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Computer-Aided Interpretation of Mammograms: Questions and Answers
1. What is CAD?
In 1998, the U.S. Food and Drug Administration first approved a device that helps radiologists display and interpret mammograms. Like a scanner, the device converts a film mammogram into a digital file that can be analyzed by computer software and displayed as a copy on a TV or computer screen. Called computer-aided detection (CAD), the software analyzes the digital representation of the mammogram image and marks suspicious areas on the screen image for the radiologist to review in association with their own reading of the original film. Preliminary studies suggested that the device helped in detection of cancer. Medicare and many insurers now reimburse for the application of CAD. The software has since spread into practice, and it is estimated that about 30 percent of mammograms are now interpreted using one of these devices.
2. What was the primary finding in the study of computer-aided interpretation of mammography?
The study reported in the April 5, 2007, issue of the New England Journal of Medicine* showed that computer-aided interpretation of mammograms was less accurate than interpretation without a computer's help and that computer software designed to improve mammogram interpretation may actually make it worse. With CAD, women were more likely to be told their mammogram was abnormal and to have undergone additional evaluation or biopsy to rule out breast cancer.
3. What were the other results reported by this study on accuracy?
With the software, 32 percent more women were recalled for additional tests and 20 percent more women had a breast biopsy even though most didn't ultimately have breast cancer. Use of the software had no clear impact on the early detection of breast cancer. If anything, the software may promote the detection of the least dangerous breast cancers, or in situ breast cancer. These sometimes grow so slowly that they would never come to clinical attention in the absence of screening. At this time, scientists are uncertain whether detection of these cancers by screening reduces a woman's chance of dying from breast cancer.
The software had no significant impact on the detection of more dangerous invasive breast cancers.
4. How is computer-aided detection different than digital mammography?
Both digital and film mammography use X-rays to produce an image of the breast. In film mammography, which has been used for over 35 years, the image is created directly on a film. Once a film mammogram is obtained, it cannot be significantly altered; if the film is underexposed, the contrast, which is the difference between the lightest and darkest areas on a display screen, is lost and cannot be regained.
Digital mammography takes an electronic image of the breast and stores it directly in a computer. Digital mammography allows improvement in image storage and transmission because images can be stored and sent electronically.
With CAD, the film mammogram is converted to a digital image. But this converted image is like a digital copy of a photograph, whereas with digital mammography, the 'picture' of the breast is directly recorded as a digital image and can be magnified and manipulated like people magnify and manipulate a digital photograph. CAD reviews patterns of the digital image to alert the radiologist to spots that could be cancer, but how it works with digital mammograms may be different than how it works with converted images of regular mammograms. This difference needs careful study and has not yet been resolved.
5. How important is reader training in interpreting CAD?
With CAD, the computer analyzes the digital image and marks suspicious areas on a copy of the mammogram shown on a screen. Radiologists must compare the marked image and the film mammogram to find cancers they might have missed with the naked eye. They cannot manipulate the image on the screen like radiologists can manipulate a digital mammogram. Radiologists are taught to use CAD, but there is no additional certification process for them to use this approach.
6. Do the study results suggest that women should avoid having their mammograms read with CAD?
Women should be informed about the strengths and weaknesses of what is known about CAD, but this study does not address whether risks outweigh the benefits of its use. That kind of study could provide better guidance about what women should do.
In medicine, there is a long tradition that physicians should "first, do no harm." This study raises the question of whether harm is being done with CAD because of increased recall rates and more biopsies. On the other hand, it remains uncertain whether CAD benefits women by uncovering serious breast cancers.
7. What does NCI recommend for screening mammograms?
NCI recommends that:
- Women in their 40s should be screened every one to two years with mammography
- Women aged 50 and older should be screened every one to two years
- Women who are at higher than average risk of breast cancer should seek expert medical advice about whether they should begin screening before age 40 and the frequency of screening.
