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evaluation

testing women on silicone models.

Film-screen mammography uses low-dose radiation, radiographic film, and equipment dedicated to breast imaging. Two plates on the machine compress the breast so that the radiation dosage is minimized. The process is highly regulated by the Mammography Quality Standards Act (MQSA), which specifies parameters for technical (radiation exposure, development, etc.) and clinical (positioning, contrast, etc.) image quality. Since the passage of the Act in 1992, technical image quality in the United States has improved,33 but there are persistent concerns about the quality of the clinical image and reader's interpretations.34-36

The American College of Radiology has created guidelines for the terminology used to describe and summarize findings on film-screen mammography.37 These guidelines encourage consistent assessments and explain how to measure interpretive performance using the following terminology: 0, need additional imaging and/or prior mammogram for comparison; 1, normal; 2, benign finding; 3, probably benign finding— initial short-interval follow-up suggested; 4, suspicious abnormality—biopsy should be considered; and 5, highly suggestive of malignancy—appropriate action should be taken.38 The American College of Radiology updated their assessment categories in 2003 and added a sixth category for images done after a breast cancer has been identified by biopsy.

Film-screen mammography sensitivity varies between 77% and 96%, although most healthcare providers expect a sensitivity of approximately 80% in clinical practice.39,40 Many factors influence the sensitivity of film-screen mam-mography, including younger age of patients, dense breast parenchyma, and having been screened within 1 or 2 years of the current mammogram.7,40,41 Specificity varies between 94% and 97% and is also influenced by these factors. For film-screen mammography, PPV is between 2% and 12% when a positive test is defined as one that leads to any additional evaluation with more mammograms, ultrasound, or visits to a clinician. The proportion of women found to have cancer among those referred to biopsy is higher (20% to 75%) because those women frequently have already been evaluated by ultrasound or additional mammographic imaging.7

Digital mammography uses equipment dedicated to breast imaging similar to film-screen mammography, but with a different receptor that creates the potential for a dynamic image.42 Photons that pass through the breast are collected by the receptor and counted. The information is digitized to allow display as a function of the photon count associated with a square micrometer of breast tissue. Unlike film-screen mammograms, which are preserved as fixed images similar to photographs, digital mammography stores data that can be manipulated by the radiologist.

The challenge in digital mammography is how to record and display the wealth of data that can be collected. For example, one receptor in digital mammography can detect and record up to 246 shades of gray. Designers make tradeoffs between information devoted to recording this contrast and information needed to establish spatial relationships.42 The radiologist can print a hard-copy image or review the image on a cathode-ray tube display (soft copy).

Digital mammography is not widely used because its advantages over film-screen techniques have not been proven.43 Recent work from a randomized trial suggests that digital and film-screen mammography have comparable accuracy, but the number of women recalled for additional imaging is lower with digital mammography.4 A randomized trial comparing digital mammography with film-screen mam-mography began in 2001.44

Computer-assisted reading uses software to analyze the output of digital mammography or the digitized image from equipment that scans a film-screen mammogram. The computer analysis identifies and marks areas of concern and displays them on an image shown on a cathode-ray tube adjacent to the view box where the original image is reviewed. Some, but not all, evaluations have shown that its use increases detection,45,46 and it has been approved by the U.S. Food and Drug Administration (FDA) for the second reading of films.45

Other technologies, such as ultrasound, are used for breast imaging but are not approved for screening use by the FDA. Ultrasound's application involves scanning the breast with a hand-held transducer that projects sound waves and captures them bouncing off the tissue. This information is processed to provide two-dimensional images that can be recorded on film and displayed on a lighted box as with any radiologic image.47 Ultrasound is time intensive and subject to some subjectivity because the operator must decide which images to record and how carefully to scan the breast, limiting this technology's effectiveness for screening. On the other hand, it is used effectively for the diagnostic workup of palpable lesions or abnormalities found via mammography.48

Although it is possible to identify individuals at high risk for breast cancer due to genetic defects, this is not recommended for the general population.49,50 As noted in Chapter 19, the search for genetic defects usually begins with the affected individual and then proceeds to the family if a defect is found. Currently, less than 10% of breast cancers are estimated to be associated with a genetic defect.50

Summary of the Evidence for the Benefit of Breast Cancer Screening Tests

Among the three screening tests commonly considered for breast cancer, only one has been shown to be efficacious: film-screen mammography. Clinical breast examination has never been studied as the sole intervention in a randomized trial.32 One randomized trial of film-screen mammography compared its benefit to regular screening with clinical breast examination and found no advantage for film-screen mam-mography.51 Some argue that the lack of film-screen mam-mography benefit in this trial supports the belief that clinical breast examination is effective. At present, however, no evidence-based guideline recommends clinical breast examination as the sole screening approach, even in countries with low resources.52 The systematic teaching of breast self-examination was examined in a large study conducted among women working in China. Groups of factory workers were trained in breast self-examination and brought back regularly for retraining. Despite demonstrating that women receiving the intervention had a higher sensitivity for finding lumps in silicone models, cumulative breast cancer mortality after 10 years was no different between the intervention and control populations.53

The evidence for the benefit of film-screen mammogra-phy, which has been collected in randomized clinical trials since the 1960s, is summarized in Table 24.2. The American Cancer Society and the USPSTF both recommend screening beginning at age 40 based on evidence for a 16% reduction in

TABLE 24.2. Evidence for the efficacy of film-screen mammography.

Years of Longest Study name intervention follow-up Site of study

Age group

(years) Study design

Exposure (intervention vs. control)

Screening Screening interval rounds (months)

Views

% Exposed to intervention (round)

Contamination

Relative risk ("odds ratio) of breast cancer mortality

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