Journal of the ICRU Vol 5 No 2 (2005) Report 74 Oxford University Press
doi:10.1093/jicru/ndi033
APPENDIX E: REVIEW OF MONTE CARLO CALCULATIONS FOR ASSESSMENT OF MEAN GLANDULAR DOSE IN MAMMOGRAPHY GENERAL
Among the quantities used for dose specification in mammography the average absorbed dose in glandular tissue DG is most appropriate for risk assessments (NCRP, 1986). Generally, DG is derived from incident air kerma Ka.i combined with conversion coefficients obtained from radiation transport calculations in mathematical models of the breast. For simplicity the notation cG Eq. (E.1) is used instead of cG,Ka,i Eq. (3.3.8) DG ¼ cG Ka;i :
ðE:1Þ
Conversion coefficients cG have been published by various authors (Stanton et al., 1984; Rosenstein et al., 1985; Dance, 1990; Wu et al., 1991, 1994; Alm Carlsson and Dance, 1992; Jansen et al., 1994; Zoetelief and Jansen, 1995; Klein et al., 1997; Dance et al., 2000). Values of cG are usually presented as a function of the HVL1 of the incident radiation. Commonly, cG values are calculated using simple breast phantoms. These breast models contain a superficial layer of 0.4 or 0.5 cm thickness representing the skin and underlying adipose tissue and a central region consisting of a mixture of adipose and glandular tissue (Figure E.1). Most of the publications deal with firm compression of the breast only, because it has been clearly demonstrated that this is mandatory in view of dose reduction and improvement of image quality (NCRP, 1986). Concerning the elemental tissue composition it should be noted that most authors employ data published by Hammerstein et al. (1979). The influence of the variation in the fraction of glandular tissue in the central region of the simulated female breast is included in several publications (Rosenstein et al., 1985; Wu et al., 1991a, 1994; Jansen et al., 1994; Klein et al., 1997; Dance et al., 2000). Alm Carlsson and Dance (1992) and Zoetelief and Jansen (1995) studied the influence of variations in elemental composition of glandular and adipose tissues for the range of values stated by Hammerstein et al. (1979)
and between Hammerstein et al. (1979) and ICRU (1989) elemental compositions, respectively. The calculations by different authors differ in radiation transport codes, photon interaction data, photon spectra, composition and thickness of superficial layer (representing skin and subcutaneous adipose tissue), presence of a compression plate, and focus-to-film distance dFFD. They are generally not performed with the tissue compositions recommended by the ICRU (1989a). A study on the influence of these differences on cG (Table E.1) was made by Zoetelief and Jansen (1995). It can be concluded from the table that there are various sources of
Plan
16 cm Elevation variable thickness
0.4 cm or 0.5 cm Adipose tissue 50 : 50 Adipose and glandular tissue Figure E.1. Mathematical model of the breast used in Monte Carlo calculations of average glandular dose. Shown are the superficial layer (either 0.4 cm of glandular tissue or 0.5 cm of adipose tissue) and the central region consisting of 50 % adipose and 50 % glandular tissue by mass (Reproduced from Dance, 1990, with permission from IOP, UK). The 50:50 breast model was first suggested by Hammerstein et al. (1979).
