Mammography is the process of using low-dose amplitude-X-rays (usually around 0.7 mSv) to examine the human breast and is used as a diagnostic as well as a screening tool. The goal of mammography is the early detection of breast cancer, typically through detection of characteristic masses and/or microcalcifications. Mammography is believed to reduce mortality from breast cancer. No other imaging technique has been shown to reduce risk, but breast self-examination (BSE) and physician examination are considered essential parts of regular breast care.
In many countries routine mammography of older women is encouraged as a screening method to diagnose early breast cancer. The United States Preventive Services Task Force recommends screening mammography, with or without clinical breast examination, every 1–2 years for women aged 50 and older.[1] Altogether clinical trials have found a relative reduction in breast cancer mortality of 20%, but the two highest-quality trials found no reduction in mortality.[2] Mammograms have been controversial since 2000, when a paper highlighting the results of the two highest-quality studies was published.[3]
Like all x-rays, mammograms use doses of ionizing radiation to create images. Radiologists then analyze the image for any abnormal findings. It is normal to use longer wavelength X-rays (typically Mo-K) than those used for radiography of bones.
At this time, mammography along with physical breast examination is the modality of choice for screening for early breast cancer. Ultrasound, ductography, positron emission mammography (PEM), and magnetic resonance imaging 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 still used in some institutions for evaluation of bloody nipple discharge when the mammogram is non-diagnostic. MRI can be useful for further evaluation of questionable findings as well as for screening pre-surgical evaluation in patients with known breast cancer to detect any additional lesions that might change the surgical approach, for instance from breast-conserving lumpectomy to mastectomy. New procedures, not yet approved for use in the general public, including breast tomosynthesis may offer benefits in years to come.
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.
Procedure
During the procedure, the breast is compressed by a dedicated mammography machine to even out the tissue, to 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, known as Full Field Digital Mammography (FFDM). This progress is some years later than in general radiology. This is due to several factors:
- the higher resolution demands in mammography,
- significantly increased expense of the equipment,
- the fact that digital mammography has never been shown to be superior to film-screen mammography for the diagnosis of breast cancer.
Computed radiography (CR) may help speed the transition. CR allows facilities to continue to use their existing screen-film units but do the cassettes with an imaging plate that acts as a digital adapter.
As of March 1, 2007, 18.3% of facilities in the United States and its territories have at least one FFDM unit.[4] (The FDA includes computed radiography units in this figure.[5])
"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, often performed via stereotactic core biopsy or ultrasound-guided core biopsy. 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 by a technologist. Depending on the nature of the finding, ultrasound may 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. These core biopsies require only local anesthesia, similar to what would be given during a small dental procedure.
One study shows that needle biopsies of liver malignancies rarely increase the likelihood that cancer will spread, and has not been found to occur with breast needle biopsies.[6]
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 U.S. women who are screened, about 7% (70) will be called back for a diagnostic session (although some studies estimate the number closer to 10%-15%). About 10 of these individuals 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. Mammogram results are often expressed in terms of the BI-RADS Assessment Category, often called a "BI-RADS score." The categories range from 0 (Incomplete) to 6 (Known biopsy – proven malignancy). In the UK mammograms are scored on a scale from 1-5 (1 = normal, 2 = benign, 3 = indeterminate, 4 = suspicious of malignancy, 5 = malignant).
While mammography is the only breast cancer screening method that has been shown to save lives, it is not perfect. Estimates of the numbers of cancers missed by mammography are usually around 10%–30%. This means that of the 350 per 100,000 women who have breast cancer, about 35-70 [sic 35-105] will not be seen by mammography. Reasons for not seeing the cancer include observer error, but more frequently it is because 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-aided diagnosis (CAD) are 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.[7] Generally, CAD systems in screening mammography have poor specificity and compare poorly to double reading (Taylor P, Champness J, Given-Wilson R, Johnston K, Potts H (2005). Impact of computer-aided detection prompts on the sensitivity and specificity of screening mammography. Health Technology Assessment, 9(6)).
While data are 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 10,000 women who will have about 40 cancers, CAD may help find an additional 8. The types of additional cancers that may be found are likely to be early and small.[citation needed] As of 2006, there have 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 whether CAD will be eventually found to have any effect on patient outcome.
A study released October 1, 2008, by British researchers revealed that using CAD in conjunction with a single reading by a physician may be as beneficial as a second reading by a physician. The study of 31,000 women, the largest of its kind to date, determined that the find rate for a single physician in conjunction with CAD as compared to two physicians was nearly identical.[8] Out of 227 cancers found, the CAD method found just one fewer than the 199 cancers found using two separate physicians.
