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Mammography Revisited

The goal of breast imaging is to reduce deaths due to breast cancer by detecting breast cancer early, when treatment is more effective and less harmful. Simply put, imaging diagnostics are designed to reduce the incidence of advanced disease. They do not prevent disease, but rather screen for pathology. However, because many types of breast imaging involve radiation exposure, their benefits must be balanced with their risks.

Despite what the medical media reports, breast cancers are rarely diagnosed at their very early stages. This is in part due to the fact that tumors smaller than 1 centimeter are not detectable by X-rays, mammography, X-ray computed tomography (CT), or ultrasound. (The exception to this size limitation is molecular breast imaging.)

Breast cancers found with high-quality, 2-D digital mammography are commonly within median size 1.0 to 1.5 cm (0.4 to 0.6 inches, or the size of a small marble).1 Approximately 10% of invasive cancers 1 cm in size or smaller have already spread to lymph nodes at the time of detection, compared to close to 35% of those 2 cm in size and 60% of those 4 cm or larger in size.2 It is true that imaging diagnostics generally detect tumors before they become palpable with breast exam. Breast cancers found by clinical breast examination, or by a woman herself, have a median size of 2 to 2.5 cm. Such cancers are more likely to be later stage breast cancers that are more likely to have already spread to the axillary lymph nodes and to be problematic.3

Despite their radiation risk, false positive, and false negative readings, mammograms are still the most commonly used breast imaging diagnostic in medicine today. Fierce debates about the benefits and harms of mammography have been played out in medical journals and the mainstream media for several decades. Many researchers believe that the benefits of screening outweigh the harms (e.g., over-diagnosis, radiation exposure), while others contend the opposite. Additionally, different organizations have considerably diverse recommendations as to frequency of mammography screening, particularly in relationship to age. Over the past few years, many organizations have changed their mammography recommendations and become more prudent and conservative. This is in part due to an independent review published in 2012 in The Lancet, which is recognized as one of the largest and longest studies of mammography to date.4 This study involved 90,000 women who were followed for a period of 25 years. The study concluded that mammograms have absolutely no impact on breast cancer mortality. These conclusions were widely publicized at the time and debated. They showed that the death rate from breast cancer was virtually identical between those who received an annual mammogram and those who did not, while 22% of screen-detected invasive breast cancers were over-diagnosed, leading to unnecessary treatment. Subsequently, several organizations changed their mammogram recommendation protocols.

The following are mammography recommendations of the United States Preventive Services Task Force5 (USPSTF) and the Canadian Task Force on Periodic Health Examination6 (CTFPHC). Both are independent scientific organizations whose volunteer panel members do not receive funding from the mammography industry or from other companies that would entail financial conflicts of interest.

The current United States Preventive Services Task Force mammography recommendations:7

  • Women, ages 75 years and older: The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of screening mammography.

  • Women, ages 50 to 74 years: biennial mammogram screening (every two years) is recommended;

  • Women, before the age of 50 years: The decision to start regular, biennial screening mammography should be an individual one and take patient context into account, including the patient’s values regarding specific benefits and harms.

The USPSTF further states on their website that

While screening mammography in women aged 40 to 49 years may reduce the risk for breast cancer death, the number of deaths averted is smaller than that in older women and the number of false-positive results and unnecessary biopsies is larger. The balance of benefits and harms is likely to improve as women move from their early to late 40s. In addition to false-positive results and unnecessary biopsies, all women undergoing regular screening mammography are at risk for the diagnosis and treatment of noninvasive and invasive breast cancer that would otherwise not have become a threat to their health, or even apparent, during their lifetime (known as “over-diagnosis”). Beginning mammography screening at a younger age and screening more frequently may increase the risk for over-diagnosis and subsequent overtreatment.

