Article: Mammography

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Like all x-rays, mammograms use doses of ionizing radiation to create this image. Radiologists then analyze the image for any abnormal growths. Despite continuous improvements and innovations, mammography has garnered a sizable opposition in the medical community because of an error rate that is still high and the amount of harmful radiation used in the procedure. (Gofinan) It is normal to use longer wavelength X-rays (typically Mo-K) than those used for radiography of bones.

Mammography has a false-negative (missed cancer) rate of at least 10 percent. This is partly due to dense tissues obscuring the cancer and the fact that the appearance of cancer on mammograms has a large overlap with the appearance of normal tissues.

At this time, mammography is still the modality of choice for screening for early breast cancer. It is the gold-standard which other imaging tests are compared with. CT has no real role in diagnosing breast cancer at the present. Ultrasound, Ductography, and Magnetic Resonance are adjuncts to mammography. Ultrasound is typically used for further evaluation of masses found on mammography or palpable masses not seen on mammograms. Ductograms are useful for evaluation of bloody nipple dischage when the mammogram is non-diagnostic. MRI can be useful for further evaluation of questionable findings, or sometimes for pre-surgical evaluation to look for additional lesions.

Procedure

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.

"Work-up" process

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 problem-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.

One study shows that needle biopsies may increase the likelihood that cancer will spread, if indeed the tumor is malignant. [1]

Results

Normal (left) versus cancerous (right) mammography image.

Often women are quite distressed to be called back for a diagnostic mammogram. Most of these recalls will be false positive results. It helps to know these approximate statistics: of every 1,000 US 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)

While data is accumulating suggesting that CAD can find a few additional cancers, this should be put in perspective. The additional find rate was 20%, thus in a group of 1000 women who will have about 4 cancers, CAD may help find an additional 0.8. The types of additional cancers that may be found are likely to be early and small. As of 2006, there has been no data to show that finding these additional cancers will have any effect on survival rate. Some feel that these cancers are likely to be found at the next screening, still at a curable stage, and therefore it remains to be proven whehter CAD will be eventually found to have any effect on patient outcome.

Risks

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 sreening 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 in order to capture as many cases of cancer as possible, and these call backs should not be regarded as errors. (see above: "results")

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 is 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).

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 which are not acted upon immediately, but are observed over periods of even several years, will have good outcomes. A group of 3184 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 6 monts for three years to guarantee no change. Of these 3184 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 currently.

Many critics of mammography cite the hazardous health effects of radiation. In 1976, the controversy over radiation and mammography reached a saturation point. At that time mammographic technology delivered five to 10 rads (radiation-absorbed doses) per screening, as compared to 1 rad in current screening methods. In women between the ages of 35 and 50, each rad of exposure increased the risk of breast cancer by one percent, according to Dr. Frank Rauscher, then-director of the NCI.

According to Russell L. Blaylock, MD, one estimate is that annual radiological breast exams increase the risk of breast cancer by two percent a year. So over 10 years the risk will have increased 20 percent. In the 1960s and 70s, women, even those who received 10 screenings a year, were never told the risk they faced from exposure. In the midst of the 1976 radiation debate, Kodak, a major manufacturer of mammography film, took out full-page ads in scientific journals entitled About breast cancer and X-rays: A hopeful message from industry on a sober topic.

Despite better technology and decreased doses of radiation, scientists still claim mammography is a substantial risk. Dr. John W. Gofman, an authority on the health effects of ionizing radiation, estimates that 75 percent of breast cancer could be prevented by avoiding or minimizing exposure to the ionizing radiation. This includes mammography, x-rays and other medical and dental sources.

Spread of Cancer:

One study shows that needle biopsies may increase the likelihood that cancer will spread, if indeed the tumor is malignant. [2]

The risk of radiation is apparently higher among younger women. The NCI released evidence that, among women under 35, mammography could cause 75 cases of breast cancer for every 15 it identifies. Another Canadian study found a 52 percent increase in breast cancer mortality in young women given annual mammograms. Dr. Samuel Epstein also claims that pregnant women exposed to radiation could endanger their fetus. He advises against mammography during pregnancy because "the future risks of leukemia to your unborn child, not to mention birth defects, are just not worth it."

While the number of deaths caused by breast cancer has decreased, the incidence of breast cancer is still rising. Since 1940, the incidence of breast cancer has risen by one to two percent every year. Between 1973 and 1991, the incidence of breast cancer in females over 65 rose nearly 40 percent in the United States.

Some researchers attribute this increase to better detection technologies; i.e., as the number of women screened for breast cancer rises, so does the number of reported cases. Other analysts say the correlation between mammographic screening and increases in breast cancer is much more ominous, suggesting radiation exposure is responsible for the growing number of cases. While the matter is still being debated, Professor Sandra Steingraber offers ways to navigate these statistics. According to Steingraber, the rise in breast cancer predates the introduction of mammograms as a common diagnostic tool. In addition, the groups of women in whom breast cancer incidence is ascending most swiftly -- blacks and the elderly -- are also least likely to get regular mammograms.

The majority of health experts agree that the risk of breast cancer for women under 35 is not high enough to warrant the risk of radiation exposure. For this reason, and because the radiation sensitivity of the breast in women under 35 is possibly greater than in older women, most radiologists will not perform screening mammography in women under 40. However, if there is a significant risk of cancer in a particular patient (BRCA posiitve, very positive family history, palpable mass), mammography may still be important. Often, the radiologist will try to avoid mammography, by using ultrasound, or MRI imaging.

Similarly, the risk of breast cancer to women over 55 very clearly justifies the risk of mammograms. The statistics about mammography and women between the ages of 40 and 55 are the most contentious. A 1992 Canadian National Breast Cancer Study showed that mammography had no positive effect on mortality for women between the ages of 40 and 50. In fact, the study seemed to suggest that women in that age group are more likely to die of breast cancer when screened regularly. This study, however, is the only study to find this result. The study's critics pointed out that there were very serious design flaws in the study that invalidated these results. While screening between 40 and 50 is still controversial, the preponderence of the evidence indicates that there is some small benefit in terms of early detection. Currently, the American Cancer Society and the American College of Radiology encourage mammograms every two years for women ages 40 to 49. Trying to settle this debate, a 1997 consensus panel appointed by the NIH was unable to come to a conclusion as to whether there was definite benefit between the ages of 40 and 50. The panel advised women to weigh the risks with their doctors and decide for themselves. (Prate)