1992, The British Journal of Radiology, 65, 523-527
The application of microdosimetry in clinical bone densitometry using a dual-photon absorptiometer By *A. J . Waker, PhD, B. Oldroyd and t M . Marco Department of Medical Physics, University of Leeds, The General Infirmary, Leeds LS1 3EX, UK and tlnstituto de Estudios de la Energia, CIEMAT, Avenida Complutense 22, 28040 Madrid, Spain
(Received 10 April 1991 and in revised form 29 October 1991, accepted 14 January 1992) Keywords: Microdosimetry, Dosimetry, X-rays, Absorptiometry
Abstract. Experimental microdosimetric methods have been used to determine absorbed dose values for a scanning dual-photon absorptiometer. Absorbed doses within the scanned field have been obtained for three different scanning speeds. For the normal speed setting used clinically, measurements have also been carried out in a water-filled phantom in order to estimate typical patient entrance, exit and midline doses. The results agree well with values obtained using thermoluminescence dosimetry and support the claims of the manufacturers with regard to upper limits placed on patient dose levels. The microdosimetric method enables changes in radiation quality to be followed and comparisons to be made with other low-energy photon fields used in medical diagnosis.
In recent times, new dual-photon absorptiometers have become commercially available, enabling selected site and whole-body scans of bone density to be carried out in the clinical environment. These devices facilitate the study of bone mineralization in groups of patients suffering from conditions known to affect bone loss. As part of one such study recently carried out in Leeds, 270 normal women between the ages of 20 and 80 years were scanned using a Lunar DPX scanner (Lunar, Madison-Wisconsin) in order to establish a set of control data for bone mineral density (BMD). In this paper we present data on the dosimetry of the scanner determined experimentally using microdosimetric techniques. As an absorbed dose integrating technique, microdosimetry is a suitable method for making measurements in a scanning beam and so can provide direct comparative data to more conventional thermoluminescent dosimetry (TLD), which has an associated uncertainty in its response to low-energy photons (Delgado et al, 1987). Furthermore, although it is currently not considered helpful to allocate different quality factors to photon fields of different energy (International Commission on Radiological Protection (ICRP), 1990), nevertheless at the secondary electron interaction level, differences in quality do exist (International Commission on Radiation Units and Measurements (ICRU), 1986a). These differences are directly revealed in microdosimetric measurements, which can then be used as input data in radiological risk analysis of the type described by Brenner and Amols (1989). Experimental methods
The DPX scanner (Fig. 1 a) is based on the use of a cerium-filtered and highly collimated beam from an •Present address: AECL Research, Chalk River Laboratories, Chalk River, Ontario, Canada, K0J 1JO. Vol. 65, No. 774
X-ray generator operated at 76 kVp, in effect supplying a dual-energy photon beam with narrow energy bands centred about 38 and 70 keV (Lunar, 1988a). The X-ray tube and sodium iodide detector are scanned across the patient over a pre-selected field size. Three
water filled trunk phantom
scale: O.5m
(a) MCB1
Rossi counter
MCA/PC
(b) Figure 1. (a) An end view of the Lunar DPX scanning dualphoton absorptiometer. (b) Schematic diagram of the microdosimetric measuring system with the tissue-equivalent proportional counter positioned in the water-filled phantom. MCB1 and MCB2 are the multichannel buffer cards and MCA/PC is a computer-based multichannel analyser.