International Commission on Radiation Units and Measurements 2005
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
E.1
PATIENT DOSIMETRY FOR X RAYS USED IN MEDICAL IMAGING Table E.1. Influence of difference in approach by various authors in the calculation of cG (Zoetelief and Jansen, 1995). Parameter
Range of variation
Spectra at same HVL1 for same tube voltage and anode-filter combination Photon interaction data: MCPLIB (1988) versus XCOM (Berger and Hubbell, 1987) Presence or absence of compression plate Composition and thickness of superficial layer Hammerstein’s tissue composition ICRU tissue composition Tissue composition Hammerstein versus ICRU Fraction of glandular tissue in central region (HVL1: 0.4 mm Al, 6-cm-thick breast, 25 % glandular tissue versus 50 % glandular tissue)
7 % 10 % 3–4 % (3 mm PMMA) 11–19 % (breast thickness 8-2 cm) 3–10 % (breast thickness 8-2 cm) 11–14 % (at breast thickness 8-2 cm) 10–13 %
HVL1
Rosenstein et al. (1985)a
mm Al 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
(Mo/Mo) 0.171 (0.160) – 0.217 (0.200) – – 0.314 –
Dance (1990)b
Wu et al. (1991)
Jansen et al. (1994)c
Wu et al. (1994)d
Dance et al. (2000)e
0.164 0.187 0.209 0.232 0.258 0.287 0.310 0.332
0.146 0.172 0.195 – – – – –
– 0.168 (Mo/Mo) 0.191–0.198 (W/Mo–Mo/Mo) 0.221–0.229 (Mo/Rh–Rh/Rh) 0.257 (Rh/Rh) 0.273 (W/Rh) 0.294 (W/Rh) —
0.151–0.153 0.174–0.180 0.196–0.209 0.220–0239 0.242–0.260 – – –
0.164 0.188–0.194 0.216–0.224 0.241–0.253 0.270 0.296 0.321 –
a
Conversion coefficients for tungsten targets; conversion coefficients given in brackets refer to Mo and Mo/W targets HVL1 range: 0.25–0.45 mm Al: Mo/Mo; 0.45–0.70 mm Al: W/Mo; 0.50–0.80 mm Al: W/Rh; 0.55–0.90 mm Al: W/Pd; 0.50–2.00 mm Al: W/Al. c Anode/filter materials include Mo/Mo, W/Mo, Mo/Rh, Rh/Rh and W/Rh. d Lower values refer to Mo/Rh and higher values to Rh/Rh. Values can differ by 5 % depending on differences in tube voltage. e Anode filter combinations also include Mo/Mo, Mo/Rh, Rh/Al, Rh/Rh and W/Rh (Dance, 1990). b
difference with values ranging from 3 to 19 %. Consequently, differences in cG-values of the order of 20 to 30 % could have occurred between results published by different authors. However, in practice maximum differences of 15 % are observed. E.2
thickness. The method does not allow for corrections due to variations in the fraction of glandular tissue present in the breast. This method will be advantageous when the distribution of compressed breast thickness cannot be reasonably represented by standard breast thicknesses of 4.5 or 5 cm. The third method (Appendix E.5) enables assessment of DG for variations in compressed breast thickness and fraction of glandular tissue. This method requires an assessment of the fraction of glandular tissue.
SCOPE
Three approaches for dose assessment in mammography are presented in this appendix. The first method (Appendix E.3) concerns determination of DG for a 4.5 (ACR, 1992) or 5-cm-thick standard breast (EC, 1996b). According to the American College of Radiology (ACR) protocol, Ka,i is measured using a mammographic phantom equivalent to 4.5 cm compressed breast tissue. The EC recommends Ka,i measurements for 5-cm-thick breasts or alternatively for a 4.5-cm-thick PMMA phantom (used in practice to mimic a ‘standard’ breast). DG then results from measured Ka,i and application of the appropriate conversion coefficient cG, selected on the basis of HVL1. The second method (Appendix E.4) also takes into account the variation in compressed breast
E.3 CONVERSION COEFFICIENTS CG FOR A 4.5 OR 5-CM-THICK ‘STANDARD’ BREAST Table E.2 shows a summary of conversion coefficients cG calculated by various authors for a standard sized breast, i.e., having a total thickness of 5 cm and a central region consisting of 50 % adipose and 50 % glandular tissue by mass, examined under firm compression. At an HVL1 of 0.30 mm Al employing a Mo/Mo anode-filter combination the largest difference is 12 %; at values of HVL1 of 0.35 and 0.4 mm Al for the same anode/filter combination these 94
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
Table E.2. Conversion coefficient cG that relates Ka,i to DG for a 5-cm-thick breast having a central region of 50 % adipose and 50 % glandular tissue by mass (tissue compositions according to Hammerstein et al., 1979).
APPENDIX E Table E.3. Differences in conversion coefficients cG due to differences in technical parameters in mammography. Technical parameter
Variation
Tube voltage at same HVL1 and target material Tube voltage at same HVL1 and anode/filter combination, but different layers of Lexan filtration Presence or absence of a compression plate Difference in anode/filter combinations at same HVL1
Within 10 % from the average (Rosenstein, 1984) Within 5 % from the average (Wu et al., 1991)
CONVERSION COEFFICIENT; CG
HVL1 / mm Al Figure E.2. Calculated conversion coefficients for mammary gland cG for a 5.0-cm-thick breast model with a central region consisting of 50 % adipose and 50 % glandular tissue as a function of the HVL1 for various anode/filter combinations (EC, 1996b). The anode the anode material is indicated by the chemical symbol, the filter thickness is given in mm and the filter material by the trailing symbol.
