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False positives
The goal of any screening procedure is to examine a large population of patients and find the small number most likely to have a serious condition. These patients are then referred for further, usually more invasive, testing. Thus a screening exam is not intended to be definitive: It is intended to have a high sensitivity so as to not miss any cancers. The cost of this high sensitivity is a relatively large number of results that would be regarded as suspicious in patients without disease. This is true of mammography. The patients called back for further testing from a screening session (about 7%) are sometimes referred to as "false positives", implying an error. In fact, it is essential to call back many healthy patients for further testing to capture as many cases of cancer as possible. These call backs should not be regarded as errors. (See "Results" above.) Nonetheless, some women who receive false-positive results become anxious, worried and distressed about the possibility of having breast cancer, feelings that can last for many years.
False negatives
At the same time, mammograms also have a rate of missed tumors, or "false negatives." Accurate data regarding the number of false negatives are very difficult to obtain, simply because we cannot perform mastectomies on every woman who has had a mammogram to determine the false negative rate accurately. Estimates of the false negative rate depend on close follow-up of a large number of patients for many years. This is difficult in practice, because many women do not return for regular mammography making it impossible to know if they ever developed a cancer. Dr. Samuel S. Epstein, in his book, The Politics of Cancer, claims that in women ages 40 to 49, one in four instances of cancer is missed at each mammography. Researchers have found that breast tissue is denser among younger women, making it difficult to detect tumors. For this reason, false negatives are twice as likely to occur in premenopausal mammograms (Prate.) This is why the screening program in the UK does not start calling women for screening mammograms until the age of 50.
The importance of these missed cancers is not clear, particularly if the woman is getting yearly mammograms. Research on a closely related situation has shown that small cancers that are not acted upon immediately, but are observed over periods of even several years, will have good outcomes. A group of 3,184 women had mammograms which were formally classified as "probably benign." This classification is for patients who are not clearly normal but have some area of minor concern. This results, not in the patient being biopsied, but having early follow up mammography every six months for three years to guarantee no change. Of these 3,184 women, 17 (0.5%) did have cancers. Most importantly, when the diagnosis was finally made, they were all still stage 0 or 1, the earliest stages. Five years after treatment, none of these 17 women had evidence of recurrence. Thus, small early cancers, even though not acted on immediately, were still entirely curable (Sickles, AJR, 179:463-468, 1991).
Regardless of the precise number of false negatives, it is very clear that even if some tumors are missed, lives are saved when they are found. Women need to understand that a negative mammogram is not a perfect guarantee that there is no breast cancer present, but it is the best method we have available.
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Important notice:
The content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other
qualified health provider with any questions you may have regarding a medical condition.
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