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Radiation doses in the UK trial of breast screening in women aged 40–48 years

National Co-ordinating Centre for the Physics of Mammography, Medical Physics Department, Royal Surrey County Hospital, Guildford GU2 7XX, UK

	Abstract

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Although data have been published on the radiation doses involved in screening women in the UK in the age range 50–64 years, data have not been published for the screening of younger women, when one might expect higher doses and a different risk benefit balance. Therefore, data on the radiation doses arising from screening younger women (age range 40–48 years) as part of the UK age trial have been collected and reviewed. Each of the screening centres participating in the trial was asked to submit measurements of doses for samples of approximately 50 or 100 women. The doses for 2296 women were received. The dose estimates were corrected to take account of variations in composition with age and breast thickness and in the spectra used. The average received dose was 2.5 mGy per oblique film and 2.0 mGy per craniocaudal film. Although these doses are about 7% higher than those calculated for the screening of older women, this was owing to differences in equipment rather than age of the women. Age itself was not found to be a significant factor affecting the dose to screened women aged over or under 50 years. An identifiable sub-group of women with larger breasts who were screened using higher dose systems received doses that were about 4.2 times the average, and should be considered in any risk benefit analysis. Where modern mammography systems with automatic beam quality selection and alternative target and filter materials had been introduced, there was a 15% reduction in average received dose and up to a 50% reduction in received dose for large breasts.

	Introduction

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The National Health Service Breast Screening Programme (NHSBSP) was started in the UK in 1988 following the recommendations of the Forrest report [1]. All women between the ages of 50 years and 64 years are invited for mammographic screening every 3 years. Forrest concluded that the benefit of screening younger women was unproven but should be the subject of research, and a UK multicentre trial began in 1991 [2]. This trial is commonly called the "age" trial. Women participating in the trial are between 40 years and 41 years of age at entry. In the first screen the women have mediolateral oblique (OB) and craniocaudal (CC) mammograms taken. Subsequent screens are annual until the age of 48 and include only oblique films. Thereafter, the women who participate in the trial are offered screening by the NHSBSP from the age of 50.

One of the issues to be considered in screening younger women is the risks and benefits of the radiation dose. The radiation doses involved in the NHSBSP have been studied in some detail [3, 4]. However, little information has been published on the doses involved in screening younger women. It is expected that doses to younger women may be somewhat higher since younger women have more glandular breasts. Law [5] has considered the relative probabilities of cancer detection and induction in the screening of younger women, where various radiation doses per film were assumed as actual data on doses per film were not available. It was therefore proposed to gather data on the radiation doses arising from screening younger women as part of the UK age trial. The method used was the same as that employed by Young and Burch [4]. Each of the 23 screening centres participating in the trial was asked to submit measurements of doses for samples of approximately 50 or 100 women attending for screening.

The aims of this study were to investigate the effect of age on radiation doses received, the range of doses received by women in the age trial and the effect of automatic beam quality selection on radiation doses.

	Method

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For each unit that participated in the study the following data were collected.

1. Date of survey.
   
2. Models of X-ray set, processor, films and screens.
   
3. Mean glandular dose (MGD) to standard breast and corresponding film density (the "standard optical density"). The MGD to the standard breast is calculated for a breast model of total thickness 45 mm that comprises a central region with a 50:50 mixture by weight of adipose and glandular tissue and a superficial region of adipose tissue 5 mm thick. MGD is estimated from measurements on a given mammography system with a 40 mm thickness of polymethylmethacrylate (PMMA), i.e. Perspex or Lucite [6].
   
4. Availability of facilities for using larger format 24 cm x 30 cm cassettes.
   
5. Age of women screened (where available).
   
6. MGD, compressed breast thickness and projection for each film in sample.
   
7. Exposure factors (tube current (mAs), tube potential, target and filter) for each film.
   
8. Method of tube potential selection; automatic or manual.
   

The survey was restricted to the basic screening situation and so any additional doses owing to assessment procedures or technical recalls were excluded. The MGD for each mammogram in the sample was estimated by local medical physicists from the records of exposure factors and compressed breast thickness using the method described in Report 59/2 of the Institute of Physical Sciences in Medicine [6]. The data were submitted on predefined spreadsheets and transferred to a central database for subsequent analysis. The view information was coded to distinguish between left and right breasts and between OB and CC projections. In addition, films were categorized as "main" films or "extra" films. Extra films mainly arose where a breast could not be completely covered with a single film. Less commonly an extra film could be a technical repeat taken at the time of screening. Unless otherwise indicated the first film was assumed to be the main film. Examinations were categorized as either one-view (1V) or two-view (2V). A 2V examination is conducted at the first screen at ages 40 years or 41 years and comprises OB and CC films. Subsequent screens involve a 1V examination comprising only OB films.