523
A. J. Waker, B. Oldroyd and M. Marco Table I. Absorbed dose values and microdosimetric averages derived from data recorded with a tissue-equivalent proportional counter free-in-air at three different scan settings of a dual-photon-energy X-ray absorptiometer Measurement name (No. of scans)
Scan data
Microdosimetric averages
Absorbed dose
Speed (mm/s)
Pixel (mm)
Dose per scan 0«Gy)
Dose-rate OiGy/s)
(keV/Aon)
(keV/Aim)
DPXl (4) DPX2 (5) DPX3 (8)
38.4 76.8 153.6
1.2 1.2 2.4
12.9 6.4 1.6
47.3 47.0 46.3
1.59 1.58 1.60
3.20 3.18 3.19
DPX4 (5) DPX5 (5) DPX6 (5)
38.4 76.8 153.6
1.2 1.2 2.4
13.0 6.4 1.6
47.7 47.0 46.3
1.58 1.57 1.57
3.14 3.14 3.17
scanning speeds are available. For the slow and medium scanning speeds the incremental step between each scan line is 1.2 mm and for the fast speed the step is 2.4 mm. Table I lists the linear velocities of the beam for the three settings. Differential absorption between the higher- and lower-energy photons provides the raw data for mapping out the bone density in the scanned field. The microdosimetric measuring system employed in the investigation is shown schematically in Fig. lb. The tissue-equivalent proportional counter (TEPC) used was a commercially available \ inch Rossi counter (Far West Technology, Goletha, California) filled with propane-based tissue-equivalent gas. Most of the measurements were carried out with the counter gas pressure set such that the TEPC simulated a unit density site size of 2 /xm, a few measurements being carried out at a 10 /xm site size. Signals from the proportional counter were recorded using two amplifiers with a gain factor of 10 between them and two multichannel buffer cards (EG&G Instruments). In this manner the complete range of pulse heights from the TEPC can be recorded in the form of two subspectra, which are subsequently joined, as part of the data analysis, to form a single microdosimetric event-size spectrum. The two multichannel buffer cards are part of an integrated pulse-height analysis and microcomputer system enabling full analysis of the microdosimetric data at the site of measurement. After calibration of the TEPC, by means of an internal 244Cm alpha source depositing a known amount of energy, the pulses from the TEPC are scaled in terms of imparted energy and the absorbed dose can be obtained by integration of the measured pulse height spectrum. In effect this is a measurement of the absorbed dose on an event-by-event basis where in this context an event is the deposition of energy in the tissueequivalent counter by a single interacting electron that has been generated in the counter wall or gas cavity by the primary X-ray field. The scan dimensions were set up by placing lead markers above and below the TEPC and observing the image on the DPX output monitor. Field sizes 5 cm by 10 cm ensured that the TEPC was completely scanned. 524
In the study, two sets of measurements were carried out with the TEPC free-in-air and with the DPX operated at three different scanning speeds. Measurements were also made -with the DPX operated at the medium scanning speed and the TEPC placed at three different depths in the trunk section of a Bush phantom filled with water. A Bush phantom consists of hollow circular and elliptical polythene cylinders simulating the various regions of the human body (Bush, 1946). This set of measurements provided an estimate of patient entrance, midline and exit doses. Further measurements were also carried out in the phantom with the TEPC operated with a 10 fim simulated diameter. Results
Table I provides the free-in-air absorbed doses and microdosimetric averages derived from the measured event-size spectra, an example of which is shown in Fig. 2. The microdosimetric spectra recorded for the three different scan settings were found to be identical and so only the spectrum for the medium speed is shown. For presentation purposes it is usual for the TEPC pulses to be scaled in terms of lineal energy, y, which is the imparted energy divided by the mean chord length of the TEPC simulated site and has the units of keV/^m. The recorded pulse-height spectrum from the TEPC is then displayed as the fraction of the absorbed dose per logarithmic interval of lineal energy. In this format, equal areas under the spectrum represent equal contributions to the absorbed dose (ICRU, 1986b) and the event-size spectrum is known as a dose distribution. Microdosimetric spectra can be characterised by two average quantities, the frequency mean, yF, and the dose mean, yD. The frequency mean is the first moment of the measured event-size frequency spectrum and the dose mean is the first moment of the recorded data converted to a dose distribution. Both these quantities are used to express the average microdosimetric quality of a radiation field. Table II provides a summary of the dosimetric data obtained from the phantom measurements that were carried out at the medium-scan-speed setting. Also shown in Table II are the dosimetric data derived from The British Journal of Radiology, June 1992
Microdosimetric measurements of a clinical dual-photon absorptiometer yd(y)
0.8 -
0.01
0.1
1.0 10 LINEAL ENERGY y
Figure 2. Microdosimetric event size spectrum yd(y) giving the absorbed dose fraction per logarithmic interval of lineal energy y for a measurement made free-inair at the position of the couch top and for a TEPC 2 fim simulated diameter. Equal areas under the curve represent equal dose fractions.