differences are reduced to 9 and 7 %, respectively. For larger values of HVL1 the differences remain approximately constant at 7 %. These differences are not large in view of those due to variations in technical parameters (Table E.3). Concerning the influence of tube voltage for the same target type and HVL1 of the incident radiation, the variations reported by Rosenstein et al. (1985) are somewhat larger than those published by Wu et al. (1991, 1994). Wu et al. varied tube voltage and also included an additional layer of Lexan (polycarbonate). This latter filtration will have an
effect similar to the introduction of a compression plate. The influence on cG of different anode/filter combinations producing the same HVL1 is dependent on the compressed breast thickness (Dance, 1990). Figure E.2 shows the conversion coefficients cG as a function of HVL1 for a 5-cm-thick ‘standard’ breast, i.e., containing a central region consisting of 50 % adipose tissue and 50 % glandular tissue by mass using various anode/filter combinations. The difference for a specific anode/filter combination from the average value amounts to 5 % at maximum. This is 95
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
3–4 % (3 mm PMMA, Dance, 1990) Few % from the average between W, Mo–W alloy and Mo targets (Rosenstein, 1984) 3 % for a 4.5-cm-thick breast, for Mo/Mo, W/Mo, W/Rh, W/Pd and W/Al combinations (Dance, 1990) 7 % difference between Mo/Rh and Rh/Rh (Wu et al., 1994) Within 5 % for a 5-cm-thick ‘standard’ breast for various anode/filter combinations (Jansen, 1994) Relative spectral correction factors are given by Dance et al. (2000). They range up to 1.061 – 0.036 for Rh/Rh
PATIENT DOSIMETRY FOR X RAYS USED IN MEDICAL IMAGING Table E.4. Conversion coefficients cG for calculating DG for a 4.5-cm-thick breast (50 % adipose/50 % glandular tissue) from Ka,i (after ACR, 1992. Reprinted with permission from the American College of Radiology). HVL1b
23 0.124 0.129 0.133 0.138 0.144 0.148 0.154 0.159 0.164 0.169 0.174 0.179
24
0.132 0.137 0.141 0.146 0.151 0.156 0.161 0.166 0.171 0.176 0.181 0.186
25
0.139 0.144 0.148 0.153 0.158 0.163 0.168 0.172 0.177 0.182 0.187 0.192
26
0.146 0.151 0.155 0.161 0.165 0.170 0.174 0.179 0.184 0.189 0.194 0.198
27
0.153 0.157 0.162 0.166 0.171 0.176 0.180 0.185 0.190 0.195 0.200 0.204
28
0.158 0.163 0.168 0.172 0.177 0.181 0.186 0.192 0.196 0.201 0.205 0.210
29
0.164 0.169 0.173 0.178 0.182 0.187 0.193 0.197 0.202 0.206 0.211 0.216
30
31
0.170 0.174 0.180 0.185 0.189 0.194 0.198 0.203 0.208 0.212 0.217 0.221
0.176 0.181 0.186 0.190 0.195 0.200 0.203 0.208 0.212 0.218 0.222
32
0.182 0.187 0.192 0.196 0.201 0.204 0.209 0.213 0.219 0.223 0.228
33
0.182 0.187 0.192 0.196 0.201 0.205 0.210 0.214 0.219 0.223 0.228 0.233
0.194 0.200 0.205 0.211 0.217 0.221 0.227 0.233 0.237 0.243 0.247 0.252 0.257 0.262 0.267 0.271
a
W/Al target-filter combination, peak tube voltage 45 kV, filter thickness 1.6 mm. HVL1 in mm Al.