Meanwhile, the CTFPHC specifically “recommends against mammography screening of women aged 40 to 49.” They contend that “because women aged 40 to 49 are at lower risk of cancer, the absolute benefit is lower for this age group than for older women. Screening in women aged 40 to 49 reduces the absolute risk of dying from breast cancer by 0.05%.”8 In the judgment of the CTFPHC:

Most women 40 to 49 should not receive screening, but many could receive it. The risk of having a false positive mammogram requiring further screening is 1 in 3 (the risk of a false-positive result from mammography is higher for women younger than 50 years). The risk of having a biopsy is 1 in 28. The risk of having all or part of the breast removed unnecessarily is 1 in 20. For every 1000 women aged 39 years and older who are screened using mammography, 5 will have an unnecessary lumpectomy or mastectomy as a result of over diagnosis. Generally, the proportion of false positives is high when screening younger women, and there have even been suggestions that early screening may increase mortality.9

The CTFPHC recommends “for women aged 50 to 69 years routinely screening with mammography every 2 to 3 years.” They contend that “the absolute benefits of screening remain small among women aged 50–69 years but are greater than those seen in women aged 40–49. Screening in women aged 50–69 reduces the absolute risk of dying from breast cancer by 0.13%.”10

Lastly, the CTFPHC recommends “for women aged 70 to 74 years old screening with mammography every 2 to 3 years.” They contend “the reduction in relative risk of death from breast cancer associated with mammography for women 70–74 years old is statistically non-significant and similar to that seen for younger women.”11 Hence, according to CTFPHC, mammography confers less than 1% reduction of risk of dying from breast cancer from ages 40 to 69.

One factor that creates mammogram recommendation differences according to age is that women under 50 typically have denser breast tissue. On a mammogram, denser breast tissue appears white, the same color as cancer. In addition to breast tissue, all other components of the breast (glands, connective tissue, tumors, calcium deposits, etc.) also appear as shades of white on a mammogram. With menopause, the dense tissue in women's breasts is replaced with fatty tissue, which looks gray on a mammogram. It is much easier to see the white cancer against this gray background. This has led to legislation, or “breast density laws”, to be passed in California, Connecticut, New York, Virginia, and Texas, which make it mandatory for radiologists to inform their patients who have dense breast tissue that mammograms are basically useless for them. A law is currently being considered at a federal level, which would inform all women across the country.

Mammography and X-ray Computed Tomography (CT) Scans: Balancing Benefits and Risks

Cancer induction is arguably the most important and the most feared radiation effect from medical imaging diagnostics. Radiation effects have a latency period between the time of exposure and the onset of the effect. For cancer induction, the latency period is on the order of years, with leukemia having the shortest latency period (5 to 15 years) and solid tumors having the longest latency period (10 to 60 years). However, it is not known how quickly low-level radiation can accelerate the growth of a precancerous lesion such as DCIS. Generally, it is very difficult to prove that a cancer is directly related to earlier radiation exposure, because other factors encountered during the latency period may be the actual cause of the cancer. This is particularly true when the exposures are at low radiation levels such as those received in diagnostic radiology studies.

Diagnostic X-rays including mammograms are the largest man-made source of radiation exposure to the general population, contributing about 14% of the total annual exposure worldwide from all sources. Although radiographic scans provide important diagnostic information, their use involves some small risk of developing cancer. Amy Berrington de González and colleagues at the Johns Hopkins Bloomberg School of Public Health in Baltimore examined the extent of this risk based on the annual number of diagnostic X-rays undertaken in the UK and in 14 other developed countries.

Their findings concluded:

There are no benefits for mammography in women under the age of 30, and only a marginal benefit for women between the ages of 30 and 34… Our results indicate that in the U.K. about 0.6% of the cumulative risk of cancer to age 75 years could be attributable to diagnostic X-rays. This percentage is equivalent to about 700 cases of cancer per year. In 13 other developed countries, estimates of the attributable risk ranged from 0.6% to 1.8%, whereas in Japan, which had the highest estimated annual exposure frequency in the world, it was more than 3%.12

In another study it was shown that breast cancer rates increased significantly in four Norwegian counties after women began getting mammograms every two years. In fact, according to background information in the study, the start of screening mammography programs throughout Europe has been associated with increased incidence of breast cancer.13