The mean glandular dose (D) supplied by local physicists was based on applying the following formula.

where K is the incident air kerma estimated at the upper surface of the breast, calculated from the post-exposure mAs and output data for the X-ray set in µGy mAs^-1, and g is the incident air kerma to mean glandular dose conversion factor (g-factor) calculated by Dance [7]. However, the g-factor was calculated for a model of the breast that assumes 50% glandularity. It has therefore been proposed [8] to extend Equation 1 to include two new factors, as follows

where the g-factor is unchanged, c corrects for any difference in composition from 50% glandularity and s corrects for any difference from the original tabulation by Dance [7] owing to using a different X-ray spectrum. This approach has the advantage that c- and s-factors can be readily applied retrospectively to breast dose survey data. Two tables of c-factors for average breast compositions have been calculated for women attending screening in the normal age range of 50–64 years and for those attending the age trial in the age range 40–49 years [8]. These c-factors have been applied to the dose data collected in this study to adjust the breast doses to account for differences in breast composition owing to ageand compressed breast thickness. s-factors have also been applied where spectra other than Molybdenum/Molybdenum have been used. The c-factors and s-factors have also been applied retrospectively to the dose data previously reported for the NHSBSP [4].

Since the system-to-system variations in dose are quite large, it was important to ensure that these differences did not mask or exaggerate any apparent relationship between age and dose. To minimize these effects a normalized dose ratio was calculated for each film. The normalized dose ratio was the ratio of the MGD for the breast divided by the MGD for the standard breast on the mammography system used.

	Results

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Data included
11 medical physics departments contributed 38 dose surveys covering 30 X-ray sets used in 19 breast screening centres. All of these screening centres also screen older women on behalf of theNHSBSP. The data were collected between January 1996 and January 2000 and include a total of 2196 women. Although most datasets were for a sample of 50 or 100 women on one screening system, the sample size varied from 15 to 105 women. The dose data for women attending the age trial are compared with those reported previously for the NHSBSP [4]. The dose data are presented with and without the application of composition and spectral correction factors.

X-ray technique and systems used
The tube potential selected ranged from 25 kV to 32 kV. For 32 of the 38 dose surveys the beam quality was selected manually, and for 6 surveys it was selected automatically. 17% of exposures in this sample from the age trial had beam quality selected automatically as compared with 12% for the sample reported previously for the NHSBSP [4]. Where beam quality was selected manually amolybdenum target material was always used. 99.9% of films were taken using a molybdenum filter and 0.1% using a rhodium filter. 81% of these films with automatically selected beam quality were taken at a tube potential of 28 kV, with 18.5% taken at a higher tube potential and 0.5% taken at a lower tube potential. For the data reported for the NHSBSP where the beam quality was selected manually, 7% were at a tube potential greater than 28 kV and 1% at a tube potential lower than 28 kV [4]. Where automatic beam quality selection was used, 70% of films were produced using a molybdenum–molybdenum target–filter combination, 23% using a molybdenum–rhodium combination and 7%using either a rhodium–rhodium or tungsten–rhodium combination. For the data as a whole, 70% of films were taken at a tube potential of 28 kV using a molybdenum–molybdenum filter combination. All units operated with an anti-scatter grid. Standard optical densities ranged from 1.42 to 1.86 and averaged 1.64. It is recommended in the NHSBSP [9] that targets for standard optical densities be set in the range 1.4 to 1.8 (including base and fog), and 91% of the systems lay within this range.

