Understanding the Limitations of Mammograms

Patient Expert

“A mammogram saved my life”  This proclamation echoes through debates about mammogram screening guidelines.  It may be true when the mammogram detects an aggressive tumor.  Sometimes it just feels true.  No one can know what would have happened if a mammogram missed a slow-growing tumor.  That tumor might have never caused any trouble.  Or it might have grown at a sluggish pace only to be discovered when a woman felt it, had a lumpectomy and moved on with her life.

Outdated data

When Dr. William Halsted started operating on breast cancer patients in the 1880’s, he believed that a tumor needs to be cut out completely along with any surrounding tissue because cancer grows out progressively from a single location.  If it can be removed before it gets into the lymphatic system, the patient has a good chance of survival.

That’s the theory that mammography screening is based on.  Catch tumors early before they can spread.  Unfortunately, that theory is not correct for all types of breast cancer.  Some tumors are so aggressive that they have spread their deadly cells beyond their original location well before they can be detected.

Comparing screening programs effectiveness

A recent study by Dr. Gilbert Welch and his colleagues in the New England Journal of Medicine illustrates this problem.  Dr. Welch wanted to see how effective cancer screening has been in detecting and reducing metastatic disease for breast and prostate cancers.  Women and men are widely screened for these cancers, so years of screening should have reduced the number of patients diagnosed at the metastatic stage.

The study looked at the rates of metastatic breast cancer diagnoses for women between 1975 and 2012.  Despite widespread screening begun between 1980 and 1990, the metastatic breast cancer rate has remained essentially unchanged.  While mammography catches many breast cancers, it hasn’t been very useful at finding the ones that spread to distant organs.  Dr. Gilbert explains, “screening mammography has been unable to identify at an earlier stage, before symptoms appear, cancers that are destined to become metastatic. In fact, the mean age at diagnosis among women 40 years of age or older hasn't changed over the past 37 years, remaining at 63.7 years.”

For prostate cancer, the results are dramatically different.  When PSA screening became widespread in about 1998, the rate of metastatic prostate cancer briefly spiked as the test picked up previously undetected cancers.  Then it plummeted by more than half in the next few years before leveling off.

There are some limitations to this kind of comparison.  Breast cancer screening looks only at the breast while prostate cancer screening is based on a blood test, so it makes sense that prostate cancer screening might be more effective at picking up aggressive forms of the disease early.

Changing guidelines

Studies like this one can help us understand why organizations are changing their guidelines for mammogram screening.  The American Cancer Society has just changed its recommendations.  Now they recommend annual mammograms from age 45-55 for a woman with average risk, and mammograms every other year for women over 55 because breast cancers tend to be slower growing in older women.  The complexity of the data also explains why different organizations are coming up with different guidelines.

Mammograms are not very effective in younger women whose breast tissue tends to be denser, so the controversial age range is from 40-50.  Most medical organizations still recommend mammograms for women over fifty although they are disagreeing about how frequently and the most effective upper age limit.

To the woman who believes that she or a family member has been saved by an annual screening mammogram, this is all nonsense.  To her there is no drawback to a false positive test that leads to a painful biopsy as long as it finds either no cancer or a tiny, treatable one.  A woman who has gone through multiple call backs and biopsies for cysts or benign tumors may welcome this opportunity to avoid the pain and anxiety of false positives.

Then there is the issue of “average risk.”  Before my inflammatory breast cancer diagnosis, I was at below average risk for breast cancer.  I faithfully had my screening mammograms.  Yet neither they nor the diagnostic mammograms I had after I experienced strange breast symptoms found my breast cancer.  Mammograms are not infallible, and a woman may ignore breast changes relying on the idea that her mammogram last month was just fine.

It is important for us to understand that the data for the widespread use of screening mammograms has not found them to be effective in reducing breast cancer mortality.  Perhaps a better use of public health dollars would be more treatment facilities that are affordable and easily accessible.  To some women, mammograms don’t need to reduce the death rate to be effective.  They consider finding a cancer in time to have less extensive surgery, chemo, or radiation to be a worthwhile goal in and of itself.

We also need to remember that none of this discussion is about diagnostic mammograms.  When a woman feels a lump or notices any sort of breast change, a mammogram is an important part of the diagnostic process in finding out what is wrong.

Yet each woman still has to sit down with a doctor and decide what screening mammogram schedule is best for her.  Some issues to include in that discussion should include:

  • What is my breast cancer risk?  If you have many breast cancer risk factors, you may want a more frequent screening schedule.  However, even if you are at low risk, be proactive about your breast health.  Low risk doesn’t meant no risk.
  • If I have other people in my family who had breast cancer, how old were they when they were diagnosed? Many doctors suggest starting mammograms ten years younger than your first degree relative (mother or sister) was when she was diagnosed.
  • How dense are my breasts? Dense breasts are a risk factor for breast cancer, but they also make mammograms less successful in detecting it.  If you have dense breasts, talk to your doctor about whether ultrasounds or breast MRI’s should be part of your screening program.
  • Do I feel more stressed by the idea of missing a small, slow-growing cancer or by undergoing a biopsy for a benign lump? The emotional component to screening decisions is important.  There is no “right” way to feel.  Much of the recent discussion of the drawbacks to mammography has focused on the emotional stress of false positives, but adult women should be respected in their ability to handle tough decisions.
  • How important is it to me to catch a possible cancer early enough to have a lumpectomy compared to more extensive surgery and treatment? Although you know that screening may not make a difference in how long you live, you may put a high priority on avoiding a mastectomy and/or chemo.  Your doctor should take your feelings on this issue into account when setting up your screening schedule.

While the varying mammogram screening guidelines are confusing, they are leading to one excellent outcome.  They are putting responsibility in the hands of women to educate themselves and to decide their own priorities in managing their health care.  Mammograms are one important part of being proactive about health.  Seeing the doctor about all breast changes is even more crucial to catching cancer in time to save your life.

For more helpful articles on mammograms:

American Cancer Society New Mammogram Guidelines – Later, Fewer, Farther Between

Breast Density:  Why You Should Care

Mammogram Research to Help Older Women Make Decisions

Your First Mammogram:  Easy as 1-2-3


Welch, G. et al.  Trends in Metastatic Breast and Prostate Cancer--Lessons in Cancer Dynamics.  The New England Journal of Medicine. October 20, 2015.  Accessed from http://www.nejm.org/doi/full/10.1056/NEJMp1510443#t=article November 6, 2015.