100 1000 (keV/jjm)
measurements carried out in the phantom at the beam entrance and exit with a 10 ^m TEPC simulated diameter.
factor given by the ratio of the beam area, Ab, to the total scanned area A^n, thus:
Discussion
For a fixed scanning speed the beam sweeps out equal areas of field in equal time intervals, so Z>scan will be the same everywhere within the scanned field although the total imparted energy to the patient does of course increase in direct proportion to the field size scanned. As ^scan ' s proportional to the scan velocity and scan time, the dose per scan will, for a fixed pixel size, be inversely proportional to the scan velocity. This is seen to be the case in Table I. Using the relation between the dose per scan and the intrinsic beam dose-rate given above, the beam dose-
Examination of Table I shows that for the two independent sets of measurements made, the dosimetric results are in good agreement. For a stationary beam the absorbed dose to a layer of uniform density and unit thickness will be given by the intrinsic dose-rate of the beam D, multiplied by the time of exposure. For a scanning beam, however, during an identical time interval, the same imparted energy is deposited in a much larger mass and therefore the absorbed dose, £>scan, in a scanned field will be less by a
Table II. Dosimetric and microdosimetric quantities derived from measurements made with a tissue-equivalent proportional counter using two different simulated diameters in a water phantom scanned by a dual-photon-energy X-ray absorptiometer Scan data
Measurement data Name
Position
4> Om)
No.
Absorbed dose
Microdosimetric averages
Speed (mm/s)
Pixel (mm)
Dose/scan OGy)
Dose rate (p.Gy/s)
(keV//im)
yD (keV/jan)
DPX7 DPX8 DPX9
ENT ML EX
2 2 2
5 5 10
76.8 76.8 76.8
1.2 1.2 1.2
7.4 1.9 0.36
54.3 13.9 2.64
1.63 1.50 1.45
3.04 2.99 3.21
DPX10 DPX11
ENT EX
10 10
6 18
76.8 76.8
1.2 1.2
7.2 0.43
52.8 3.16
1.06 0.99
1.88 1.81
ENT = entrance, base of phantom: 17 cm depthln water. ML = midline, centre of phantom: 8.5 cm depth in water. EX = exit, surface of phantom: centre line of counter at zero depth. (f> Tissue-equivalent proportional counter simulated unit density diameter. Vol. 65, No. 774
525
A. J. Waker, B. Oldroyd and M. Marco
rate can be determined from the measured scan doses. The ratio (AXJA^ has been estimated from the DPX manufacturer's data concerning the beam size and, for the field sizes used, is 396.3. Derived beam dose-rates are given in Table I and to within less than 2% a constant value is found, as should be the case for a machine parameter independent of the selected scanning velocity. At the edges of the field the beam is almost stationary as it increments forward before commencing the next line. Therefore at the field edges the dose-rate will be higher than the scan dose and will approach that of the intrinsic beam dose-rate. The microdosimetric event-size spectrum corresponding to the free-air measurements is shown in Fig. 2. Although the beam is composed of two principal photon energies around 70 keV and 38 keV, because of the change in proportion of photoelectric effect to Compton scattering that takes place between these two energies, the mean energy of the secondary electrons produced in tissue-like materials remains almost the same for the two energies at around 12 keV. At these energies and for a 2 /im simulated diameter, the electrons are mostly crossing the counter and the device still behaves as a good lineal energy spectrometer with a peak in the event-size spectrum at round 3.9 keV/^m. This is in agreement with that found with other measurements recorded in this range of photon energies (Schmitz et al, 1989). The constancy of the microdosimetric averages for the various scanning conditions shown in the final columns of Table I is a consequence of the similarity between the measured spectra and indicates no change in the beam microdosimetric quality. As the beam passes through a phantom the proportion of the 70 keV X-ray energy peak changes from about 20% to 60% of the total influence (Sorenson et al, 1989). However, as discussed above, the mean secondary electron energy does not change significantly and therefore the microdosimetric spectra for the inphantom measurements are the same regardless of depth of measurement. Comparison of the microdosimetric averages for the 2 /mi measurements