(Radiology. 1999;210:345-351.)
© RSNA, 1999
Mammography in the 1990s: The United States and Canada
Orhan H. Suleiman, MS, PhD1,
David C. Spelic, MS, PhD1,
John L. McCrohan, MS1,
Gordon R. Symonds, PEng2 and
Florence Houn, MD, MPH1
1 U.S. Food and Drug Administration, Center for Devices and Radiological Health (HFZ-240), Division of Mammography Quality and Radiation Programs, 1350 Piccard Dr, Rockville, MD 20850 (O.H.S., D.C.S., J.L.M., F.H.)
2 Consumer and Clinical Radiation Hazards Division, Radiation Protection Bureau, Health Canada, Ottawa, Ontario, Canada (G.R.S.).
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Abstract
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PURPOSE: To evaluate trends in mammography quality before and after the implementation of the Mammography Quality Standards Act (MQSA) of 1992 and to compare technical data collected in the United States with corresponding data obtained from the first survey of mammography facilities conducted in 1994–1995 in Canada.
MATERIALS AND METHODS: Data from MQSA inspections conducted in 1995–1997 were analyzed and compared with survey data on U.S. mammography facilities acquired before the MQSA. Technical indicators of mammography quality such as radiation dose, phantom image score, film processing, and darkroom fog were analyzed.
RESULTS: In the United States, phantom image scores, along with other technical measures of performance such as film processing, darkroom fog, and x-ray beam quality, have improved continuously since 1985. The U.S. mean glandular dose has increased to 1.6 mGy compared with the Canadian dose of 1.1 mGy. The mean total phantom image score with artifact subtraction was 11.1 in Canada in 1994–1995 and 11.8 in the U.S. in 1997.
CONCLUSION: Mammography quality is better today than it has been at any other time in the United States. With the exception of radiation dose, Canadian technical measures of performance are comparable to measures before MQSA in the United States.
Index terms: Breast radiography, quality assurance, 00.11 • Quality assurance
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Introduction
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Mammography has been undergoing remarkable change in recent years. In a previous article (1), we reported on the technical trends in mammography from 1975 to 1992, with special emphasis on the results of the comprehensive 1985, 1988, 1992 Nationwide Evaluation of X-ray Trends (NEXT) surveys. The NEXT surveys, conducted collaboratively by the state and federal governments, were coordinated by the Conference of Radiation Control Program Directors. Since then, essentially all mammography facilities in the United States have been subject to the Mammography Quality Standards Act (MQSA) of 1992. Our purpose was to assess what has happened in mammography since 1992 in both the United States and Canada and to compare some of the common measures of performance in mammography with those in other radiologic procedures. Although the MQSA is a comprehensive law that covers many aspects of mammography, we will limit our focus to technical measures of performance.
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MATERIALS AND METHODS
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Sample Selection
The methods of selecting samples for the NEXT surveys have been previously reported (2). The MQSA data were collected as part of the annual facility inspection. The Canadian survey was conducted in 1994 and 1995 as part of a first-time comprehensive national survey of 338 Canadian mammography facilities. These 338 facilities represented 60% of all mammography facilities in Canada (3).
Radiation Exposure Measurements
Radiation exposure measurements were obtained by using calibrated MDH radiation dosimeters (Radcal, Monrovia, Calif). X-ray beam quality measurements were performed by using type 1100 aluminum alloy for the non-MQSA surveys and type 1145 aluminum alloy for the MQSA surveys.
Mammographic Phantom
The mammographic phantom used to perform standard measurements is the same phantom used by the accreditation bodies, by the U.S. Food and Drug Administration for MQSA inspections, and in the NEXT surveys. This phantom, which has previously been described in detail (1), is equivalent to a 4.2-cm compressed breast that consists of 50% glandular tissue and 50% adipose tissue, which is the nominal equivalent to the average-sized breast.
Film Processing
Film processing quality was tested by using an empirical test known as the sensitometric technique for the evaluation of processing, or STEP (4,5). This test was performed by using a sheet of control film from the same emulsion batch (Eastman Kodak, Rochester, NY), which was exposed in each surveyed facility's darkroom to a calibrated sensitometer (X-rite, Grandville, Mich) and developed in the processor being surveyed. The resultant optical densities were determined with a calibrated densitometer (X-rite).
A processing speed of 100 corresponds to a processor that develops the control film in a manner that is equivalent to the film manufacturer's recommendations. Equivalence here means that the resultant optical densities are the same as those that result when film from the same emulsion batch is developed according to the film manufacturers' specifications. Film from the control film emulsion batch is tested in each film manufacturer's recommended processing environment. Although the control film used to conduct the STEP test may not be the same as that used at the facility, the control film selected has response characteristics representative of the film used in mammography. For routine in-house quality assurance testing, it is important that the facility use the same type of film used in the clinical setting.