In general, radiation dose from all medical imaging has come under recent scrutiny in the medical and lay press. This is primarily the result of recent articles on the increased cancer risks associated with cumulative mammography exposure and CT scans.14, 15 In another study on cancer risks from CT scans, Berrington de González and colleagues estimated that 29,000 future cancers (approximately 2% of the cancers diagnosed annually in the U.S.) could be related to CT performed in the U.S. in 2007.16 This is comparable to recent estimates of 1.5% to 2.0% by Brenner and Hall.17

The late Dr. John W. Gofman, an authority on the health effects of ionizing radiation, estimated that 75% of breast cancer could be prevented by avoiding or minimizing exposure to ionizing radiation.18 This included mammography, X-rays, CTs, and other medical and dental sources. Often called the father of the antinuclear movement, Dr. Gofman and his colleague Arthur R. Tamplin at Lawrence Livermore National Laboratory, developed data in 1969 showing that the risk from low doses of radiation was 20 times higher than stated by the government.19 Their publication of the data, despite strong efforts to censor it, led them to lose virtually all their research funding and, eventually, their positions at the government laboratory. To this day, the cumulative risk of medical radiation exposure is considerably understated.

Ionizing radiation is a known cause of cancer and genetic mutation, and the effects of small amounts of radiation have a negative cumulative effect on the body. So, it seems amazing that mainstream medicine frequently dismisses the idea that medical imaging tests from mammograms to CT scans could play much of a role in causing breast cancer. This does not mean that an individual should never have X-rays or radiographic scans, but rather, that it is wise to be thoughtful about radiation exposure. The risk of harm from radiation is highest in tissue where cells are rapidly changing, such as the growing breast tissue of adolescent females. The chance of ionizing radiation causing genetic damage or increasing the risk of cancer is related to the total amount of radiation accumulated by a person.20 The greatest risk comes from:

  • larger doses of radiation21, such as CT scans;

  • cumulative exposure to radiation, such as over several years;

  • high-strength forms of radiation, as seen in radiation therapy.

False Positives and Over-diagnosis

When a mammogram shows an abnormal area that appears to look like cancer but turns out to be normal, it is called a false positive. Of course, the good news is this means no breast cancer, but the suspicious area, usually involving calcifications, requires follow-up with more than one doctor, extra tests involving more radiation exposure, and possibly an invasive biopsy.

In 2001, it was reported that as many as three quarters of all post-mammogram biopsy results are merely benign lesions.22 A 2012 study published in the New England Journal of Medicine analyzed the effects of mammogram screening in the U.S. over the past three decades, and concluded that 1.3 million women were misdiagnosed and mistreated as a result.23 The researchers found that the number of early-stage breast cancers detected have doubled over the past 30 years since the advent of mammography, from 112 to 234 cases per 100,000. The authors concluded:

After excluding the transient excess incidence associated with hormone-replacement therapy and adjusting for trends in the incidence of breast cancer among women younger than 40 years of age, we estimated that breast cancer was over-diagnosed (i.e., tumors were detected on screening that would never have led to clinical symptoms) in 1.3 million U.S. women in the past 30 years. We estimated that in 2008, breast cancer was over-diagnosed in more than 70,000 women; this accounted for 31% of all breast cancers diagnosed. Despite substantial increases in the number of cases of early-stage breast cancer detected, screening mammography has only marginally reduced the rate at which women present with advanced cancer. Although it is not certain which women have been affected, the imbalance suggests that there is substantial over-diagnosis, accounting for nearly a third of all newly diagnosed breast cancers, and that screening is having, at best, only a small effect on the rate of death from breast cancer.

This conclusion concurs with the Cochrane Collaboration Review, published in 2013, which also found no evidence that mammography screening influences overall mortality.24 These studies cast a shadow on mammography efficacy and calls into question whether mammography screening really benefits women. According to the authors of the Cochrane Review:

If we assume that screening reduces breast cancer mortality by 15% and that over-diagnosis and overtreatment is at 30%, it means that for every 2000 women invited for screening throughout 10 years, one will avoid dying of breast cancer and 10 healthy women, who would not have been diagnosed if there had not been screening, will be treated unnecessarily. Furthermore, more than 200 women will experience important psychological distress including anxiety and uncertainty for years because of false positive findings.