in Table II shows that the beam quality is indeed the same as for the free-air measurements. The absorbed dose per scan, however, is reduced by a factor of around 20 as one moves from the beam entrance to the exit on the far side of the phantom. It is interesting to note that the dose per scan at the phantom entrance is higher than that recorded in air for the same speed by around 14%, owing to the presence of scattering material above the point of measurement. In previous work carried out to investigate the energy response of the TEPC to measure absorbed dose at low photon energy (Evazi, 1990), it was seen that with a 10 /mi simulated diameter the TEPC had a marginally flatter energy response than with a 2 /«n site size. This prompted the measurements to be made at 10 /mi, and these were found to be in very good agreement with those given for the 2 /mi simulated diameter. 526
Conclusions
The objective of the work reported in this paper has been to investigate the application of a tissue-equivalent proportional counter and microdosimetric technique to the dosimetry of a dual-photon absorptiometer The scan absorbed doses agree well with TLD dosimetry carried out by hospital radiation protection staff, who measured scan doses of between 8 and 9 /*Gy for eight measurements at the medium speed. These measurements were made at the base of an aluminium spine phantom set in a block of wax, and so correspond most closely with the measurements reported here for the phantom entrance dose. The recorded doses-perscan are significantly lower than the DPX manufacturer's data of upper limits: 15 ftGy for the medium speed, 30 fiGy for the slow speed and 5 /iGy for the fast setting (Lunar, 1988b). The measurement of dose using microdosimetric methods has the advantage that changes in radiation quality can be followed and comparisons made with other applications of low-energy X rays in medicine. A comparative quality factor to that of mammography X rays can be derived in the manner suggested by Brenner and Amols (1989) and discussed by Waker and Marco (1991), by incorporating the ICRU 40 quality factor versus lineal energy function (ICRU, 1986a) into the measured event-sized spectra. Quality factors derived in this way are 1.18 and 0.88 for the mammography beam and DPX beam, respectively. Acknowledgments This work has been funded by the British Council, whose support is gratefully acknowledged, under the Acciones Integradas scheme. References BRENNER, D. J. & AMOLS, H. I., 1989. Enhanced risk from low energy screen-film mammograph X-rays. British Journal of Radiology, 62, 910-914. BUSH, F., 1946. Energy absorption in radium therapy. British Journal of Radiology, 19, 14-21. DELGADO, V., GONZALEZ, L., MARCO, M., MORAN, P. & VANO,
E., 1987. Experimental determination of the relative TL response of TLD-100 chips for 60 kVp X- and ^Co gammarays. Nuclear Instruments and Methods, A255, 238-241. EVAZI, M. T., 1990. Application of experimental microdosimetric techniques to the dosimetry of X-rays in the diagnostic energy range. PhD Thesis, University of Leeds. ICRP. 1990. Recommendations of the International Commission on Radiological Protection, Publication 60. Annals of the ICRP, 27(1-3), 81. ICRU, 1986a. The Quality Factor in Radiation Protection, Report 400 (International Commission on Radiation Units and Measurements, Bethesda, Maryland). 1986b. Microdosimetry, Report 36 (International Commission on Radiation Units and Measurements, Bethesda, Maryland). LUNAR, 1988a. Technical Note No. 1. Introduction to DPX and Basic Principles. (Lunar, 313 W. Beltline Hwy, Madison, Wisconsin 53713, USA). 1988b. Technical Note No. 3. DPX Performance: Dose (Lunar, 313 W. Beltline Hwy, Madison, Wisconsin 53713, USA).
The British Journal of Radiology, June 1992
Microdosimetric measurements of a clinical dual-photon absorptiometer SCHMITZ, TH., KRAMER, H. M. & Booz, J., 1989. Assessment of the photon response of a TEPC: implementation of operational quantities for dose equivalent. Radiation Protection Dosimetry, 29, 69-74. SORENSON, J. A., DUKE, P. R. & SMITH, S. W., 1989.
Vol. 65, No. 774
Simulation studies of dual energy X-ray absorptiometry. Medical Physics, 16(1), 75-80. WAKER, A. J. & MARCO, M., 1991. The application of microdosimetry to the metrology of low-energy X rays used in mammography. British Journal of Radiology, 65, 258-261.
