Exam Results
What is a false negative result?
A false negative result occurs when a cancer is present but is not diagnosed. A breast cancer may be present at the time of the mammogram and be large enough to detect yet still not be spotted by the radiologist who reads and interprets the film. Other small cancers may be present but may be too small to see (less than 5 mm, or the size of a grain of rice) at the time. Yet other cancers are large enough to be seen on a mammogram but are not interpreted as an abnormal finding.
What causes a false negative result?
There are two factors that lead to false negatives: breast density and doctor error.
Breast cancers are easier to locate in postmenopausal women because their breasts have less dense breast tissue and more fat, which the mammogram can "see" through. Most younger (premenopausal) women have dense breasts: the milk ducts occupy much of the space within the breast and have the same density as a solid breast cancer. This means that the mammogram film shows the same overall white color for normal ductal tissue as it shows for a cancer, so real (invasive) cancers are harder to locate. Pre-cancers or benign breast conditions are easier to locate, even in dense breasts, because they may have calcium deposits that appear as sparkly white dots against a dull white background.
The experience, skill, and training of the doctor reading a mammogram also play a role. Radiologists have different criteria for calling a mammogram abnormal, in addition to different levels of experience in interpreting mammograms. Of breast cancers identified at the time of a second mammogram, 25–40% were actually visible on the first mammogram taken two to four years previously. Most were not considered sufficiently suspicious to be called abnormal at the time. A few were obvious problems but were overlooked by the first radiologist.
How can I lower my risk for a false negative mammogram?
No test is 100% accurate. If you notice a lump, nipple discharge, any other unusual symptoms, see your doctor, even if you had a negative mammogram last week, or even if you are scheduled for a routine mammogram next week. A screening test is not a substitute for a physician's appraisal of existing symptoms.
Make sure that your mammography facility is accredited by the American College of Radiology and certified by the Food and Drug Administration under the Mammography Quality Standards Act of 1992. All such facilities (other than Veterans Administration facilities, which have their own stringent standards) are now required to meet minimum standards for equipment and image quality and to be operated by trained personnel. Most facilities have met these professional standards.
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What is a false positive mammogram?
Many mammograms are read as "suspicious." Only a small fraction of suspicious abnormalities on mammograms (5–10%) turn out to be breast cancer. However, each suspicious mammogram requires a follow-up medical visit. Generally, a woman is asked to return for another mammogram, since the first mammogram may have been unclear or difficult to read. Other visits may require a doctor's appointment for a physical exam and possibly other tests. Common follow-up test procedures include sonograms, needle biopsies, or surgical biopsies. Most women who undergo these procedures find out that no breast cancer is present, so the original suspicious mammogram is classified as a "false positive" test result. In some instances, the follow-up can involve repeated testing and months of uncertainty before the doctor is able to tell for sure that no cancer is present. The experience is often frightening and stressful. If you have a suspicious mammogram, remember that it is not all that uncommon. Nor does it mean that you have cancer.
How common are false positive test results?
The rate varies from one mammography center to another. On average, in the United States, one mammogram in 10 results in the patient being called back for further testing. A woman who gets 10 mammograms at an average testing site will have a 65% chance of being called back at least once because of a false positive test result. A woman who gets 10 mammograms at one of the more accurate testing sites will have a 40–50% chance of having at least one false positive test result. Each such false positive test involves a 30% chance of having a sonogram (ultrasound), a 30% chance of a repeated physical exam or surgical consultation, and a 30% chance of having a biopsy.
The false positive rate averages 5–15% at most mammography facilities in the United States. A few sites report rates much higher than this, upwards of 50%, but this rate may have fallen as accreditation standards have been tightened. Quality-of-care guidelines for mammography facilities recommend aiming for a callback rate of 5–10%.
How can I lower my risk for a false positive mammogram?
To lower the risk of a false positive mammogram, have the test done at a location where a board-certified radiologist is present at all times and interprets the films as soon as they are taken. If the film is incomplete or fuzzy, the mammogram can be repeated immediately before the patient gets dressed and goes home. Most callbacks for abnormal mammograms result from a mistake made during the test procedure that made it impossible to read the mammogram. A few breast-screening centers provide instant access to a follow-up sonogram (ultrasound) if the mammogram reveals a solid mass. Most of these masses are run-of-the-mill cysts, and the sonogram can show that immediately. This capability keeps the patient from worrying needlessly. Lower-cost mass screening programs generally perform a large number of mammograms quickly, then have all of the films reviewed together at a later time. This means that the women whose mammogram films are unreadable will have to be called back for another test session. The waiting period before the next appointment can create a lot of anxiety if the woman is not told that the problem is only with the quality of the film.
Another way to lower the risk of a false positive mammogram is to locate a mammography facility where each film is read independently by two board-certified radiologists who limit their practice to breast imaging. When two highly experienced specialists agree that a spot or shadow on a mammogram film is worth closer attention, it probably is.