Darkroom Fog
Photographic fog is defined as the net increase in film optical density when the film is exposed to 2 minutes of ambient conditions in the photographic darkroom (6). This darkroom fog test was conducted by preexposing a sheet of film that the facility normally used in the clinical setting to x rays and then exposing one-half of this film to ambient conditions in the darkroom. The increase in optical density that corresponded to this ambient exposure is defined as fog.
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RESULTS
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The measures of technical performance from surveys and inspections of mammography facilities since 1988 in the United States and from the 1994-1995 Canadian survey are listed in Table 1. The 1988 and 1992 U.S. NEXT surveys were conducted before the MQSA was passed (1–4,7–9). The mean film processing speeds for film processors from the NEXT and MQSA studies are listed in Table 2 (1,3,4,7–12). The darkroom fog levels from studies of radiographic darkrooms associated with examination-specific surveys are listed in Table 3 (1,3,6,9,11,12).
The exposure reproducibility of the x-ray machine was measured as the coefficient of variation, which is defined as the SD divided by the mean, for a series of exposures. This measure of performance demonstrates how precisely or consistently the x-ray equipment performs. Coefficients of variation for different types of x-ray equipment, including mammographic equipment, are shown in Table 4.
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TABLE 4. Coefficients of Variation for Exposure Reproducibility of Various Diagnostic X-ray Procedures by Survey Year
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DISCUSSION
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Phantom Radiation Dose and Image Quality
Image quality continues to improve in the United States, whereas the overall radiation dose has recently started to increase slightly (Table 1, Fig 1). Mean phantom doses for screen-film mammography in the United States increased from 1.33 mGy in 1988 to 1.60 mGy in 1997. This correlates with an increase in mean phantom film optical density from 0.96 to 1.52 for the same period. Total phantom image quality scores, without artifact subtraction, have improved since the 1992 baseline NEXT study score of 11.2 to the 1997 MQSA inspection score of 12.2. Studies (13) suggest that higher film optical densities improve image quality and increase cancer detection (14,15). Consequently, some of the improvement in phantom image scores could be attributable to the observed higher film optical densities.
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Figure 1. Mean glandular radiation dose and phantom image scores by survey year. Phantom image scores without artifact subtraction are reported. The following phantoms were used: in 1985, Radiation Measurements, Inc (RMI) 152 (4.7 cm) with "C" insert; in 1988, RMI 156 (4.2 cm) with "C" insert; 1992 to present, RMI 156 with "D" insert.
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The Canadian survey results showed a lower mean radiation dose of 1.13 mGy, which is lower than the lowest mean dose observed in any U.S. survey, including the 1.33-mGy mean dose observed in 1988. Dose distributions in the United States and Canada are shown in Figure 2.
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Figure 2a. (a) Distribution of mean glandular radiation doses at U.S. mammography facilities. (b) Distribution of mean glandular radiation doses at Canadian mammography facilities.
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Figure 2b. (a) Distribution of mean glandular radiation doses at U.S. mammography facilities. (b) Distribution of mean glandular radiation doses at Canadian mammography facilities.
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Phantom image scores from surveys conducted before 1995 were not corrected for artifacts; consequently, phantom image scores obtained by using MQSA data were recalculated without artifact subtraction for comparison with the earlier surveys. According to survey data, phantom image scores with and without artifact subtraction were slightly lower at Canadian mammography facilities compared with U.S. facilities in 1995 and 1997, but the Canadian facility phantom scores were higher than those in the United States in 1992. The associated phantom score distributions are shown in Figure 3.
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Figure 3a. (a) Distribution of mammographic phantom image scores at U.S. mammography facilities. The protocol used to score phantom images in 1992 did not include artifact subtraction. The corresponding 1997 scores were recalculated without artifact subtraction to allow comparison. (b) Distribution of mammographic phantom image scores at Canadian facilities. Phantom images were scored by using the same protocol applied in the 1992 NEXT mammography survey (ie, without artifact subtraction).
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Figure 3b. (a) Distribution of mammographic phantom image scores at U.S. mammography facilities. The protocol used to score phantom images in 1992 did not include artifact subtraction. The corresponding 1997 scores were recalculated without artifact subtraction to allow comparison. (b) Distribution of mammographic phantom image scores at Canadian facilities. Phantom images were scored by using the same protocol applied in the 1992 NEXT mammography survey (ie, without artifact subtraction).
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In the United States, phantom film optical density has increased consistently over the years (Table 1). The mean Canadian value falls between the 1992 and 1995 U.S. values. Optical density distributions from the 1992 and 1997 U.S. surveys and the 1994-1995 Canadian survey are shown in Figure 4. The lower film optical densities at Canadian facilities are one likely reason for the slightly lower phantom image scores.