527
The application of microdosimetry in clinical bone densitometry using a dual-photon absorptiometer By *A. J . Waker, PhD, B. Oldroyd and t M . Marco Department of Medical Physics, University of Leeds, The General Infirmary, Leeds LS1 3EX, UK and tlnstituto de Estudios de la Energia, CIEMAT, Avenida Complutense 22, 28040 Madrid, Spain
(Received 10 April 1991 and in revised form 29 October 1991, accepted 14 January 1992) Keywords: Microdosimetry, Dosimetry, X-rays, Absorptiometry
Abstract. Experimental microdosimetric methods have been used to determine absorbed dose values for a scanning dual-photon absorptiometer. Absorbed doses within the scanned field have been obtained for three different scanning speeds. For the normal speed setting used clinically, measurements have also been carried out in a water-filled phantom in order to estimate typical patient entrance, exit and midline doses. The results agree well with values obtained using thermoluminescence dosimetry and support the claims of the manufacturers with regard to upper limits placed on patient dose levels. The microdosimetric method enables changes in radiation quality to be followed and comparisons to be made with other low-energy photon fields used in medical diagnosis.
In recent times, new dual-photon absorptiometers have become commercially available, enabling selected site and whole-body scans of bone density to be carried out in the clinical environment. These devices facilitate the study of bone mineralization in groups of patients suffering from conditions known to affect bone loss. As part of one such study recently carried out in Leeds, 270 normal women between the ages of 20 and 80 years were scanned using a Lunar DPX scanner (Lunar, Madison-Wisconsin) in order to establish a set of control data for bone mineral density (BMD). In this paper we present data on the dosimetry of the scanner determined experimentally using microdosimetric techniques. As an absorbed dose integrating technique, microdosimetry is a suitable method for making measurements in a scanning beam and so can provide direct comparative data to more conventional thermoluminescent dosimetry (TLD), which has an associated uncertainty in its response to low-energy photons (Delgado et al, 1987). Furthermore, although it is currently not considered helpful to allocate different quality factors to photon fields of different energy (International Commission on Radiological Protection (ICRP), 1990), nevertheless at the secondary electron interaction level, differences in quality do exist (International Commission on Radiation Units and Measurements (ICRU), 1986a). These differences are directly revealed in microdosimetric measurements, which can then be used as input data in radiological risk analysis of the type described by Brenner and Amols (1989). Experimental methods
The DPX scanner (Fig. 1 a) is based on the use of a cerium-filtered and highly collimated beam from an •Present address: AECL Research, Chalk River Laboratories, Chalk River, Ontario, Canada, K0J 1JO. Vol. 65, No. 774
X-ray generator operated at 76 kVp, in effect supplying a dual-energy photon beam with narrow energy bands centred about 38 and 70 keV (Lunar, 1988a). The X-ray tube and sodium iodide detector are scanned across the patient over a pre-selected field size. Three
water filled trunk phantom
scale: O.5m
(a) MCB1
Rossi counter
MCA/PC
(b) Figure 1. (a) An end view of the Lunar DPX scanning dualphoton absorptiometer. (b) Schematic diagram of the microdosimetric measuring system with the tissue-equivalent proportional counter positioned in the water-filled phantom. MCB1 and MCB2 are the multichannel buffer cards and MCA/PC is a computer-based multichannel analyser.