8. How would a woman know whether her mammogram is interpreted using CAD or digital mammography?
Women can ask their doctors or contact local hospitals or imaging centers to find out if CAD is routinely used. Mammogram reports will typically contain a statement of whether or not CAD was used to assist in image interpretation. Film mammography is still much more common than digital mammography.
9. Should women who live in communities without newer technologies delay their next mammograms until they can have digital or CAD-read mammograms?
Women should have their next mammogram when they are scheduled for it. At the individual level, the overall accuracy of mammography using each of these techniques is more similar than different. It would be better to have a film mammogram when a woman is supposed to have her next mammogram than for her to delay her screening on the basis of her preference for receiving screening with or without CAD or digital mammography.
10. Does getting a mammogram with CAD feel similar to getting a mammogram without CAD?
CAD only impacts the interpretation of the images after they are obtained. From a woman's perspective, the positioning and compression of the breast are identical.
11. What other breast imaging techniques might be useful for breast cancer screening?
In addition to mammography, ultrasound and magnetic resonance imaging (MRI) are both sometimes used to screen for breast cancer in women at very high risk of the disease. A large, NCI-sponsored study comparing ultrasound vs. mammography in high- risk women is currently in progress.
There are no multi-center clinical trials that have proven the superiority of MRI or ultrasound compared to mammography as a means of screening for breast cancer among the general population of women over age 40.
12. How many women are screened with mammography annually in the United States?
The U.S. FDA reports that there are about 33.5 million mammography procedures performed per year in the United States. Data from 2000-2002 show that about 70 percent of all mammograms that are performed annually are for screening purposes (to detect cancer in women with no symptoms as opposed to following cancer once it has been diagnosed). This translates to about 23.5 million screening procedures every year.
13. Does Medicare reimburse for mammography, including CAD?
Reimbursement by Medicare in 2005 for film-screen mammograms was $85.65 and for digital screening mammography (for women with two breasts, as opposed to those who have undergone mastectomy) was $135.29. Actual cost for mammograms will vary by region and the form of reimbursement. Medicare provides an additional $20 for reimbursement if CAD is used to supplement interpretation of screening mammography.
14. What are the other implications of this research?
Researchers need to conduct larger studies to clarify whether and how CAD software improves cancer detection. Studies are also needed of whether any possible benefits of the software outweigh its harms and costs. There is also the potential for research to improve the accuracy of the CAD software.
15. Should women get screening MRIs instead of mammograms?
Screening MRI has never been tested in women who are at average-risk for breast cancer, so it is not known whether it is effective and safe. Women should continue to get regular screening mammograms. On March 28, 2007, the American Cancer Society issued guidelines for the use of MRIs for breast cancer screening among women at high-risk for breast cancer. Those guidelines can be found at http://caonline.amcancersoc.org/cgi/content/full/57/2/75.
16. Who were the manufacturers of the CAD devices used in this study, where are they located, and how much, on average, do the devices cost?
There are several manufacturers of FDA-approved CAD devices in the United States, including R2 Technology, Inc., Santa Clara, Calif. (ImageChecker), iCAD, Inc., Nashua, NH (SecondLook), and Kodak, Inc. The R2 Technology product was the first to receive FDA approval and manufactured the devices used by facilities in this study. In one comparative study, the R2 Technology and iCAD products had similar effects on radiologists' interpretation of mammograms.
In December, 2001, R2, Inc. cited a typical list price for the ImageChecker CAD System of $160,000 to $240,000.
*Fenton JJ, Taplin SH, et al. Influence of Computer-Aided Detection on Performance of Screening Mammography. NEJM, Vol. 356, No. 14. April 5, 2007
For a press release on this finding, please go to http://www.cancer.gov/newscenter/pressreleases/CADmammographyRelease.
For more information about cancer, visit http://www.cancer.gov, or call NCI's Cancer Information Service at 1-800-4 CANCER.
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