These studies are bringing mainstream attention to the possibility that mammography may have caused more harm than good in the millions of women who have employed it over the past 30 years as their primary strategy in detecting breast cancer. The adverse health effects associated with over-diagnosis and consequent invasive biopsies, and even overtreatment with lumpectomy, radiation, chemotherapy, and hormone-suppressive treatments, cannot be underestimated, especially when one considers the profound psychological trauma that follows each stage of diagnosis and treatment.

False Negatives

Mammograms detect tumors but can miss more than a quarter of all breast cancers.25 Dr. Samuel S. Epstein, in his book The Politics of Cancer,26 claims that in women ages 40 to 49, one in four instances of cancer are missed during each mammography. In general, breast screening with full-field digital mammography (FFDM) fails to detect 15-30% of cancers.27 This figure is higher for women with dense breasts. Hence, false negatives are twice as likely to occur in mammograms of premenopausal women.

Newer Mammography Machines

Digital cameras have replaced the older, film-based cameras for most North Americans. Mammography has also gone digital. Clinics now offer 2-D digital mammograms which replace X-ray film with solid-state detectors that convert X-rays into electric signals. As of October 2015, over 95% of accredited mammography machines in the U.S. were digital.28 The primary advantage of 2-D digital over film mammography is that the electrical signals used to produce images can be electronically manipulated. A radiologist or physician can zoom in to magnify and optimize different parts of breast tissue without having to take an additional image. The electronic images can also be readily shared with clinicians at other locations, which may particularly benefit rural and underserved communities using telemedicine for reading and interpreting these mammograms. 2-D digital mammograms have a slightly lower radiation dose than film. Hence, digital mammograms are also an improvement in clinical efforts to reduce women’s exposure to radiation, but they still carry risk of radiation exposure and false positive and negative results.

When digital imaging was first introduced as an alternative to analog film-screen radiography, technology enabled only small “spot views”. Larger digital detectors were then developed that permitted imaging an entire small breast, but multiple images were required for larger breasts, requiring added radiation and time. The latter limitation was partially overcome by mammographic equipment using fan-beam technology. Eventually, larger digital detectors became available, thus enabling full-field imaging or full-field digital mammography (FFDM).

Millions of women are now undergoing a newer type of mammography, known as digital breast tomosynthesis (DBT). Sometimes called 3-D mammography, DBT takes many X-rays at different angles to create a three-dimensional image of the breast. In both 3-D and 2-D mammograms, the breast is compressed between two plates. In 2-D mammograms, which take images only from the front and side, this may create images with overlapping breast tissue. Because 3-D mammography provides images of the breast in slices from many different angles, finding abnormalities and determining which abnormalities may be important may be easier with 3-D imaging. On the other hand, 3-D mammography is more expensive than 2-D. In one study it was concluded the radiation dose of patients undergoing a single-view DBT was comparable to a single-view full-field digital mammography (FFDM). For patients with thicker breasts, the radiation dose of DBT was slightly lower than FFDM.29 Basically, the negative issues with DBT as a screening tool include additional reading time, IT storage and connectivity limitations, over-diagnosis, and cost effectiveness.30

Cancer screening does not equal cancer prevention, and although early detection is important, using a screening method that in and of itself increases the risk of developing cancer is simply not good medicine. After advancements in MRI, ultrasound imaging, and thermography (infrared and contact), mammograms have become an antiquated and questionably appropriate cancer screening diagnostic. However, most clinics and hospitals are financially invested in mammographic equipment. It will take some time to change over to MRI in place of mammography, but it will eventually happen. In North America, women are still urged to get an annual mammogram starting at the age of 40, even though updated guidelines set forth by the U.S. Preventive Services Task Force in 2009 urge women to wait to get a mammogram until the age of 50, and to only get bi-annual screening thereafter. Unfortunately, many women are completely unaware that current research does not support the use of routine mammograms to prevent breast cancer mortality.