523
A. J. Waker, B. Oldroyd and M. Marco Table I. Absorbed dose values and microdosimetric averages derived from data recorded with a tissue-equivalent proportional counter free-in-air at three different scan settings of a dual-photon-energy X-ray absorptiometer Measurement name (No. of scans)
Scan data
Microdosimetric averages
Absorbed dose
Speed (mm/s)
Pixel (mm)
Dose per scan 0«Gy)
Dose-rate OiGy/s)
(keV/Aon)
(keV/Aim)
DPXl (4) DPX2 (5) DPX3 (8)
38.4 76.8 153.6
1.2 1.2 2.4
12.9 6.4 1.6
47.3 47.0 46.3
1.59 1.58 1.60
3.20 3.18 3.19
DPX4 (5) DPX5 (5) DPX6 (5)
38.4 76.8 153.6
1.2 1.2 2.4
13.0 6.4 1.6
47.7 47.0 46.3
1.58 1.57 1.57
3.14 3.14 3.17
scanning speeds are available. For the slow and medium scanning speeds the incremental step between each scan line is 1.2 mm and for the fast speed the step is 2.4 mm. Table I lists the linear velocities of the beam for the three settings. Differential absorption between the higher- and lower-energy photons provides the raw data for mapping out the bone density in the scanned field. The microdosimetric measuring system employed in the investigation is shown schematically in Fig. lb. The tissue-equivalent proportional counter (TEPC) used was a commercially available \ inch Rossi counter (Far West Technology, Goletha, California) filled with propane-based tissue-equivalent gas. Most of the measurements were carried out with the counter gas pressure set such that the TEPC simulated a unit density site size of 2 /xm, a few measurements being carried out at a 10 /xm site size. Signals from the proportional counter were recorded using two amplifiers with a gain factor of 10 between them and two multichannel buffer cards (EG&G Instruments). In this manner the complete range of pulse heights from the TEPC can be recorded in the form of two subspectra, which are subsequently joined, as part of the data analysis, to form a single microdosimetric event-size spectrum. The two multichannel buffer cards are part of an integrated pulse-height analysis and microcomputer system enabling full analysis of the microdosimetric data at the site of measurement. After calibration of the TEPC, by means of an internal 244Cm alpha source depositing a known amount of energy, the pulses from the TEPC are scaled in terms of imparted energy and the absorbed dose can be obtained by integration of the measured pulse height spectrum. In effect this is a measurement of the absorbed dose on an event-by-event basis where in this context an event is the deposition of energy in the tissueequivalent counter by a single interacting electron that has been generated in the counter wall or gas cavity by the primary X-ray field. The scan dimensions were set up by placing lead markers above and below the TEPC and observing the image on the DPX output monitor. Field sizes 5 cm by 10 cm ensured that the TEPC was completely scanned. 524
In the study, two sets of measurements were carried out with the TEPC free-in-air and with the DPX operated at three different scanning speeds. Measurements were also made -with the DPX operated at the medium scanning speed and the TEPC placed at three different depths in the trunk section of a Bush phantom filled with water. A Bush phantom consists of hollow circular and elliptical polythene cylinders simulating the various regions of the human body (Bush, 1946). This set of measurements provided an estimate of patient entrance, midline and exit doses. Further measurements were also carried out in the phantom with the TEPC operated with a 10 fim simulated diameter. Results
Table I provides the free-in-air absorbed doses and microdosimetric averages derived from the measured event-size spectra, an example of which is shown in Fig. 2. The microdosimetric spectra recorded for the three different scan settings were found to be identical and so only the spectrum for the medium speed is shown. For presentation purposes it is usual for the TEPC pulses to be scaled in terms of lineal energy, y, which is the imparted energy divided by the mean chord length of the TEPC simulated site and has the units of keV/^m. The recorded pulse-height spectrum from the TEPC is then displayed as the fraction of the absorbed dose per logarithmic interval of lineal energy. In this format, equal areas under the spectrum represent equal contributions to the absorbed dose (ICRU, 1986b) and the event-size spectrum is known as a dose distribution. Microdosimetric spectra can be characterised by two average quantities, the frequency mean, yF, and the dose mean, yD. The frequency mean is the first moment of the measured event-size frequency spectrum and the dose mean is the first moment of the recorded data converted to a dose distribution. Both these quantities are used to express the average microdosimetric quality of a radiation field. Table II provides a summary of the dosimetric data obtained from the phantom measurements that were carried out at the medium-scan-speed setting. Also shown in Table II are the dosimetric data derived from The British Journal of Radiology, June 1992
Microdosimetric measurements of a clinical dual-photon absorptiometer yd(y)
0.8 -
0.01
0.1
1.0 10 LINEAL ENERGY y
Figure 2. Microdosimetric event size spectrum yd(y) giving the absorbed dose fraction per logarithmic interval of lineal energy y for a measurement made free-inair at the position of the couch top and for a TEPC 2 fim simulated diameter. Equal areas under the curve represent equal dose fractions.