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 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.
Research shows[9] that false-positive mammograms may affect women's well-being and behavior. Some women who receive false-positive results may be more likely to return for routine screening or perform breast self-examinations more frequently. However, 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 mastectomies cannot be performed 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).
Other risks
The radiation exposure associated with mammography is a potential risk of screening. The risk of exposure appears to be greater in younger women. The largest study of radiation risk from mammography concluded that for women 40 years of age or older, the risk of radiation-induced breast cancer was minuscule, particularly compared with the potential benefit of mammographic screening, with a benefit-to-risk ratio of 48.5 lives saved for each life lost due to radiation exposure.[10] Organizations such as the National Cancer Institute and United States Preventive Task Force take such risks into account when formulating screening guidelines.[11]
The majority of health experts agree that the risk of breast cancer for asymptomatic 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 positive, 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.
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 (conducted in the 1980s) had no positive effect on mortality for women between the ages of 50 and 60.[12] 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.[who?]
There is a body of evidence that clearly shows that there is overdiagnosis of cancer when women are screened. These cancers would never have affected these women in their lifetimes. An estimate of this overdiagnosis is 10 breast cancers diagnosed and unnecessarily treated per life saved when 2000 women are screened for 10 years.[13]
While screening between 40 and 50 is still controversial, the preponderance of the evidence indicates that there is some small benefit in terms of early detection. Currently, the American Cancer Society, the National Cancer Institute, and the American College of Radiology encourage mammograms every two years for women ages 40 to 49.[14] In contrast, the American College of Physicians, a large internist group, has recently encouraged individualized screening plans as opposed to wholesale biannual screening of women aged 40 to 49.[15] Recently the U.S. Preventive Services Task Force said that it might not be necessary for women between the ages of 40 and 50 to get routine screenings. Their report says that the benefits of screenings before the age of 50 don't outweigh the risks.[16]
Critique of screening mammography
The use of mammography as a screening tool for the detection of early breast cancer continues to be debated. Critics point out that a large number of women need to be screened to find cancer. Keen and Keen indicated that repeated mammography starting at age 50 saves about 1.8 lives over 15 years for every 1,000 women screened.[17] This result has to be seen against the negatives of errors in diagnosis, overtreatment, and radiation exposure. Countercritics argue that the benefit is greater. The Cochrane analysis of screening indicates that it is "not clear whether screening does more good than harm". According to their analysis one in 2,000 women will have her life prolonged by 10 years of screening, however, another 10 healthy women will undergo unnecessary breast cancer treatment.[18] Newman points out that screening mammography does not reduce death overall, but causes significant harm by inflicting cancer scare and unnecessary surgical interventions.[19]
Alternatives to mammography
While the cost of mammography is relatively low, its sensitivity is not ideal, with reports listing the range from 45% to about 90% depending on factors such as the density of the breast. Neither is the X-ray based technology completely benign, as noted above. Therefore there is considerable ongoing research into the use of alternative technologies.
One approach, contrast enhanced magnetic resonance imaging (MRI), has shown substantial progress. In this method, the breast is scanned in an MRI device before and after the intravascular injection of a contrast agent (Gadolinium DTPA). The pre-contrast images are "subtracted" from the post-contrast images, and any areas that have increased blood flow are seen as bright spots on a dark background. Since breast cancers generally have an increased blood supply, the contrast agent causes these lesions to "light up" on the images. The available literature suggests that the sensitivity of contrast-enhanced breast MRI is considerably higher than that of either radiographic mammography or ultrasound and is generally reported to be in excess of 95% (though not all reported studies have been as encouraging). The specificity (the confidence that a lesion is cancerous and not a false positive) is only fair, thus a positive finding by MRI should not be interpreted as a definitive diagnosis. The reports of 4,271 breast MRIs from eight large scale clinical trials were reviewed recently by CD Lehman.[20] Overall the sensitivity ranged from 71% to 100% in these reports, however the call-back rates were low at 10% and the risk of having a benign biopsy was reported at 5%, a significant improvement over mammography.
Several medical instrument vendors have entered this arena with breast MRI solutions. One company, Aurora Systems, is the only manufacturer to make a breast-dedicated unit and as the exclusive patent holder of certain solutions to fat signal suppression that appear to be more or less essential.[citation needed] Siemens, General Electric and Philips Medical, main manufacturers of MRI instruments, offer breast MRI products or add-ons, and several third-party companies offer aftermarket products to enable breast MRI on conventional MRI instruments.