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Figure 4a. (a) Distribution of phantom image optical densities at U.S. mammography facilities. (b) Distribution of phantom image optical densities at Canadian mammography facilities.
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Figure 4b. (a) Distribution of phantom image optical densities at U.S. mammography facilities. (b) Distribution of phantom image optical densities at Canadian mammography facilities.
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We are also aware that there was an active effort to lower mammography radiation doses in Canada with the supporting observation of more extended cycle processing, a technique that can reduce the radiation dose while maintaining constant film optical density. In 1995, 63% of the Canadian facilities used extended cycle processing compared with 43% that used this technique in the U.S.
Film Processing
Mammography facilities inspected under the MQSA performed substantially better on average than did all other facilities, that is, the nonmammography facilities in 1984–1995; the U.S. mammography facilities in 1985, 1988, and 1992; and the Canadian mammography facilities in 1994 and 1995 (Table 2). The distributions of processing speeds at mammography facilities in the United States in 1992 and 1997 and in Canadian mammography facilities in 1995 are shown in Figure 5. The U.S. data show an increase in film processing speed between 1992 and 1997, with a substantial reduction in facilities that were underprocessing—that is, having processing speeds less than 80 in a standard processing cycle. Twenty-three percent of Canadian mammography facilities did not meet the current MQSA standards for standard cycle film processing compared with 6% of the MQSA-inspected facilities that did not meet the standards in 1995, which dropped to 1% of MQSA-inspected facilities in 1997 (Table 2).
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Figure 5a. (a) Distribution of standard cycle processing speeds at U.S. mammography facilities. (b) Distribution of standard cycle processing speeds at Canadian mammography facilities.
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Figure 5b. (a) Distribution of standard cycle processing speeds at U.S. mammography facilities. (b) Distribution of standard cycle processing speeds at Canadian mammography facilities.
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The results of the U.S. NEXT survey of nonmammography facilities also revealed consistently poorer processing (Table 2). After 1992, 50% of dental facilities surveyed in 1993, 10% of facilities conducting chest radiography in 1994, and 19% of facilities conducting abdominal radiography in 1995 did not meet MQSA processing speed standards. If the 1994 chest survey data from hospitals and from private practice are differentiated, 4.5% of hospitals (n = 134) and 15.5% (n = 148) of private practice facilities did not meet current MQSA processing speed standards. Similarly, the results of the 1995 survey of abdominal and lumbosacral spine facilities showed that 7.8% of hospitals (n = 141) and 27.0% (n = 178) of private practice facilities were not meeting the current MQSA processing speed standards.
It is important to understand that the level in which an MQSA facility is actually cited for underprocessing is a log relative exposure difference of 0.08 from that recommended by the film manufacturers, which corresponds to a nominal 20% difference in speed. When a facility's processor is developing film according to the film manufacturer's recommendations or in an equivalent manner, a processor speed of 100 is assigned for standard processing. Facilities are cited for noncompliance of the MQSA standards only when their automatic film processor's speed is less than 80. This is a very loose tolerance (20% of speed) and is comparable to a 2.2°C (ie, 4°F) temperature change. Today, the developer temperature of most automatic film processors can be regulated to within 0.5°F (0.3°C). Although 6% of the MQSA-inspected facilities in 1995 deviated by 20% or more from the processing speed standard compared with 23% of the Canadian facilities, 30% of U.S. MQSA-inspected facilities and 39% of Canadian mammography facilities were underprocessing by 10% or more—that is, their processing speed was less than 90. This level of processing performance corresponds to a nominal 1.1°C (ie, 2°F) temperature difference and is easily attainable.
Stated another way, a 10% change (ie, log relative exposure difference of 0.04) in processing corresponds to a change in film optical density of 0.12 for a film with a contrast (gradient or gamma) of 3 and to a change in film optical density of 0.16 for a film with a contrast of 4. Although film processing today is better at MQSA-inspected facilities, there is potential for more improvement in film processing quality control.
Darkroom Fog
Perhaps the simplest indicator of a facility's commitment to quality assurance is whether its darkroom fog level is 0.05 or less. Table 3 and Figure 6 show distributions of darkroom fog levels in U.S. and Canadian facilities. According to the data, U.S. mammography facilities inspected under MQSA in 1995–1997 had mean fog levels of 0.03 or lower. In 1996 and 1997, 2.3% of facilities had fog levels higher than 0.10. The results of all surveys of mammography and nonmammography facilities conducted before the MQSA was passed showed mean darkroom fog levels higher than 0.05. After MQSA was implemented for U.S. mammography facilities, higher levels of fog continued to be observed in non–MQSA-regulated facilities, such as those that performed dental radiography in 1993, chest radiography in 1994, abdominal radiography in 1995, and mammography in Canada in 1995. In 1992, the fog levels in 37% of the facilities that participated in the voluntary mammography accreditation program of the American College of Radiology exceeded the current MQSA standard fog level of 0.05 even though the voluntary standards of the American College of Radiology were more stringent in that they required the facility to have a fog level no higher than 0.02 (16).