Average doses and dose distribution
The average MGD per film and compressed breast thickness for women screened at the usual age range in the NHSBSP, and those for women screened as part of the age trial, are shown without composition and spectral correction in Table 1. The average MGD for OB films was 2.37 mGy for the age trial compared with 2.03 mGy for the NHSBSP. As found previously, the MGD [4] was lower for CC films. Mean thickness was 52.3 mm for OB films and 52.0 mm for CC films in the age trial compared with 54.3 mm for OB films and 51.5 mm for CC films in the NHSBSP [4]. The normalized dose ratios shown in Table 1 imply that, after correcting for the dose to the standard breast, doses to women in the age trial were about 9% higher than those in the NHSBSP.

View larger version (31K): [in this window] [in a new window]   Figure 1. Histogram of meanglandular dose per film for mediolateral oblique views. Doses are corrected for composition and spectra used. Axis labels represent the mid-point of 0.2 mGy bands.

View larger version (23K): [in this window] [in a new window]   Figure 2. Histogram of total mean glandular doses per woman forone-view examinations. Axis labelsrepresent the mid-point of 0.5 mGybands. Composition and spectral correction factors were applied.

Effect of breast thickness on dose
The average MGD per film as a function of breast thickness is shown in Figure 3. Where manual tube potential selection was employed the MGD increased rapidly with breast thickness. Also shown in Figure 3 is the effect on this relationship of using automatic beam quality selection with three General Electric DMR, twoGeneral Electric 800T (General Electric Medical Systems, Paris, France) and one Siemens Mammomat 3000 (Siemens-Elema AB, Solma, Sweden) mammography X-ray sets. For these systems, doses rose less with increasing breast thickness than under manual tube potential control. For the largest breasts, doses were up to 50% lower when these automated systems were used than when tube potential was selected manually. The average doses per OB film, including all thicknesses, were found to be about 15% lower where X-ray sets were used with automatic selection as shown in Table 4. The difference was found to be 17% for the normalized dose ratios, which minimize the confounding effect of other variables such as film speed.

View larger version (14K): [in this window] [in a new window]   Figure 3. Mean glandular dose per film (main mediolateral oblique films only) as a function of compressed breast thickness for manual (•) and automatic () beam quality selection. The error bars show 95% confidence limits; for some data points the errors are too small to be seen. Composition and spectral correction factors were applied.

View larger version (18K): [in this window] [in a new window]   Figure 4. The average mean glandular dose (MGD) for the main mediolateral oblique films plotted against the MGD to the standard breast for each unit. Composition and spectral correction factors were applied. The error bars show 95% confidence limits.

	Discussion

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Women and X-ray systems included
The data reviewed are broadly representative of the age trial, since 19 of the 23 screening centres participating in the age trial provided data. The equipment used included a wide range of X-ray models and film–screen systems. This variation in equipment is the main reason for the wide variations in the dose to the standard breast. The doses to individual women also depended on the radiographic techniques used, and in particular the beam quality selection. Most of the X-ray sets were operated with manual selection oftube potential with a molybdenum filter and molybdenum target material. In most cases the tube potential selected was 28 kV, which represents the traditional selection for mammography in the UK. For a substantial minority of exposures (approximately 19%) a higher tube potential (typically 30 kV) was manually selected to reduce doses and shorten exposure times. In the data for the NHSBSP, a smaller proportion of exposures (approximately 7%) had a tube potential greater than 28 kV selected manually. At the time the data were collected only a minority of systems used in the age trial had the facility for automatic beam quality selection. The use of these more modern X-ray sets resulted in a much greater variety of tube potentials being used and the introduction of alternative filter, and to a lesser extent, target materials. These changes have previously been shown to lead to increased doses forsome types of breast and reduced doses for othersdepending on the choices made [10, 11]. The average standard optical density for the mammography systems in this study of doses in the age trial is 1.64. This is identical to that reported in the review of radiation doses in the NHSBSP [4].

Average doses
Composition and spectral correction factors were used to improve the accuracy of dose estimates. It should be noted that one effect of using these factors was to increase the estimates of average doses in the NHSBSP by approximately 15%, and in the age trial by approximately 5% (Tables 1 and 2). Composition correction had a different effect on doses for the two groups because of the change in breast composition with age. As found previously, doses for CC films were about 19% lower than for OB films. The small differences found in the compressed breast thickness can explain only part of this dose difference. The other cause of this difference is likely to be the effect of pectoral muscle in the oblique views overlying the automatic exposure control chamber and causing an increase in exposure.

