British Journal of Radiology (2005) 78, 816-820
© 2005 British Institute of Radiology
doi: 10.1259/bjr/52648102
Breast dose from magnification films in mammography
J Law, PhD
Medical Physics, University of Edinburgh, Chancellor's Building, 49 Little France Crescent, Edinburgh EH16 4SB, UK
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Abstract
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In mammography, much attention has been given to estimating breast dose from exposures made in conventional "contact" geometry, but much less attention has been given to doses from magnification geometry. Estimation based on contact film dose is difficult because the grid is removed, the geometry is different, and calculation of scatter reaching the film is complex. In this paper, exposures of Perspex blocks of standard thickness to give the same film density in both geometries are compared on 20 X-ray sets of various designs with a nominal magnification 1.8. The ratio derived between doses in each geometry is 2.2±0.15, giving an average magnification film dose of 5.0±0.3 mGy.
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Introduction
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Patient breast dose is obviously important in breast screening programmes as in any other branch of radiology. Not only should it be kept as low as reasonably achievable and consistent with satisfactory film quality, but the balance of benefit and risk also has to be considered, especially for younger women. Much attention has been given to breast-dose measurement for conventional mammograms taken in contact geometry, but doses from magnification films as used at the first assessment stage in a screening programme have received considerably less attention. This is reasonable, at least in the early years of a programme, because numbers of contact films taken greatly exceed those of magnification films. Also, a simplified consideration of the benefit/risk relationship strongly suggests that the assessment stage is more beneficial than the initial screening stage. Nevertheless, the UK Breast Screening Programme (NHSBSP) was started more than 15 years ago, and no estimate of breast dose from magnification films appears to have been published in that time. This paper will attempt to estimate that dose, at least within the NHS BSP, and in relation to contact film doses on the same X-ray units. The results will then be used in a later paper to consider the balance of benefit and risk at the assessment stage.
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Methods
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Methods for direct estimates of patient dose, based on mAs values recorded for mammographic exposures to groups of 50–100 women, are well established in the NHSBSP [1, 2]. That is a very reasonable and worthwhile approach for conventional contact films, for which large quantities of data can be obtained within a very few days on each X-ray unit. For magnification films, where the numbers of films taken are very much smaller, much longer times would be needed, with correspondingly more scope for variability in related parameters such as film processing.
An alternative approach is to derive an estimate for the ratio of magnification and contact film doses from observations with phantoms such as plain blocks of polymethyl methacrylate (PMMA), also known as Perspex. This ratio, if it is derived from results obtained on a sufficiently wide range of X-ray unit designs, can then be applied to deduce an estimated magnification film dose from a national dose average for contact films. In the UK that national dose average is well established [3] and is normally used in benefit/risk assessments for breast screening [4]. This approach leads to a national average magnification film dose rather than ones for specific X-ray units, though the latter could also be estimated given enough data on each design of unit. As with contact films, it is the national average dose which seems most appropriate for benefit/risk discussions.
A simple calculation of magnification film dose based on conventional contact film dose and an inverse square law correction for the closer distance of the breast from the tube is not possible. With magnification films the grid is removed, and an indeterminate proportion of scatter will reach the film and contribute to its eventual density, though the proportion is small because of the large air gap between breast and film. A small proportion of scatter will also penetrate the grid in contact films, but there is no basis for supposing that these proportions are the same. Moreover, magnification films are always taken using fine focus, for which X-ray tube outputs, in mGy/mAs at a standard distance, and half value layers may both differ from their values on broad focus on the same X-ray unit. In estimating dose, half value layer (HVL) is used as a parameter which affects dose at depth, and hence mean glandular dose, in relation to entrance air kerma.
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Equipment used and data obtained
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Films were taken of 4 cm thick blocks of PMMA (Perspex) under automatic exposure control (AEC) on both broad and fine focus on a number of different X-ray units. Film densities and mAs values were recorded. On 10 of these units, extra films had been taken earlier for a range of mAs values, by varying the fine control of the AEC system. Graphs were plotted for these 10 sets of results of optical density (OD) aginst mAs. These graphs were plotted on linear scales because of the relatively narrow range of OD values concerned. They yielded an average slope of 2.74 mAs/0.1 OD, i.e. approximately 5% in mAs per 0.1 in OD. The standard error on this mean of 2.74 was ±3.8%. This value was then applied to the broad and fine focus block film densities and mAs values on all the X-ray units to derive the pairs of mAs values, broad and fine, corresponding to a standard film density.
This "standard" film density was not the same for all the X-ray units; for comparisons to be valid it is only necessary that the density chosen as standard should be the same within each pair of films, i.e. contact and magnification, on each X-ray unit. Densities were chosen as standard which were as close as possible to those observed, so that the corrections for density differences were as small as possible in order to minimize errors arising from this calculation. All such standard densities lay between 1.5 and 2.0 and thus will have been within the straight line portion of the film–screen characteristic curve.
