Mammography is the process of using low-dose X-rays (usually around 0.7 mSv) to examine the human breast. It is used to look for different types of tumors and cysts. Only mammography has been proven to reduce mortality from breast cancer. In some countries routine (annual to five-yearly) mammography of older women is encouraged as a screening method to diagnose early breast cancer.
At this stage mammography is still the modality of choice for screening of early breast cancer. It is the gold-standard for other imaging methods such as ultrasound and NMR-tomography. CT has no real role in diagnosing breast cancer at the present. Ultrasound is useful as an adjunct to mammography in some cases, as is Magnetic resonance imaging, but neither should be used as a screening method. Only Mammography has been proven useful for screening.
During the procedure, the breast is compressed by a dedicated mammography machine, in order to even out the tissue, increase image quality, and to hold the breast still (preventing motion blur). Both front and side images of the breast are taken. Deodorant, talcum powder or lotion may show up on the X-ray as calcium spots, and women are discouraged from applying these on the day of their investigation.
Until some years ago, mammography was typically performed with screen-film cassettes. Now, mammography is undergoing transition to digital detectors. This progress is some years later than in general radiology. This is due to several factors: 1) the higher resolution demands in mammography, 2) significantly increased expense of the equipment, 3) the fact that digital mammography has never been shown to be superior to film-screen mammography for the diagnosis of breast cancer.
As of 2005, only a small percentage of facilities have digital mammography.
In the past several years, the "work-up" process has become quite formalized. It generally consists of screening mammography, diagnostic mammography, and biopsy when necessary. After a screening mammogram, some women may have areas of concern which can't be resolved with only the information available from the screening mammogram. They would then be called back for a "diagnostic mammogram". This phrase essentially means a problems solving mammogram. During this session, the radiologist will be monitoring each of the additional films as they are taken to determine the cause of the abnormal appearance. Ultrasound is often used at this point, as well.
Generally the cause of the unusual appearance is found to be benign. If the cause cannot be determined to be benign with sufficient certainty, a biopsy will be recommended. The biopsy procedure will be used to obtain actual tissue from the site for the pathologist to examine microscopically to determine the precise cause of the abnormality. In the past, biopsies were most frequently done in surgery, under local or general anesthesia. The majority are now done with needles using either ultrasound or mammographic guidance to be sure that the area of concern is the area that is biopsied.
Often women are quite distressed to be called back for a diagnostic mammogram. It helps to know these approximate statistics: of every 1,000 women who are screened, about 7% (70) will be called back for a diagnostic session. About 10 of these will be referred for a biopsy, the remaining 60 are found to be of benign cause. Of the 10 referred for biopsy, about 3.5 will have a cancer, and 6.5 will not. Of the 3.5 who do have cancer, about 2 have a low stage cancer that will be essentially cured after treatment.
It is important to note that while mammography is the only screening method which has been shown to actually save lives, it is not perfect. Estimates of the numbers of cancers missed by mammography are usually around 10-20%. This means that of the 350 per 100,000 women who currently have a breast cancer, about 35-70 will not be seen by mammography. Reasons for not seeing the cancer include observer error, but more frequently it is due to the fact that the cancer is hidden by other dense tissue in the breast and even after retrospective review of the mammogram, cannot be seen. Furthermore, one form of breast cancer, lobular cancer, has a growth pattern that produces shadows on the mammogram which are indistinguishable from normal breast tissue.
Computer-assisted analysis is being tested to decrease the number of cases of cancer that are missed in mammograms. In one test, a computer identified 71% of the cases of cancer that had been missed by physicians. However, the computer also flagged twice as many non-cancerous masses than the physicians did. In a second study of a larger set of mammograms, a computer recommended six biopsies that physicians did not. All six turned out to be cancers that would have been missed. (Destounis, et al., 2004)
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