Average doses in the age trial were about 2.5 mGy per OB film and 2.0 mGy per CC film, where composition and spectral correction factors have been applied. These doses are about 24% higher for the OB film and 22% higher for CC films than previously reported for the NHSBSP [4]. These higher doses are partly explained by the use of composition correction factors and partly by a systematic difference in the doses to the standard breast between the age trial and the NHSBSP. The use of spectral correction factors had only a minor effect because the factors are all close to 1 and owing to the relatively small proportion of exposures that used alternative target filter combinations.

Age
The women in the age trial had significantly higher normalized dose ratios if composition correction factors were not applied (Table 1). However, when composition correction factors were applied, the normalized dose ratios for women in the age trial were not significantly different from those in the NHSBSP (Table 2). Age therefore appears to have a negligible effect on dose between the ages of 40 years and 64 years. This is consistent with the previous finding that when women attending breast screening in the NHSBSP were grouped in 5-year age bands, there was no significant difference in dose per film between 50 years and 64 years [4]. It can therefore be concluded that although breast composition changes with age, which can lead to greater exposure factors, the net effect on average doses per film is negligible. It should be remembered, however, that women in the age trial do receive higher overall radiation doses because screening is conducted annually compared with 3-yearly in the NHSBSP.

Effect of breast thickness on dose
As previously found, compressed breast thickness is a major determinant of dose per film, with thicker breasts receiving much higher doses. However, the magnitude of doses to large breasts was strongly affected by the method used to select beam quality. Where beam quality was manually selected, the average dose per OB film rose from just above 1 mGy for a 20 mm thick compressed breast to 7.1 mGy for a 100 mm thick compressed breast. The largest breasts received doses that were about 2.8 times that for average sized breasts. Where more modern X-ray sets with automatic beam quality selection were used, doses were up to 50% lower for large breasts, resulting in a reduction in average doses of about 15%. These dose savings appear to be mainly attributable to the increased use of a rhodium filter and also, occasionally, an alternative target material (rhodium or tungsten). In the data for the age trial, 17% of exposures were made using automatic beam quality selection compared with 12% in the data for the NHSBSP [4]. This difference in the use of automatic beam quality selection would account for less than 1% of any difference in the average doses in the age trial and in the NHSBSP.

Dose to the standard breast
MGDs to the standard breast were calculated for each mammography system by combining the mAs for correct exposure of a 40 mm thick block of PMMA with values of tube output and beam quality [6]. MGD to the standard breast allows the effect of equipment factors to be assessed, whilst eliminating patient variables. As one would expect, the standard breast dose was correlated with the average MGD for each mammography system. However, the average dose for an oblique film was about 70% higher than the dose to the standard breast on the same system. This difference is explained by the fact that the standard breast is only 45 mm thick whilst the breasts inthe age trial were, on average, approximately 52 mm thick. This difference is higher than reported previously for the NHSBSP and is attributable to the introduction of the composition correction factor [4]. For a few systems a very different relationship between the standard breast dose and the average MGD was found, raising questions about the accuracy of one or both of the measurements. As discussed previously, systematic and sampling errors in estimating breast thickness introduce errors into dose estimation [3, 4]. Most of the data presented here represent an average for many systems and it is expected that these sources of error will have been averaged down.

Sub-groups of women
While the average dose per OB film was approximately 2.5 mGy, there were subgroups of women for whom doses were much higher. One identifiable sub-group of women who received larger doses than average comprises those women with relatively thick breasts on compression. It has been shown here that the small sub-group of women with compressed breasts greater than 95 mm thick had doses of approximately 2.8 times the average for a given mammography system if the beam quality was selected manually. Another factor that affects doses to women was the dose to the standard breast for the equipment used, which ranged from 0.82 mGy to 2.20 mGy. Although quality assurance guidelines for the NHSBSP [9] require that the MGD to the standard breast should not exceed 2 mGy, a few systems were slightly above this limit. For the mammography system with the highest standard breast dose of 2.2 mGy, breast doses of about 1.5 times the average can be expected. Thus, using this system one can expect that a few women with large breasts (100 mm thick) would receive doses of about 10.5 mGy per film, i.e. 2.8 x 1.5 x 2.5 mGy. 99.6% of OB films had doses of less than 10.5 mGy, and this dose per film can be regarded as an upper limit of what can normally be expected for a screening mammogram in the age trial. Figure 1 shows that most doses per film fell well below this. Some doses per film exceeded 10.5 mGy and the highest was 16.5 mGy. A large breast which was denser than usual could explain such a high dose. Note that in such a case the c-factor used will be higher than it should be and could result in an overestimate by as much as 20%. The reverse is also true, in that a breast that is less dense than assumed would have a dose that could be underestimated by up to 20%. Such errors in individual estimates are reduced where the doses for a number of women are averaged. Alternative explanations could be an error in recording exposure factors or in making calculations. By using modern equipment with automatic beam quality selection such high doses per film should be avoided altogether. In this study the highest dose per film was 7.1 mGy when automatic beam quality selection was used.