In order to illustrate the method of calculation which has been used, specimen results for one model of X-ray unit are shown in Table 1
, with corrected values and the first part of the subsequent calculation.
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Table 1. (b) Calculation of distance factors and depth dose corrections for various models of X-ray unit, for 4 cm Perspex blocks, and nominal magnification 1.8
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Output values, in mGy or µGy per mAs, were obtained on both broad and fine focus, and initially corrected to a standard focal distance such as 50 cm or 100 cm. Corrected mAs values on broad and fine focus films were then multiplied by the corresponding output figures to give the values of air kerma at the standard distance. Perspex block doses can then be calculated for the actual distances of the blocks from the focus by inverse square law, and breast dose for the corresponding breast thickness could be calculated by applying a further factor gc taken from standard tables [1, 5]. Values of this factor gc are determined by the half value layer (HVL) on each X-ray unit. Measurements of HVL had been made on both broad and fine focus. Differences between the two values on each X-ray unit were small but were taken into account at a later stage of the calculation.
Because the eventual aim of the calculation is a ratio of breast dose for magnification films and contact geometry films, the question of dosemeter calibration does not need to be considered. Similarly, the actual value of the factor gc should be the same on broad and fine focus, subject only to two corrections, both quite small. The first of these allows for possible differences in HVL between broad and fine focus and has already been mentioned.
In Scotland, data were available on 13 X-ray units having magnification facilities and fine foci. For one of these units the magnification factor was 1.6, and data from this unit were omitted at this stage. On seven of the remaining 12 units the magnification factor was a fixed 1.8, while on the other 5 units it could be set at 3 different values, but all block films had been obtained at a factor of 1.8. In Yorkshire, data were available on a further eight X-ray units, all at a nominal magnification of 1.8. The ratio of notional block doses on broad and fine focus was then calculated for all these 20 X-ray units.
A further correction is then necessary for the systematic error arising from the variation of percentage depth dose with focus–skin distance (FSD). The gc factor referred to above links entrance air kerma to mean glandular dose in a breast of standard composition and standard thickness, in this case 5 cm. This factor is itself a combination of several factors, including one for X-ray attenuation within the breast material and one for the fall-off of dose with distance by inverse square law, so that the effect of inverse square law itself for contact films at a conventional FFD of 60 cm or 65 cm is already included in the gc factor. However, at a substantially different FSD, as for magnification films, the fall-off of dose with depth is somewhat different, given by a factor
where d is depth in the breast and F1 and F2 are the two FSD values. For example, taking a mean depth in a 4 cm Perspex block as 2 cm, and F1 and F2 as 65 cm and 39 cm respectively, this correction factor is 1.04, with depth dose being relatively greater at longer FSD.
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Results
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Specimen results from five X-ray sets of one particular model are shown in Table 1a
. This table illustrates the initial stages of the calculation. These stages lead to a simple ratio of µGy values, but uncorrected for differences in distance and for differences in dose at a depth due to differences in FSD for equipment produced by different manufacturers. These corrections now have to be applied (Table 1b). They depend on the particular dimensions of each X-ray unit, i.e. distances from tube focus to film cassette, and to the surface of the Perspex in contact and in magnification geometries. These distances were measured on several X-ray units of each model, where available, and averaged to yield a simple multiplying factor for each model to be applied to the µGy ratios on each unit of that model. For the Siemens 3000 units (Siemens, Erlangen, Germany) the resulting factor was 2.78 (Table 1a, b).
A similar procedure was followed for all X-ray units in Scotland, and for the further eight X-ray units in Yorkshire. In Scotland all data refer to exposures with Molybdenum target and filter at 28 kV; the Yorkshire results were obtained at various tube voltages between 25 kV and 28 kV. All these results are summarized in Tables 2 and 3, grouped according to tube voltage, model of X-ray unit, and region. Names of manufacturers and models of X-ray units are given in Table 1b. Further details of variations in some models are given in Table 4. These variations are slight and extremely unlikely to have any effect on dose estimations.
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Table 2. Dose ratios, magnification/contact geometry. Means for various X-ray unit models in two regions grouped according to tube voltage. Ratios calculated as illustrated in Table 1
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Table 3. Dose ratios, magnification/contact geometry. Means for various X-ray unit models in two regions. Ratios calculated as illustrated in Table 1. 28 kV, Mo/Mo unless indicated
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Table 4. Comparison of magnification/contact film dose ratios for Perspex blocks of 2, 4 and 6 cm thickness. 28 kV, Mo/Mo unless indicated
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Further results were obtained on most of the X-ray units for 2 cm and 6 cm Perspex, in the same way as for 4 cm blocks. The distance factors and depth dose corrections were slightly different for each thickness. These results are given in Table 4, where values for 2 cm and 6 cm are compared with those for 4 cm.