100 1000 (keV/jjm)
measurements carried out in the phantom at the beam entrance and exit with a 10 ^m TEPC simulated diameter.
factor given by the ratio of the beam area, Ab, to the total scanned area A^n, thus:
Discussion
For a fixed scanning speed the beam sweeps out equal areas of field in equal time intervals, so Z>scan will be the same everywhere within the scanned field although the total imparted energy to the patient does of course increase in direct proportion to the field size scanned. As ^scan ' s proportional to the scan velocity and scan time, the dose per scan will, for a fixed pixel size, be inversely proportional to the scan velocity. This is seen to be the case in Table I. Using the relation between the dose per scan and the intrinsic beam dose-rate given above, the beam dose-
Examination of Table I shows that for the two independent sets of measurements made, the dosimetric results are in good agreement. For a stationary beam the absorbed dose to a layer of uniform density and unit thickness will be given by the intrinsic dose-rate of the beam D, multiplied by the time of exposure. For a scanning beam, however, during an identical time interval, the same imparted energy is deposited in a much larger mass and therefore the absorbed dose, £>scan, in a scanned field will be less by a
Table II. Dosimetric and microdosimetric quantities derived from measurements made with a tissue-equivalent proportional counter using two different simulated diameters in a water phantom scanned by a dual-photon-energy X-ray absorptiometer Scan data
Measurement data Name
Position
4> Om)
No.
Absorbed dose
Microdosimetric averages
Speed (mm/s)
Pixel (mm)
Dose/scan OGy)
Dose rate (p.Gy/s)
(keV//im)
yD (keV/jan)
DPX7 DPX8 DPX9
ENT ML EX
2 2 2
5 5 10
76.8 76.8 76.8
1.2 1.2 1.2
7.4 1.9 0.36
54.3 13.9 2.64
1.63 1.50 1.45
3.04 2.99 3.21
DPX10 DPX11
ENT EX
10 10
6 18
76.8 76.8
1.2 1.2
7.2 0.43
52.8 3.16
1.06 0.99
1.88 1.81
ENT = entrance, base of phantom: 17 cm depthln water. ML = midline, centre of phantom: 8.5 cm depth in water. EX = exit, surface of phantom: centre line of counter at zero depth. (f> Tissue-equivalent proportional counter simulated unit density diameter. Vol. 65, No. 774
525
A. J. Waker, B. Oldroyd and M. Marco
rate can be determined from the measured scan doses. The ratio (AXJA^ has been estimated from the DPX manufacturer's data concerning the beam size and, for the field sizes used, is 396.3. Derived beam dose-rates are given in Table I and to within less than 2% a constant value is found, as should be the case for a machine parameter independent of the selected scanning velocity. At the edges of the field the beam is almost stationary as it increments forward before commencing the next line. Therefore at the field edges the dose-rate will be higher than the scan dose and will approach that of the intrinsic beam dose-rate. The microdosimetric event-size spectrum corresponding to the free-air measurements is shown in Fig. 2. Although the beam is composed of two principal photon energies around 70 keV and 38 keV, because of the change in proportion of photoelectric effect to Compton scattering that takes place between these two energies, the mean energy of the secondary electrons produced in tissue-like materials remains almost the same for the two energies at around 12 keV. At these energies and for a 2 /im simulated diameter, the electrons are mostly crossing the counter and the device still behaves as a good lineal energy spectrometer with a peak in the event-size spectrum at round 3.9 keV/^m. This is in agreement with that found with other measurements recorded in this range of photon energies (Schmitz et al, 1989). The constancy of the microdosimetric averages for the various scanning conditions shown in the final columns of Table I is a consequence of the similarity between the measured spectra and indicates no change in the beam microdosimetric quality. As the beam passes through a phantom the proportion of the 70 keV X-ray energy peak changes from about 20% to 60% of the total influence (Sorenson et al, 1989). However, as discussed above, the mean secondary electron energy does not change significantly and therefore the microdosimetric spectra for the inphantom measurements are the same regardless of depth of measurement. Comparison of the microdosimetric averages for the 2 /mi measurements in Table II shows that