Mammography was one of the first cancer screening tests, preceded only by the Pap test for cervical cancer. Modern mammography techniques were developed in the 1960s and officially recommended as a screening method by the American Cancer Society in 1976. A key tool in the fight against cancer is early detection and treatment, and effective cancer screening tests are a big deal, in part because there aren’t many of them. (See my previous post on lung cancer CT screening for more about what makes a good screening test)
For decades, mammography has been recommended as an effective breast cancer screening test for all women beginning at age 40. This recommendation began following the results of initial mammography screening trials (performed in New York State in the 1960s) and other early trials in Europe, which showed a consistent decrease in breast cancer mortality. In the early 1980s, Canada threw its hockey puck in the ring (rink?) with its own large-scale mammography trial, and the trouble began. In this study, despite the detection of more small breast cancers in the mammography group, there was, surprisingly, no difference in survival between women who obtained screening annual mammograms and women who did not.
These results certainly made doctors say, “Hmmm.” I confess I was unaware of some of the history behind screening mammography before researching this post, and it seems quite difficult to decide what to do given the conflicting results of these trials. But before we see what happened next, and talk about more recent controversy, we should probably back up and describe what it means to get a mammogram.
If you have never had a private body part squished by a total stranger (in a medical facility, that is), let me enlighten you about screening mammography.
- Why do we do it? Screening mammography is designed to detect breast cancer as early as possible in order to maximize the chance of survival.
- What is a mammogram? It is simply an X-ray of the breast. There are some technical differences from a typical X-ray, but this is not important to our discussion.
- How is it done? A technologist specially trained to perform mammography obtains the pictures, taken of each breast from two different angles. These are called CC (craniocaudal, taken looking down on the breast from above) and MLO (mediolateral oblique, taken from the side at a slight angle). During the mammogram, the breast sits on a flat plate, and a plastic paddle is pressed on top of the breast to flatten it. I will take the word of others that this is uncomfortable, but it only lasts a few seconds.
- What is the radiologist looking for? Primarily two things: masses or calcifications. Either can indicate that a cancer is present. A mass usually appears as a density, a brighter region on the mammogram image. Masses can be benign (non-cancerous), such as a fluid-filled cyst or a fibroadenoma (a benign solid tumor); they can also be malignant, i.e., cancer. Calcifications, which appear as bright white dots and lines, can also be benign or malignant. The calcifications themselves are not the problem, but they can form near cancers (as well as near benign processes) in the breast. Tiny calcifications of a certain shape are particularly concerning for cancer, and the distribution of calcifications in the breast can also be more or less concerning. When these small calcifications are grouped together, it can indicate an underlying precancerous condition called ductal carcinoma in situ, or DCIS.
- What happens after the pictures are taken? If the radiologists finds something concerning on your annual screening mammogram, you will be “recalled” for a diagnostic mammogram, meaning you will return to the radiology department to further evaluate the abnormality. This will include additional mammogram pictures—sometimes magnified or from different angles—and almost always an ultrasound of the region of concern. You will then meet the radiologist and discuss your findings. Based on what is seen, the radiologist will decided the next step from three options: biopsy the area of concern; wait six months and repeat the mammogram; or continue annual mammograms in twelve months. MRI sometimes plays a troubleshooting role in cases in which it is difficult to decide which course is best.
(First, some disclosures. I am not an expert in mammography; my training in diagnostic radiology included mammography, but it is not a regular part of my current practice. Additionally, I may be inherently biased because I am a radiologist. I will endeavor to discuss the issues as objectively as possible, but I cannot promise a complete lack of influence from opinions in my profession and its governing body, the American College of Radiology.)
As you recall from the introductory cliffhanger, early mammography trials in New York state and Europe contradicted the results of a subsequent large trail in Canada. How is it possible that these large, well-regarded trials came to nearly opposite conclusions? Part of this puzzle stems from the population being screened with mammography: healthy women. Only a minority of women (~12%) develop breast cancer in their lives, and an even smaller percentage will actually die due to breast cancer—less than 1% in all of the large screening trials, closer to 0.1% in most of them. Small numbers can result in fickle conclusions, with merely a few more or less breast cancer deaths in either study group drastically altering the outcome.
To further complicate matters, mammography screening—like most screening tests—comes with benefits and downsides, which can vary depending on the age of the screened patient. The primary benefit is obvious: discovering breast cancers early enough for definitive, life-saving treatment. The downsides are more subtle, and include arguments that screening is not cost effective and may do more harm than good. What harm can screening mammography do? Therein lies the crux of this controversy.