Table 2 also shows data for slightly different values of tube voltage, where these are available on the same model of X-ray unit. It appears that varying tube voltage from 28 kV to 25 kV has no detectable effect on the magnification dose ratio, as far as can be seen from the limited quantity of data available on this point. Accordingly, data from all tube voltage values and all X-ray units are included in Table 3, where numbers in brackets indicate the number of X-ray units which have contributed to each value. Uncertainties quoted are standard errors on the means (68% confidence level) where four or more results were available; ranges are stated where n was less than four. Table 3 suggests that there is no statistically significant variation in dose ratio between the four X-ray units concerned, with the possible exception of the Diamond unit. There are not enough results available to confirm that point, but it appears unlikely that any difference that may exist will amount to as much as 10%, and is therefore unlikely to be of any practical importance.
Results were also obtained on some X-ray units with 2 cm and 6 cm Perspex blocks. Distance factors and depth dose corrections were slightly different for each thickness, and these differences were allowed for. These results are given in Table 4, which compares dose ratios for the three thicknesses. There appears to be no significant difference between the results for 4 cm and 6 cm blocks, but for 2 cm blocks the magnification dose ratio is probably smaller by between about 5% and 10%. Results from the Alpha RT unit with nominal magnification 1.6 have been included here and agree with those for the other units at 1.8 magnification. Excluding this one set of results would not affect any subsequent conclusions.
The overall mean dose ratio, based on results for 20 X-ray units having nominal magnification 1.8, is 2.13±0.04 for 25 kV to 28 kV and Molybdenum target and filter. To this value it is now desirable to make a small correction for the slight difference in HVL between broad and fine foci. For the 13 X-ray units for which HVL values on both foci were available, the mean difference was 0.005 mm Al. Fine focus HVL values are slightly higher than for broad focus, presumably because in some X-ray tubes there is a difference in target angle. This HVL difference results in a 1% difference in the gc factor converting entrance air kerma into mean dose [1, 5]. Accordingly the mean dose ratio given above should be increased from 2.13 to 2.15±0.04.
So far, all results have been obtained using Perspex blocks, but these results show that there is no significant variation in magnification dose ratio between 4 cm and 6 cm Perspex. The mean compressed breast thickness found in the NHSBSP is 5.5 cm [2, 6], which is considered to be equivalent to between 4.5 cm and 5 cm Perspex. Thus the dose ratio values found at 4 cm and 6 cm Perspex should apply also to compressed breasts of average thickness, and will probably apply to the greater part of the observed breast thickness range without appreciable additional error. Because the results for 6 cm Perspex blocks are on average 2% greater than 4 cm blocks, the average result for this range of Perspex thickness is taken as 1% greater than the 4 cm result. This increases the result for breast thickness around 5.6 cm from 2.15 to 2.17±0.04. This uncertainty will be discussed further in the next section.
To convert Perspex block measurements to entrance air kerma and then to estimates of breast dose it is necessary to use factors such as those tabulated in references [1, 5]. Those factors have not been used in the present study, apart from one small correction based on small differences in HVL, because the aim has been to arrive at a ratio of dose with magnification and contact geometries.
All results described so far refer to a magnification of 1.8. Results were also obtained on a few X-ray sets for magnifications of 1.5, 1.6 and 2.0. These showed that, relative to results for 1.8, there was little difference apart from the obvious major difference due to distance from the X-ray focus. At 1.5 magnification, the dose ratio of 2.17 for 1.8 would be reduced to approximately 1.5; at magnifications of 1.6 and 2.0 the corresponding ratios would be about 1.7 and 2.9, respectively.
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Discussion
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This section is mainly concerned with discussion of uncertainties, both random and systematic. The random uncertainty arising from results in Table 3 has been shown to be less than ±2% (at 68% confidence level). Thus, systematic errors need only be considered if they are comparable with or exceed this magnitude at the same confidence level.
The gc factors used in calculation here are intended for use with measurements on 4.5 cm Perspex block phantoms, and to apply to the "new standard breast", whereas the previous pg factors apply to 4 cm Perspex and the "old standard breast". The old standard assumed 50%–50% adipose and glandular tissue whereas the new standard varies these proportions with breast thickness. If the broad and fine focus beams have the same HVL, then whichever set of factors is used, there is no effect on the ratio of doses because the same factor is used in each. The greatest difference in HVL observed in the 12 X-ray units considered here is 0.02 mmAl and the average difference is only 0.005 mmAl. For an HVL difference of 0.02 mmAl the difference in effect between using the new gc and the old pg factors is only 0.2% in the dose ratio. This is negligible.