b
in agreement with the findings by the other investigators (Table E.2). In addition, Wu et al. (1991) report a dependence of cG on tube voltage waveform amounting to 4 % between one phase, two pulse, and constant potential generators. This finding is less relevant today as modern mammography units are equipped with high-frequency tube-voltage generators, which provide approximately constant potentials. Alm Carlsson and Dance (1992) report variations in dose up to 4 % across an 8-cm-thick breast for a homogeneous x-ray field. This variation is due to a lack of side scatter and does not include a heel effect. It is concluded that the differences in results obtained by various investigators are rather small, i.e., 10 cm (EC, 1996b). The attenuation in a reference PMMA phantom is somewhat smaller than that in the corresponding 5-cm-thick standard breast model used for numerical dosimetry. Therefore, the conversion coefficients from Ka,i to DG are somewhat larger than those for the 5-cm-thick standard breast and are given in Table E.5 (EC, 1996b). E.4 CONVERSION COEFFICIENTS CG AS A FUNCTION OF BREAST THICKNESS Almost all authors provide cG as a function of HVL1 and compressed breast thickness. As an 96
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.30 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.40 0.41 0.42 0.43 0.44 0.45
W/Ala
Mo/30 mm Mo target-filter combination X-ray peak tube voltage/kV
APPENDIX E Table E.5. Conversion coefficients for calculating DG for a 5.0 cm ‘standard’ breast from Ka,i for 4.5-cm-thick standard PMMA phantom (EC, 1996b). cG
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
0.177 0.202 0.223 0.248 0.276 0.304 0.326 0.349
Ratio of conversion factors
HVL1/mm Al
1.4
0% glandularity
1.2
1.0
0.8 100% glandularity 0.6 2
6 cm
7 cm
8 cm
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
0.135 0.154 0.172 0.192 0.214 0.236 0.261 0.282
0.114 0.130 0.145 0.163 0.177 0.202 0.224 0.244
0.098 0.112 0.126 0.140 0.154 0.175 0.195 0.212
0.390 0.433 0.473 0.509 0.543 0.573 0.587 0.622
0.274 0.309 0.342 0.374 0.406 0.437 0.466 0.491
0.207 0.235 0.261 0.289 0.318 0.346 0.374 0.399
0.183 0.208 0.232 0.258 0.285 0.311 0.399 0.363
0.164 0.187 0.209 0.232 0.258 0.287 0.310 0.332
6
8
Figure E.3. Ratios of conversion coefficients cG for breasts with glandularities different from 50 % for a HVL1 of 0.35 mm Al. The upper curve is the ratio for 0–50 % and the lower curve is the ratio for 100–50 %. In each case the solid line corresponds to the data of [Dance et al. (2000), reproduced with permission from IOP, UK], the open circles to the data of Wu et al. (1991), and the closed circles to the data of Klein et al. (1997).
tube-current exposure-time product for radiographs of phantoms of various thickness having central regions with compositions of 0, 50, and 100 % glandular tissue. By comparing the tube-current exposure-time product measured for the phantoms of different thickness and composition with that determined during actual patient examinations at known compressed breast thickness the fraction of glandular tissue for a patient can be derived. Klein et al. (1997) determined the fractions of glandular tissue in German women. They found that the average composition ranged from 75 % at a compressed breast thickness of 25 mm through 40 % at 50 mm breast thickness to an approximately constant level of 20 % for breast thicknesses in excess of 70 mm. Similar work has been done by Geise and Palchevsky (1996) and by Young et al. (1998) with excellent and reasonable agreement with the results of Klein et al. (1997), respectively. An alternative method to take the fraction of glandular tissue into account for assessment of DG has been presented by Dance et al. (2000). Dance et al. (2000) propose to extend Eq. (E.1) to
example, Table E.6 shows cG values as a function of thickness of ‘standard’ breasts, i.e., mathematical models having a superficial layer of 0.5 cm adipose tissue and a central region of 50 % adipose tissue and 50 % glandular tissue by mass. Application of these conversion coefficients implies that the fraction of glandular tissue is independent of compressed breast thickness. This, however, is not the case in practice (Geise and Palchevsky, 1996; Klein et al., 1997; Young et al., 1998).
E.5 CONVERSION COEFFICIENTS CG FOR ASSESSMENT OF DG TAKING BREAST COMPOSITION INTO ACCOUNT Conversion coefficients cG for fractions of glandular tissue in the central region of the compressed breast differing from 50 % have been presented by various investigators (Rosenstein et al., 1985; Wu et al., 1991, 1994; Jansen et al., 1994; Klein et al., 1997; Dance et al., 2000). Figure E.3 shows ratios of conversion coefficients cG for breasts with glandularities different from 50 % for a HVL1 of 0.35 mm Al. A method for the assessment of the fraction of glandular tissue has been presented by Klein et al. (1997). Measurements were made of the
DG ¼ Ka;i cG cs,
ðE:2Þ
where c corrects for any difference in breast composition from 50 % glandular tissue, and the factor s for any difference from the cG tabulation by Dance (1990) due to the use of a different x-ray spectrum. Correction factors c are tabulated against HVL1 and breast thickness. They are available as a function of glandularity and for breasts of typical glandularity at a given breast thickness for women in the age groups 40–49 years and 50–64 years. 97
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
HVL1/ cG for breast thicknesses of mm Al 2 cm 3 cm 4 cm 4.5 cm 5 cm
4
Breast thickness (cm)
Table E.6. The conversion coefficient cG that relates Ka,i to DG for ‘standard’ breast phantoms (Reproduced from Dance, 1990, with permission from IOP, UK).