Exposure Time
The increase in mean exposure time for imaging the mammographic phantom (Fig 7) is of concern. This phantom is nominally equivalent to a 4.2-cm compressed breast consisting of 50% glandular tissue and 50% adipose tissue. Although the longer exposure times associated with this phantom correlate with increased film optical densities, these exposure times now exceed 1 second. This suggests that in approximately 50% of patients, even longer exposure times may be required, assuming there is no change in beam quality.
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Figure 7. Mean exposure times for mammographic phantom imaging in the United States by survey year. Data points from the Canadian study are also shown. Data values reflect the exposure times for imaging a 4.2-cm equivalent phantom. The mean exposure times are on the left axis, and the percentage of facilities that had mean exposure times longer than 1 second are on the right axis.
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Exposure times of longer than 1 second may introduce image quality problems related to patient motion and film reciprocity law failure. To penetrate breasts with higher attenuation and minimize exposure times, higher beam qualities are probably being used. However, there are no available data on this group of patients to support this hypothesis. Information on the mammographic techniques and exposure times used to perform imaging of a thicker phantom that corresponds to a breast with higher attenuation would be extremely useful.
Exposure reproducibility measurements are a measure of x-ray equipment performance consistency. It is encouraging to observe that the mammography equipment inspected under the MQSA in 1995–1997 was extremely consistent (Table 4), with a coefficient of variation in the order of 1.0%, which was consistently better than the exposure reproducibility measurements of mammography equipment before the MQSA and those of nonmammography equipment. Only the CT equipment in 1990 had a better coefficient of variation of 0.7%.
In summary, the quality of mammography in the United States has continued to improve since 1985, with improvements in many of the technical measures of performance associated with good quality such as phantom image scores, film processing, and darkroom fog levels. This is in contrast to the results of recent surveys of nonmammography facilities in which these common measures of performance are not as good as those at mammography facilities. Once mandatory regulatory standards were established under the MQSA, which mandated facility accreditation and certification, more facilities met the previously voluntary standards. The quality of mammography in the United States, with phantom image scores as the measure of performance, is better today than that in any other period of observation, although the average mean glandular dose has increased slightly. The authors believe that this increase in dose is justifiable considering the improved phantom image scores.
The 1994–1995 Canadian mammography facilities had phantom image scores that were higher than those of U.S. facilities in 1992 but lower than those of U.S. facilities in 1995. The radiation doses for mammography in Canada were lower than those from any previously reported survey. In general, most of the technical measures of performance at Canadian facilities, with the exception of dose, were comparable to those before MQSA in the U.S. Because of these concerns, the Canadian government recently conducted a workshop to address the concerns raised on the basis of the survey results (17). One concern is that the survey is now several years old, and thus its results do not represent the current practice in Canada. Therefore, one of the recommendations is to repeat the survey on a periodic basis to monitor the status of mammography in Canada. Other recommendations are to establish national technical standards for mammography and to implement a national mandatory certification-accreditation program.
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Acknowledgments
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The authors are extremely grateful for the efforts of many individuals and organizations, without whom the quality of mammography would not be what it is today. Our thanks to Fred Rueter, who retired several years ago from the U.S. Food and Drug Administration; he was actively involved with the NEXT surveys, and his most recent contribution was assisting the Canadian government in conducting this survey; the many individuals from the state governments who collected these data over the years either as part of the NEXT surveys or most recently as MQSA inspectors; and the NEXT committee members, who donated many hours over the course of many years to establish this critical database. We are thankful for the efforts of these individuals and countless others.
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Footnotes
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Address reprint requests to O.H.S.
The mention of commercial products, their sources, or their use in connection with material reported herein is not to be construed as either actual or implied endorsement of such products by the U.S. Department of Health and Human Services.
Abbreviations: MQSA = Mammography Quality Standards Act
NEXT = Nationwide Evaluation of X-ray Trends
Author contributions: Guarantor of integrity of entire study, O.H.S.; study concepts and design, O.H.S.; data analysis, D.C.S., G.R.S.; statistical analysis, D.C.S., G.R.S.; manuscript preparation, O.H.S.; manuscript editing, O.H.S., D.C.S.; manuscript review, O.H.S., D.C.S., F.H., G.R.S., J.L.M.
Received February 2, 1998;
revision requested April 9, 1998; revision received August 4, 1998;
accepted September 28, 1998.
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References
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Abstract
Introduction
