Back in 1977, the first mammography screening study showed that women who had been screened had a lower mortality than those that had not. This led us to conclude that early detection was the key to curing all breast cancers. It was a reasonable hypothesis at a time when it was thought that all breast cancers were the same and that they slowly grew to a certain size, then “got out” and metastasized elsewhere in the body, leading to death. Since then, there have been several studies of mammography on different age groups with different screening schedules. Even the most encouraging results showed only a 26% decrease in mortality regardless of whether the screening was with the old style mammograms of the 80’s or the fancy new digital mammograms of today.
This week, we heard about the 25-year follow-up of a study done in Canada where women aged 40-59 were randomized to mammogram and physical breast exams or strictly physical breast exams for five years. The data showed no difference in mortality between the two groups. This suggests that a clinical breast exam is the same as mammography in asymptomatic Canadian women 40-59 years old. The women who had mammograms had smaller tumors detected, but that did not shift the mortality.
What this tells us is not that mammography is worthless but rather, that the hypothesis of early detection may need reevaluation. Over the last 37 years, we have learned that not all breast cancers are the same. There are at least five different kinds based on their molecular biology! Some of them are very slow-growing and others are very aggressive. Some are sensitive to treatment and others are not. Some can be found at an early stage on mammography and some have spread by the time they are visible. The good news is that we now have targeted treatments for some of these tumors (HER2/neu positive and hormone positive) that significantly impact mortality. We need more effective treatments for triple negative breast cancers and metastatic disease.
And what is the cost of mammography screening? There are false negatives, which result in normal mammograms when women actually have cancer, and false positives in which mammograms thought to be abnormal result in biopsies to double check. There is also the over-diagnosis of lesions (22% in this study) that never would have become clinical cancers in the first place, not to mention the cumulative risk of yearly radiation.
So, should we throw out mammography? No! It is a great diagnostic tool for women with lumps and an important diagnostic option for high-risk women as well. However, it is time that we recognize that the premise that early detection is the only factor between life and death may be flawed.
Think about it. Prior to the 16th century, people thought the earth was flat because that is how it looked. It was counterintuitive to think that it was round and public opinion didn’t change until it was proven. In the same way, we previously hypothesized that all breast cancers could be detected “early” through screening and that early detection was the answer to curing the disease. Like TSA screening in the airport, having an annual mammogram makes us feel like we are doing something to prevent catastrophe. But just as TSA screening does not guarantee there will be no terrorist attack, this study is another piece of evidence that mammographic screening is no guarantee that you won’t die from breast cancer. In both situations, refocusing investments from early detection to prevention would save more lives in the long run.