doi:10.1093/jicru/ndi033
APPENDIX E: REVIEW OF MONTE CARLO CALCULATIONS FOR ASSESSMENT OF MEAN GLANDULAR DOSE IN MAMMOGRAPHY GENERAL
Among the quantities used for dose specification in mammography the average absorbed dose in glandular tissue DG is most appropriate for risk assessments (NCRP, 1986). Generally, DG is derived from incident air kerma Ka.i combined with conversion coefficients obtained from radiation transport calculations in mathematical models of the breast. For simplicity the notation cG Eq. (E.1) is used instead of cG,Ka,i Eq. (3.3.8) DG ¼ cG Ka;i :
ðE:1Þ
Conversion coefficients cG have been published by various authors (Stanton et al., 1984; Rosenstein et al., 1985; Dance, 1990; Wu et al., 1991, 1994; Alm Carlsson and Dance, 1992; Jansen et al., 1994; Zoetelief and Jansen, 1995; Klein et al., 1997; Dance et al., 2000). Values of cG are usually presented as a function of the HVL1 of the incident radiation. Commonly, cG values are calculated using simple breast phantoms. These breast models contain a superficial layer of 0.4 or 0.5 cm thickness representing the skin and underlying adipose tissue and a central region consisting of a mixture of adipose and glandular tissue (Figure E.1). Most of the publications deal with firm compression of the breast only, because it has been clearly demonstrated that this is mandatory in view of dose reduction and improvement of image quality (NCRP, 1986). Concerning the elemental tissue composition it should be noted that most authors employ data published by Hammerstein et al. (1979). The influence of the variation in the fraction of glandular tissue in the central region of the simulated female breast is included in several publications (Rosenstein et al., 1985; Wu et al., 1991a, 1994; Jansen et al., 1994; Klein et al., 1997; Dance et al., 2000). Alm Carlsson and Dance (1992) and Zoetelief and Jansen (1995) studied the influence of variations in elemental composition of glandular and adipose tissues for the range of values stated by Hammerstein et al. (1979)
and between Hammerstein et al. (1979) and ICRU (1989) elemental compositions, respectively. The calculations by different authors differ in radiation transport codes, photon interaction data, photon spectra, composition and thickness of superficial layer (representing skin and subcutaneous adipose tissue), presence of a compression plate, and focus-to-film distance dFFD. They are generally not performed with the tissue compositions recommended by the ICRU (1989a). A study on the influence of these differences on cG (Table E.1) was made by Zoetelief and Jansen (1995). It can be concluded from the table that there are various sources of
Plan
16 cm Elevation variable thickness
0.4 cm or 0.5 cm Adipose tissue 50 : 50 Adipose and glandular tissue Figure E.1. Mathematical model of the breast used in Monte Carlo calculations of average glandular dose. Shown are the superficial layer (either 0.4 cm of glandular tissue or 0.5 cm of adipose tissue) and the central region consisting of 50 % adipose and 50 % glandular tissue by mass (Reproduced from Dance, 1990, with permission from IOP, UK). The 50:50 breast model was first suggested by Hammerstein et al. (1979).
International Commission on Radiation Units and Measurements 2005
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
E.1
PATIENT DOSIMETRY FOR X RAYS USED IN MEDICAL IMAGING Table E.1. Influence of difference in approach by various authors in the calculation of cG (Zoetelief and Jansen, 1995). Parameter
Range of variation
Spectra at same HVL1 for same tube voltage and anode-filter combination Photon interaction data: MCPLIB (1988) versus XCOM (Berger and Hubbell, 1987) Presence or absence of compression plate Composition and thickness of superficial layer Hammerstein’s tissue composition ICRU tissue composition Tissue composition Hammerstein versus ICRU Fraction of glandular tissue in central region (HVL1: 0.4 mm Al, 6-cm-thick breast, 25 % glandular tissue versus 50 % glandular tissue)
7 % 10 % 3–4 % (3 mm PMMA) 11–19 % (breast thickness 8-2 cm) 3–10 % (breast thickness 8-2 cm) 11–14 % (at breast thickness 8-2 cm) 10–13 %
HVL1
Rosenstein et al. (1985)a
mm Al 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