the beam quality is indeed the same as for the free-air measurements. The absorbed dose per scan, however, is reduced by a factor of around 20 as one moves from the beam entrance to the exit on the far side of the phantom. It is interesting to note that the dose per scan at the phantom entrance is higher than that recorded in air for the same speed by around 14%, owing to the presence of scattering material above the point of measurement. In previous work carried out to investigate the energy response of the TEPC to measure absorbed dose at low photon energy (Evazi, 1990), it was seen that with a 10 /mi simulated diameter the TEPC had a marginally flatter energy response than with a 2 /«n site size. This prompted the measurements to be made at 10 /mi, and these were found to be in very good agreement with those given for the 2 /mi simulated diameter. 526
Conclusions
The objective of the work reported in this paper has been to investigate the application of a tissue-equivalent proportional counter and microdosimetric technique to the dosimetry of a dual-photon absorptiometer The scan absorbed doses agree well with TLD dosimetry carried out by hospital radiation protection staff, who measured scan doses of between 8 and 9 /*Gy for eight measurements at the medium speed. These measurements were made at the base of an aluminium spine phantom set in a block of wax, and so correspond most closely with the measurements reported here for the phantom entrance dose. The recorded doses-perscan are significantly lower than the DPX manufacturer's data of upper limits: 15 ftGy for the medium speed, 30 fiGy for the slow speed and 5 /iGy for the fast setting (Lunar, 1988b). The measurement of dose using microdosimetric methods has the advantage that changes in radiation quality can be followed and comparisons made with other applications of low-energy X rays in medicine. A comparative quality factor to that of mammography X rays can be derived in the manner suggested by Brenner and Amols (1989) and discussed by Waker and Marco (1991), by incorporating the ICRU 40 quality factor versus lineal energy function (ICRU, 1986a) into the measured event-sized spectra. Quality factors derived in this way are 1.18 and 0.88 for the mammography beam and DPX beam, respectively. Acknowledgments This work has been funded by the British Council, whose support is gratefully acknowledged, under the Acciones Integradas scheme. References BRENNER, D. J. & AMOLS, H. I., 1989. Enhanced risk from low energy screen-film mammograph X-rays. British Journal of Radiology, 62, 910-914. BUSH, F., 1946. Energy absorption in radium therapy. British Journal of Radiology, 19, 14-21. DELGADO, V., GONZALEZ, L., MARCO, M., MORAN, P. & VANO,
E., 1987. Experimental determination of the relative TL response of TLD-100 chips for 60 kVp X- and ^Co gammarays. Nuclear Instruments and Methods, A255, 238-241. EVAZI, M. T., 1990. Application of experimental microdosimetric techniques to the dosimetry of X-rays in the diagnostic energy range. PhD Thesis, University of Leeds. ICRP. 1990. Recommendations of the International Commission on Radiological Protection, Publication 60. Annals of the ICRP, 27(1-3), 81. ICRU, 1986a. The Quality Factor in Radiation Protection, Report 400 (International Commission on Radiation Units and Measurements, Bethesda, Maryland). 1986b. Microdosimetry, Report 36 (International Commission on Radiation Units and Measurements, Bethesda, Maryland). LUNAR, 1988a. Technical Note No. 1. Introduction to DPX and Basic Principles. (Lunar, 313 W. Beltline Hwy, Madison, Wisconsin 53713, USA). 1988b. Technical Note No. 3. DPX Performance: Dose (Lunar, 313 W. Beltline Hwy, Madison, Wisconsin 53713, USA).
The British Journal of Radiology, June 1992
Microdosimetric measurements of a clinical dual-photon absorptiometer SCHMITZ, TH., KRAMER, H. M. & Booz, J., 1989. Assessment of the photon response of a TEPC: implementation of operational quantities for dose equivalent. Radiation Protection Dosimetry, 29, 69-74. SORENSON, J. A., DUKE, P. R. & SMITH, S. W., 1989.
Vol. 65, No. 774
Simulation studies of dual energy X-ray absorptiometry. Medical Physics, 16(1), 75-80. WAKER, A. J. & MARCO, M., 1991. The application of microdosimetry to the metrology of low-energy X rays used in mammography. British Journal of Radiology, 65, 258-261.
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