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June 2010 - December 2013
II - Mammography
10. X-ray mammography inaccuracy risks
As anyone who saw them knows, mammographic images of breast tissue are all but clear. Layers of overlapping tissue structures are projected onto a 2-D film (or digital) detector and, sometimes, small abnormal tissue structures do not show, or are inconspicuous enough to remain undetected. In such case, test result is called false negative, i.e. does not detect an existing, objectively detectable malignant tissue growth.
Obviously, the consequence is that the missed breast cancer (BC) gets detected at a latter time, and at a more advanced stage.
On the other hand, often times what does appear to be a suspicious spot on the mammogram turns out to be a false alarm. Such test result is called false positive. The consequences are stress and additional test, or tests required.
However, not all false positive test results are corrected at the additional testing. It's been found that breast cancer incidence rates are consistently higher in screened populations than in those not screened, that are comparable with respect to the risk of developing breast cancer. The only explanation for this data is that a portion of the confirmed positive test results by screening mammography are so called pseudo disease, or quasi breast cancer: it appears to be abnormal, malignant growth, but would not have become symptomatic during woman's lifetime.
There are two consequences of this inaccuracy:
(1) an excess of diagnosed cancers that are actually pseudo disease, usually referred to as overdiagnosis, and
(2) correspondingly higher rate of unnecessary treatment, which is a part of mammography-related overtreatment.
Overdiagnosis is only in part caused by the inaccuracy of mammography. The test cannot differentiate between the actually malignant disease, and the one that is not. But sometimes it is also the limitation of the accessory tests used to confirm mammography result.
By itself, overdiagnosis is undesirable for the same reason false positives are, and that is causing unnecessary stress to women labeled with breast cancer. But its consequences are much more serious, since such a woman never learns that what test detected as a malignant growth is, in effect, a benign one, which wouldn't have affected her in any way.
Inevitably, such woman takes on the entire burden of one that actually does have breast cancer, and that includes full-blown breast cancer treatment. This is not only unpleasant experience as any invasive treatment can be, it also can result in lasting damage to her health and wellbeing, such as disfigurement, or increased risk of heart-related death due to the negative effect of radiation therapy on cardiovascular system.
Related to the first two indicators of test accuracy are:
Sensitivity (odds of having existing BC test-detected):
Specificity (odds of having true BC-free status