(Mo/Mo) 0.171 (0.160) – 0.217 (0.200) – – 0.314 –
Dance (1990)b
Wu et al. (1991)
Jansen et al. (1994)c
Wu et al. (1994)d
Dance et al. (2000)e
0.164 0.187 0.209 0.232 0.258 0.287 0.310 0.332
0.146 0.172 0.195 – – – – –
– 0.168 (Mo/Mo) 0.191–0.198 (W/Mo–Mo/Mo) 0.221–0.229 (Mo/Rh–Rh/Rh) 0.257 (Rh/Rh) 0.273 (W/Rh) 0.294 (W/Rh) —
0.151–0.153 0.174–0.180 0.196–0.209 0.220–0239 0.242–0.260 – – –
0.164 0.188–0.194 0.216–0.224 0.241–0.253 0.270 0.296 0.321 –
a
Conversion coefficients for tungsten targets; conversion coefficients given in brackets refer to Mo and Mo/W targets HVL1 range: 0.25–0.45 mm Al: Mo/Mo; 0.45–0.70 mm Al: W/Mo; 0.50–0.80 mm Al: W/Rh; 0.55–0.90 mm Al: W/Pd; 0.50–2.00 mm Al: W/Al. c Anode/filter materials include Mo/Mo, W/Mo, Mo/Rh, Rh/Rh and W/Rh. d Lower values refer to Mo/Rh and higher values to Rh/Rh. Values can differ by 5 % depending on differences in tube voltage. e Anode filter combinations also include Mo/Mo, Mo/Rh, Rh/Al, Rh/Rh and W/Rh (Dance, 1990). b
difference with values ranging from 3 to 19 %. Consequently, differences in cG-values of the order of 20 to 30 % could have occurred between results published by different authors. However, in practice maximum differences of 15 % are observed. E.2
thickness. The method does not allow for corrections due to variations in the fraction of glandular tissue present in the breast. This method will be advantageous when the distribution of compressed breast thickness cannot be reasonably represented by standard breast thicknesses of 4.5 or 5 cm. The third method (Appendix E.5) enables assessment of DG for variations in compressed breast thickness and fraction of glandular tissue. This method requires an assessment of the fraction of glandular tissue.
SCOPE
Three approaches for dose assessment in mammography are presented in this appendix. The first method (Appendix E.3) concerns determination of DG for a 4.5 (ACR, 1992) or 5-cm-thick standard breast (EC, 1996b). According to the American College of Radiology (ACR) protocol, Ka,i is measured using a mammographic phantom equivalent to 4.5 cm compressed breast tissue. The EC recommends Ka,i measurements for 5-cm-thick breasts or alternatively for a 4.5-cm-thick PMMA phantom (used in practice to mimic a ‘standard’ breast). DG then results from measured Ka,i and application of the appropriate conversion coefficient cG, selected on the basis of HVL1. The second method (Appendix E.4) also takes into account the variation in compressed breast
E.3 CONVERSION COEFFICIENTS CG FOR A 4.5 OR 5-CM-THICK ‘STANDARD’ BREAST Table E.2 shows a summary of conversion coefficients cG calculated by various authors for a standard sized breast, i.e., having a total thickness of 5 cm and a central region consisting of 50 % adipose and 50 % glandular tissue by mass, examined under firm compression. At an HVL1 of 0.30 mm Al employing a Mo/Mo anode-filter combination the largest difference is 12 %; at values of HVL1 of 0.35 and 0.4 mm Al for the same anode/filter combination these 94
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
Table E.2. Conversion coefficient cG that relates Ka,i to DG for a 5-cm-thick breast having a central region of 50 % adipose and 50 % glandular tissue by mass (tissue compositions according to Hammerstein et al., 1979).
APPENDIX E Table E.3. Differences in conversion coefficients cG due to differences in technical parameters in mammography. Technical parameter
Variation
Tube voltage at same HVL1 and target material Tube voltage at same HVL1 and anode/filter combination, but different layers of Lexan filtration Presence or absence of a compression plate Difference in anode/filter combinations at same HVL1
Within 10 % from the average (Rosenstein, 1984) Within 5 % from the average (Wu et al., 1991)
CONVERSION COEFFICIENT; CG
HVL1 / mm Al Figure E.2. Calculated conversion coefficients for mammary gland cG for a 5.0-cm-thick breast model with a central region consisting of 50 % adipose and 50 % glandular tissue as a function of the HVL1 for various anode/filter combinations (EC, 1996b). The anode the anode material is indicated by the chemical symbol, the filter thickness is given in mm and the filter material by the trailing symbol.