Regulation
Mammography facilities in the United States and its territories (including military bases) are subject to the Mammography Quality Standards Act (MQSA). The act requires annual inspections and accredition every 3 years through an FDA-approved body. Facilities found deficient during the inspection or accreditation process can be barred from performing mammograms until corrective action has been verified or, in extreme cases, can be required to notify past patients that their exams were sub-standard and should not be trusted.
At this time MQSA applies only to traditional mammography and not related scans such as breast ultrasound, stereotactic breast biospy, or breast MRI.
See also
References
- ^ U.S. Preventative Task Force. Screening for Breast Cancer. Summary of the Evidence. Agency for Healthcare Research and Quality.
- ^ Gøtzsche PC, Nielsen M (2006). "Screening for breast cancer with mammography". Cochrane Database Syst Rev (4): CD001877. doi:10.1002/14651858.CD001877.pub2. PMID 17054145.
- ^ Miller AB (2003). "Is mammography screening for breast cancer really not justifiable?". Recent Results Cancer Res. 163: 115–28; discussion 264–6. PMID 12903848.
- ^ Mammography Quality Scorecard, from the Food and Drug Administration. Updated April 2, 2007. Accessed April 9, 2007.
- ^ Mammography Frequently Asked Questions, from the American College of Radiology. Revised January 8, 2007; accessed April 9, 2007.
- ^ http://bmj.bmjjournals.com/cgi/content/full/328/7438/507
- ^ Destounis SV, DiNitto P, Logan-Young W, Bonaccio E, Zuley ML, Willison KM (2004). "Can computer-aided detection with double reading of screening mammograms help decrease the false-negative rate? Initial experience". Radiology 232 (2): 578–84. doi:10.1148/radiol.2322030034. PMID 15229350.
- ^ http://www.cnn.com/2008/HEALTH/conditions/10/01/computer.mammogram.help.ap/index.html
- ^ Noel T. Brewer, PhD; Talya Salz, BS; and Sarah E. Lillie, MPH (University of North Carolina at Chapel Hill) (3 April 2007). "Systematic Review: The Long-Term Effects of False-Positive Mammograms". Annals of Internal Medicine (Annals of Internal Medicine) 146 (7): 502. PMID 17404352. http://www.annals.org/cgi/content/abstract/146/7/502. Retrieved 2008-02-17.
- ^ Feig S, Hendrick R. "Radiation risk from screening mammography of women aged 40-49 years". J Natl Cancer Inst Monogr: 119–24. PMID 9709287.
- ^ Screening for Breast Cancer: Recommendations and Rationale. From the United States Preventive Task Force, a section of the Agency for Healthcare Research and Quality. Released February 2002; accessed April 9, 2007.
- ^ http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Abstract&list_uids=1423088
- ^ [1]
- ^ Screening Mammograms: Questions and Answers, from the National Cancer Institute. Released May 2006; accessed April 9, 2007.
- ^ Screening Mammography for Women 40 to 49 years of age: A Clinical Practice Guideline from the ACP from the American College of Physicians. Released April 2007; accessed April 11, 2007.
- ^ [|Sammons, Mary-Beth] (November 2009). "New Mammogram Guidelines Spark Controversy". AOL Health. http://www.aolhealth.com/condition-center/breast-cancer/screening-guidelines. Retrieved November 2009.
- ^ Nick Mulcahy (April 2, 2009). "Screening Mammography Benefits and Harms in Spotlight Again". http://www.medscape.com/viewarticle/590535.
- ^ Gøtzsche PC, Nielsen M (October 23, 2001 (updated June 1, 2005)). "Screening for breast cancer with mammography". http://www.cochrane.org/reviews/en/ab001877.html. Retrieved April 25, 2009.
- ^ David H. Newman. Hippocrates' Shadow. Scibner (2008). p. 193. ISBN 1-4165-5153-0.
- ^ Lehman CD (2006). ""Role of MRI in screening women at high risk for breast cancer"". Journal of Magnetic Resonance Imaging 24 (5): 964. doi:10.1002/jmri.20752. PMID 17036340.
- Daniel Kopans, Meyer JE, Sadowsky N (1984). ""Breast Imaging"". New England Journal of Medicine 310 (15): 960–7. PMID 6366562.
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