It stayed up for several years and then fell back to 1990 levels where it has remained (43) while deaths have continued to fall for women. Breast cancer therapies are similar for men and women, but breast cancer in men is found at a larger size and later stage. Men are not screened. Therapy has improved, but lives are saved when breast cancer is treated earlier.
There is no question that early detection saves lives for women ages 40-74.
Future posts will go into greater detail. I look forward to any questions that you might have.