differences are reduced to 9 and 7 %, respectively. For larger values of HVL1 the differences remain approximately constant at 7 %. These differences are not large in view of those due to variations in technical parameters (Table E.3). Concerning the influence of tube voltage for the same target type and HVL1 of the incident radiation, the variations reported by Rosenstein et al. (1985) are somewhat larger than those published by Wu et al. (1991, 1994). Wu et al. varied tube voltage and also included an additional layer of Lexan (polycarbonate). This latter filtration will have an
effect similar to the introduction of a compression plate. The influence on cG of different anode/filter combinations producing the same HVL1 is dependent on the compressed breast thickness (Dance, 1990). Figure E.2 shows the conversion coefficients cG as a function of HVL1 for a 5-cm-thick ‘standard’ breast, i.e., containing a central region consisting of 50 % adipose tissue and 50 % glandular tissue by mass using various anode/filter combinations. The difference for a specific anode/filter combination from the average value amounts to 5 % at maximum. This is 95
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
3–4 % (3 mm PMMA, Dance, 1990) Few % from the average between W, Mo–W alloy and Mo targets (Rosenstein, 1984) 3 % for a 4.5-cm-thick breast, for Mo/Mo, W/Mo, W/Rh, W/Pd and W/Al combinations (Dance, 1990) 7 % difference between Mo/Rh and Rh/Rh (Wu et al., 1994) Within 5 % for a 5-cm-thick ‘standard’ breast for various anode/filter combinations (Jansen, 1994) Relative spectral correction factors are given by Dance et al. (2000). They range up to 1.061 – 0.036 for Rh/Rh
PATIENT DOSIMETRY FOR X RAYS USED IN MEDICAL IMAGING Table E.4. Conversion coefficients cG for calculating DG for a 4.5-cm-thick breast (50 % adipose/50 % glandular tissue) from Ka,i (after ACR, 1992. Reprinted with permission from the American College of Radiology). HVL1b
23 0.124 0.129 0.133 0.138 0.144 0.148 0.154 0.159 0.164 0.169 0.174 0.179
24
0.132 0.137 0.141 0.146 0.151 0.156 0.161 0.166 0.171 0.176 0.181 0.186
25
0.139 0.144 0.148 0.153 0.158 0.163 0.168 0.172 0.177 0.182 0.187 0.192
26
0.146 0.151 0.155 0.161 0.165 0.170 0.174 0.179 0.184 0.189 0.194 0.198
27
0.153 0.157 0.162 0.166 0.171 0.176 0.180 0.185 0.190 0.195 0.200 0.204
28
0.158 0.163 0.168 0.172 0.177 0.181 0.186 0.192 0.196 0.201 0.205 0.210
29
0.164 0.169 0.173 0.178 0.182 0.187 0.193 0.197 0.202 0.206 0.211 0.216
30
31
0.170 0.174 0.180 0.185 0.189 0.194 0.198 0.203 0.208 0.212 0.217 0.221
0.176 0.181 0.186 0.190 0.195 0.200 0.203 0.208 0.212 0.218 0.222
32
0.182 0.187 0.192 0.196 0.201 0.204 0.209 0.213 0.219 0.223 0.228
33
0.182 0.187 0.192 0.196 0.201 0.205 0.210 0.214 0.219 0.223 0.228 0.233
0.194 0.200 0.205 0.211 0.217 0.221 0.227 0.233 0.237 0.243 0.247 0.252 0.257 0.262 0.267 0.271
a
W/Al target-filter combination, peak tube voltage 45 kV, filter thickness 1.6 mm. HVL1 in mm Al.