Random errors in HVL measurement could be up to ±0.01 mmAl, giving up to a 4% error in dose ratio in the worst case [1, 5], but errors from this cause would be included in the random error on the mean dose ratio. Systematic errors in tube output measurement on any one tube, e.g. ±3 mm in distance measurement, would also be included in the random variation between tubes already observed in the dose ratio values.
A number of simplifying assumptions have been made in this paper. First, in using the factors derived by Dance et al [5] for magnification geometry, no allowance has been made for any greater non-uniformity in the incident beam, due either to heel effect or to beam divergence, or for any difference in back scatter from the image receptor. Similarly no allowance has been made for the fact that with lower tube current in magnification mode there could be a theoretical effect due to reciprocity law failure. All these effects are considered to be small, probably each of them less than about 2–3%, and not necessarily all in the same direction. We are concerned here only with the differences that might arise in these effects between conventional and magnification geometries. In the case of back scatter for example, this is around 8% at mammographic energies and field sizes. Accordingly a systematic uncertainty of ±5% will be included to allow for all effects mentioned in this paragraph.
A further question is whether, on modern X-ray units with automatic selection of target/filter and tube voltage, the same combination will necessarily be chosen for both contact and magnification modes on any given breast. To investigate that point in detail would be a separate exercise, beyond the scope of this paper. However, in Scotland at present almost all magnification films are taken at 28 kV with Mo target and filter, so any differences would arise in the contact film. Results shown in Table 2 suggest that small variations in tube voltage have negligible effect. It should be emphasised that this paper only aims to estimate magnification film dose where the same or very similar exposure factors are selected for films taken in the same mode.
Hence the overall uncertainty on the dose ratio of 2.17 is probably in the region of ±0.15, if random and systematic uncertainties are combined by simple addition, so that a final value of 2.2±0.15 may be the best estimate.
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Conclusion
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The final value for the ratio of breast dose under magnification to that with conventional contact geometry is estimated to be 2.2±0.15. The mean breast doses in the NHSBSP have been shown to be 2.5 mGy for lateral oblique views and 2.0 mGy for craniocaudal views. Applying this ratio yields corresponding magnification doses of 4.4 mGy and 5.5 mGy. Either view may be used in magnification techniques. Thus the best estimate of mean dose for magnification is 5.0±0.3 mGy.
The consequences for risk of cancer induction and for benefit/risk calculations will be discussed in another paper, but it may be remarked here that magnification films are normally taken on one breast only, with half the risk which would arise if the same dose was given to both breasts. Moreover, this is the dose for a full field film. Many magnification films, perhaps the majority, are taken with the irradiated field considerably reduced, ("paddle films") to about one third of the full field, with a corresponding further reduction in effective dose and risk.
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Acknowledgments
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This research was partly supported by the European Commission's Radiation Protection Research Programme (Dimond III).
I wish to thank Mr A Watt (Edinburgh) and Mrs C Clayton (Leeds) for providing data. Together with Miss D A Shaw, Mrs J Coupar, Miss A Mumby, Mrs C Deas, Mrs S Simpson and Mrs H Macdonald (Scotland), they kindly made extra measurements or provided films at my request. I also wish to thank Dr K Faulkner (Newcastle) for suggesting that this project be undertaken, and for helpful discussion.
Received for publication February 4, 2005.
Revision received April 11, 2005.
Accepted for publication April 20, 2005.
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References
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- Institute of Physical Sciences in Medicine. The Commissioning and Routine Testing of Mammographic X-ray Systems (IPSM Report 59 2nd edition) York: IPSM, 1994.
- Faulkner K, Law J, Robson KJ. Assessment of mean glandular dose in mammography. Br J Radiol 1995;68:877–81.[Abstract/Free Full Text]
- Young KC, Burch A, Oduko JM. Radiation doses received in the UK Breast Screening Programme in 2001 and 2002. Br J Radiol 2005;78:207–18.[Abstract/Free Full Text]
- Law J, Faulkner K. Two view screening and extending the age range: the balance of benefit and risk. Br J Radiol 2002;75:889–94.[Abstract/Free Full Text]
- Dance DR, Skinner CL, Young KC, Beckett JC, Kotre CJ. Additional factors for the estimation of mean glandular breast dose using the UK mammography dosimetry protocol. Phys Med Biol 2000;45:3225–40.[CrossRef][Medline]
- Burch A, Law J. A method for estimating compressed breast thickness during mammography. Br J Radiol 1995;68:394–9.[Abstract/Free Full Text]
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Abstract
Introduction