Doses for whole examinations were higher than doses per film, particularly for 2V examinations. Using the same arguments as above, the maximum normally expected dose for a CC film is 8.4 mGy, i.e. 2.8 x 1.5 x 2 mGy. Therefore, the maximum dose that may normally be expected for a 2V examination is 18.9 mGy if there is one film per view. In practice the maximum dose per woman out of the 123 who underwent a 2V examination was 14.3 mGy, even when all extra films were included. One may therefore conclude that a very small proportion of women will receive about 4.2 times, i.e. 2.8 x 1.5, the average dose in either a 1V or 2V screening programme, and that this identifiable sub-group should be considered in any risk benefit analysis. A similar conclusion was reached for the high dose sub-group of women screened by the NHSBSP [4]. A few women will receive doses higher than this, but this is very rare and impossible to predict in advance.

Films per view
Many of the screening centres did not have facilities for larger format 24 cm x 30 cm mammography film. Some women's breasts were too large to fit on standard sized film and had to be imaged using a mosaic of standard sized films. This procedure involved a higher dose than if a single large format film were used. The extra dose is approximately proportional to the area of overlap of the films over the breast. In calculating the dose per examination it was assumed here that there was 100% overlap between films and this will somewhat overestimate the true additional dose. For OB views, approximately 5.3% of women required more than one film if larger format film was not available. Even where the larger format film was available, approximately 1.6% of women still required more than one film per OB view. Fewer extra films were taken for the CC views, with only about 0.8% of women having extra films where larger format film was not available. It is the use of these extra films that causes average dose per examination to be slightly higher (3%) than average dose per film. Although this is a small effect for the women screened as a whole, it is larger for the few women who needed extra films. Although the potential dose saving achievable by ensuring that all centres had large format film available would be small for the screened population as a whole, it would be substantial for those women who currently need extra films. Women who had large compressed breast thicknesses were more likely to require extra films. Almost 20% of the women with compressed breasts with a thickness greater than 80 mm required two or three OB films per breast. Therefore dose reduction owing to having large format film available most benefits those women receiving the highest doses. However, a change in practice that ensured that all screening centres had large format film available would also need to take into account other issues such as cost, convenience and image quality.

	Conclusions

Top Abstract Introduction Method Results Discussion Conclusions References

 
The range of doses received in the age trial has been evaluated and found to average 2.5 mGy for OB films and 2.0 mGy for CC films. A small sub-group of women (0.5%) with compressed breasts greater than 95 mm thick generally received the highest doses. A few of these women who were screened using the highest dose systems received doses approximately 4.2 times these averages and this should be considered in any risk benefit analysis. When doses for women aged 40–48 years were compared with doses for women aged 50–64 years, no significant dose variation with age was found. The use of mammography X-ray sets with automatic beam quality selection resulted in a 15% reduction in average doses and up to a 50% reduction for large breasts. The use of automatic beam quality selection is effective in lowering doses to large breasts and should be employed where possible, especially when screening younger women. However, the benefits of such dose reduction should be balanced by the consideration of any losses in image quality.

	Acknowledgments

 
The author would like to acknowledge the workof the many radiographers and physicists throughout the UK who collected the raw data and calculated the doses analysed in this paper. The National Co-ordinating Office of the NHSBSP funds the work of the National Co-ordinating Centre for the Physics of Mammography.

Received for publication July 9, 2001. Revision received October 26, 2001. Accepted for publication November 9, 2001.

	References

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This Article Abstract Figures Only Full Text (PDF) Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Young, K C Search for Related Content PubMed PubMed Citation Articles by Young, K C