b
in agreement with the findings by the other investigators (Table E.2). In addition, Wu et al. (1991) report a dependence of cG on tube voltage waveform amounting to 4 % between one phase, two pulse, and constant potential generators. This finding is less relevant today as modern mammography units are equipped with high-frequency tube-voltage generators, which provide approximately constant potentials. Alm Carlsson and Dance (1992) report variations in dose up to 4 % across an 8-cm-thick breast for a homogeneous x-ray field. This variation is due to a lack of side scatter and does not include a heel effect. It is concluded that the differences in results obtained by various investigators are rather small, i.e., 10 cm (EC, 1996b). The attenuation in a reference PMMA phantom is somewhat smaller than that in the corresponding 5-cm-thick standard breast model used for numerical dosimetry. Therefore, the conversion coefficients from Ka,i to DG are somewhat larger than those for the 5-cm-thick standard breast and are given in Table E.5 (EC, 1996b). E.4 CONVERSION COEFFICIENTS CG AS A FUNCTION OF BREAST THICKNESS Almost all authors provide cG as a function of HVL1 and compressed breast thickness. As an 96
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.30 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.40 0.41 0.42 0.43 0.44 0.45
W/Ala
Mo/30 mm Mo target-filter combination X-ray peak tube voltage/kV
APPENDIX E Table E.5. Conversion coefficients for calculating DG for a 5.0 cm ‘standard’ breast from Ka,i for 4.5-cm-thick standard PMMA phantom (EC, 1996b). cG
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
0.177 0.202 0.223 0.248 0.276 0.304 0.326 0.349
Ratio of conversion factors
HVL1/mm Al
1.4
0% glandularity
1.2
1.0
0.8 100% glandularity 0.6 2
6 cm
7 cm
8 cm
0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65
0.135 0.154 0.172 0.192 0.214 0.236 0.261 0.282
0.114 0.130 0.145 0.163 0.177 0.202 0.224 0.244
0.098 0.112 0.126 0.140 0.154 0.175 0.195 0.212
0.390 0.433 0.473 0.509 0.543 0.573 0.587 0.622
0.274 0.309 0.342 0.374 0.406 0.437 0.466 0.491
0.207 0.235 0.261 0.289 0.318 0.346 0.374 0.399
0.183 0.208 0.232 0.258 0.285 0.311 0.399 0.363
0.164 0.187 0.209 0.232 0.258 0.287 0.310 0.332
6
8
Figure E.3. Ratios of conversion coefficients cG for breasts with glandularities different from 50 % for a HVL1 of 0.35 mm Al. The upper curve is the ratio for 0–50 % and the lower curve is the ratio for 100–50 %. In each case the solid line corresponds to the data of [Dance et al. (2000), reproduced with permission from IOP, UK], the open circles to the data of Wu et al. (1991), and the closed circles to the data of Klein et al. (1997).
tube-current exposure-time product for radiographs of phantoms of various thickness having central regions with compositions of 0, 50, and 100 % glandular tissue. By comparing the tube-current exposure-time product measured for the phantoms of different thickness and composition with that determined during actual patient examinations at known compressed breast thickness the fraction of glandular tissue for a patient can be derived. Klein et al. (1997) determined the fractions of glandular tissue in German women. They found that the average composition ranged from 75 % at a compressed breast thickness of 25 mm through 40 % at 50 mm breast thickness to an approximately constant level of 20 % for breast thicknesses in excess of 70 mm. Similar work has been done by Geise and Palchevsky (1996) and by Young et al. (1998) with excellent and reasonable agreement with the results of Klein et al. (1997), respectively. An alternative method to take the fraction of glandular tissue into account for assessment of DG has been presented by Dance et al. (2000). Dance et al. (2000) propose to extend Eq. (E.1) to
example, Table E.6 shows cG values as a function of thickness of ‘standard’ breasts, i.e., mathematical models having a superficial layer of 0.5 cm adipose tissue and a central region of 50 % adipose tissue and 50 % glandular tissue by mass. Application of these conversion coefficients implies that the fraction of glandular tissue is independent of compressed breast thickness. This, however, is not the case in practice (Geise and Palchevsky, 1996; Klein et al., 1997; Young et al., 1998).
E.5 CONVERSION COEFFICIENTS CG FOR ASSESSMENT OF DG TAKING BREAST COMPOSITION INTO ACCOUNT Conversion coefficients cG for fractions of glandular tissue in the central region of the compressed breast differing from 50 % have been presented by various investigators (Rosenstein et al., 1985; Wu et al., 1991, 1994; Jansen et al., 1994; Klein et al., 1997; Dance et al., 2000). Figure E.3 shows ratios of conversion coefficients cG for breasts with glandularities different from 50 % for a HVL1 of 0.35 mm Al. A method for the assessment of the fraction of glandular tissue has been presented by Klein et al. (1997). Measurements were made of the
DG ¼ Ka;i cG cs,
ðE:2Þ
where c corrects for any difference in breast composition from 50 % glandular tissue, and the factor s for any difference from the cG tabulation by Dance (1990) due to the use of a different x-ray spectrum. Correction factors c are tabulated against HVL1 and breast thickness. They are available as a function of glandularity and for breasts of typical glandularity at a given breast thickness for women in the age groups 40–49 years and 50–64 years. 97
Downloaded from http://jicru.oxfordjournals.org/ at York University Libraries on July 6, 2012
HVL1/ cG for breast thicknesses of mm Al 2 cm 3 cm 4 cm 4.5 cm 5 cm
4
Breast thickness (cm)
Table E.6. The conversion coefficient cG that relates Ka,i to DG for ‘standard’ breast phantoms (Reproduced from Dance, 1990, with permission from IOP, UK).