If a woman has an abnormal screening mammogram, she will return for a diagnostic mammogram—additional mammogram and ultrasound images. Most of the time, the radiologist can be confident there is nothing concerning after these additional images. In about 1-2% of cases, a needle biopsy will be necessary to determine if a cancer is present, but even then most of the biopsies will not show cancer. Some believe this “false positive” mammogram—an initial screening mammogram was “positive” but no cancer was ultimately present—represents a potential harm of mammography. Women experience anxiety, and sometimes an invasive procedure, when no cancer is present. Others, including most radiologists, believe that the benefit of reduced breast cancer deaths outweighs the short-term anxiety produced in these situations. Additionally, the delayed diagnosis of breast cancer that would occur without screening could mean more women will present with larger tumors or cancer which has already spread, with the associated physical, emotional, and financial costs of treating these late-stage cancers. In the mind of policymakers, the benefits of widespread screening mammography must sufficiently outweigh the large costs and the perceived potential harms.
The group in which these recommendations is most controversial is women ages 40-49. These women have lower rates of breast cancer, and resulting higher “false positive” screening mammograms. Younger women also have denser breasts, making abnormal findings more difficult to visualize.
Even as I write this post, a new article has been published in the New England Journal of Medicine questioning the effectiveness of screening mammography, particularly as it pertains to small tumors. They argue that these small tumors usually don’t grow into larger tumors, and may even regress on their own; when these small tumors are removed, overall patient survival may not be improved. Critics of this article point out that the screening mammography techniques evaluated by the researchers were from 15 years ago. Newer techniques, such as 3-D mammography and breast MRI, can help radiologists further classify these small tumors in ways not possible 15 years ago. Further, advances in pathology can help doctors determine whether these small tumors will be more or less aggressive, and perhaps prevent unnecessary surgery.
Why don’t we sort this out with a new screening mammography trial?
Good question. We might be able to get a definitive answer with a new randomized controlled trial. However, in these types of clinical trials, half of the women would receive routine mammography, but half of the women would not. Tens of thousands of participants would be necessary for an adequate assessment of the effectiveness of screening mammography. It is very difficult to ask tens of thousands of women to defer mammography and risk a higher chance of dying from breast cancer. Most doctors believe there will never be another widespread screening mammography trial.
What does this all mean?
As this controversy has unfolded, several major health organizations have modified recommendations for breast cancer screening, particularly in regard to women ages 40-49. Some current recommendations for women in this age group are as follows:
- American College of Radiology and Society of Breast Imaging: Routine annual screening mammography beginning at age 40.
- National Comprehensive Cancer Network: Routine annual screening mammography beginning at age 40.
- American Cancer Society: Routine annual screening mammography beginning at age 45. Women ages 40-44 should discuss the benefits and downsides of screening with their health care provider, and make an informed decision on annual screening mammography.
- U.S. Preventive Services Task Force: Women ages 40-49 should discuss the benefits and downsides of screening with their health care provider, and make an informed decision on annual screening mammography.
My take on this
I wish there were a clearer answer on the effectiveness of screening mammography. The current controversy and the data surrounding it has been difficult for me to sift through and process, let alone someone without a background in medicine. Allow me to end with a few points of emphasis reflecting my own opinion—shared with other proponents of screening mammography—as to why it should continue to be an option for women beginning at age 40.
- First, we can’t forget the very real decisions women face. Numerous breast cancers and precancerous lesions are discovered each day. Is it possible some of these small cancers may not grow in size, or even regress on their own? The answers seems to be yes. Would I personally want to take that chance, or have a family member take it, with the “watch and wait” approach? Would it be better to be unaware that a small cancer is present (if a screening mammogram was not obtained)? My answer would be to both these questions would be no. I believe women should be able to make individual, informed decisions regarding these choices, with the most information possible.
- Another important point is that screening detects more early cancers, whereas without screening we would arguably see more late-stage, incurable cancers, with the associated pain, suffering, and cost. The counterargument would be that the cost of screening mammography—financially, as well as in the form of anxiety and invasive testing that can accompany “false positive” mammograms—is greater that these downsides of treating late-stage cancer. I don’t agree with this counterargument, and think that the potential risk of increased late-stage cancers outweighs the costs of screening mammography.
- Finally, it is difficult to state how much breast imaging has improved since both the early screening trials and the more recent published criticisms of screening mammography. The advent of digital mammography—not to mention 3-D mammography, advanced ultrasound, and breast MRI—can help to better classify these small tumors and precancerous lesions, and perhaps preclude the need for surgery.
The controversy surrounding screening mammography is, for better or worse, sure to continue. Ultimately, all women should be encouraged to discuss the issues surrounding screening mammography with their health care providers.