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Calculation of radiation dose enhancement factors for dose enhancement therapy of brain tumours
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1992 Phys. Med. Biol. 37 439 (http://iopscience.iop.org/0031-9155/37/2/010) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 171.67.34.69 The article was downloaded on 29/06/2012 at 10:51
Please note that terms and conditions apply.
Phys. Med. Biol., 1992, Vol. 37, No 2, 439-443. Printed in the UK
I
Calculation of radiation dose enhancement factors for dose enhancement therapy of brain tumours T D Solberg, K S Iwamoto and A Norman Departments of Radiation Oncology and Radiological Sciences and Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90024, USA Received 27 August 1991, in final form 18 October 1991 Abstnd. W h e n brain tumoun are loaded with iodinated contrast media (CM)and exposed to x-rays, the photoelenrons, Auger electrons and fluorescent x-rays irom the iodine enhance the radiation dose absorbed by the tumour. A modified CT scanner, the cTX, can be used to localize the tumour and to deliver the dose enhancement therapy. Monte Carla calculations are presented here of the central-axis radiation depth dose in a brain containing a tumour loaded with an iodine concentration of 5 mg ml-' and irradiated with the CTX operated at various kV settings. The dose enhancement factor ( D E F ) is also calculated for various held sizes and lor 5 mg mlr' of gadolinium in the tumour when the CTX is operated at 140 kV. The calculated values of the DEE are close to published experimental results.
1. Introduction
Irradiating cells in iodinated contrast media (CM) enhances the radiation dose absorbed from diagnostic x-rays (Adams et a1 1977, Matsudaira et a1 1980, Mello et al 1983, Dawson et a1 1987). The potential usefulness of this phenomenon for treating brain tumours was shown by the increased survival of rabbits after radiation therapy to a VX-2 brain tumour when irradiation was preceded by an infusion of CM (Iwamoto et al 1987). The CM thus serves two functions: to help localize the tumour and to increase the absorbed radiation dose. The CTX, a CT scanner with an added collimator, provides a practical method of delivering multiarc rotational x-ray therapy and thus of taking advantage of the dose enhancement factor (DEF) provided by the CM (Iwamoto et a1 1990). The CTX is currently receiving an extended trial via treatment of spontaneous brain tumours in dogs; but there remains some question about the DEF because of the lack of calculated values. We present here the first calculations of the DEF for a CT scanner operated at various kV and field sizes, and compare these with experimental data. 2. Methods The Monte Carlo data for this work were generated using the MCNP4 electron/ photon/neutron transport code, a multimaterial, three-dimensional, arbitrary geometry Monte Carlo code developed at the Los Alamos National Laboratory. The electron transport in this code has been adapted from the Integrated Tiger Series (ITS) code which in turn is based on ETRAN (Hableib and Vandevender 1976, Berger and Seltzer 1968). M)31-9155/92/020439+05$04.50 @ 1992 IOP Publishing Ltd
439
440
T D Solberg et al
0.5 rm
6
Figure 1. Geometry for Monte Carlo calculations.
8
10
12
Figure 2. Central-axis depth-dose data calculated, for a 200 kV x-ray spectrum. Shown is the relative depth dose DIDo
The geometry used is shown in figure 1. An isotropic photon source subtends a 0.5" angie on the piana: end of a cyiinder 40 c ~ l lin dizmeter at B distance af 63 cm from the source. This results in a field 1.1 cm in diameter at the surface of the cylinder. The cylinder is subdivided into a 1 cm diameter inner cylinder and an outer annulus. The inner cylinder is in turn divided into 5 mm thick sections in which the deposited energy is tallied. The composition of the section from 5 mm to 10 mm in depth is bone; the absorption cross sections for composite bone were generated from the data of Johns and Cunningham (1983). The sections from SO to 60 mm in depth, representing the tumour, contain 5 mg ml-' of iodine in water. The remainder of the cylinder is water. The cylinder thus models a skull and brain with a tumour containing CM: the first 5 mm (water) represents skin, the next 5 mm, the skull bone, the rest (water) brain with a 1 cm thick tumour containing iodinated CM. The photon energy distributions for a CT scanner operated at 80, 120 and 140 kV were taken from Matscheko and Carlsson (1989); the distribution for 200 kV was taken from Kahn (1984). Published experimental values of DEF were plotted against the iodine concentration, the equation for the regression line for each set of data was obtained, and the m f at 5 mg ml-' was then calculated from the equation.
3. Results Figure 2 shows the results of a typical calculation for the depth dose from a 200 kV x-ray spectrum when the tumour had 5 mg ml-' of iodine in water or only water. From these runs carried out at a field size of 1.5 cm (diameter) the DEF was 1.64. A similar calculation using the 140 kV CT spectrum yielded a DEF of 1.83 for 5 mg ml-' of iodine in water and 1.64 for 5 mg ml-' of gadolinium in water. Figure 3 shows the results obtained at four field sizes for the depth dose from a 140 kV CT x-ray spectrum when the tumour had 3 mg m1-l of iodine in water. The increased depth-dose for increased field sizes reflects the contribution of the scattered radiation. The OEF increased Only from 1.6 for fields of 1.5 and 3.0 cm to 1.7 for 5.0 and 10 cm diameter fields because
Radiation dose enhancement
-
3.0 on diameter field size 5.0 m diameter field sire 10.0m diameter fieid size
4
6
f
0
2
8
10
441
12
10 20 30 .o 50 IODINE CONCENTRATION(mglml)
Depth (cm) Figure 3. Central-aiis depth-dose data for various field sizes and a 140 kV CT x-ray s p e a r " Shown is the relative depth and dose.
60
Figure 4. Measured values of D E F for various iodine concentrations. Cells were exposed either to x-rays at the isacentre of a CT scanner operated at 140 kV or to a fixed source of 250 kV x-rays.
Table 1. Calculated versus measured values of DEF at various kV kV
80
100
120
140
200
250
Calculated Measured
1.98 -
1.9'
1.90
1.83 1.7b
1.64
-
1.4'
1.26
1.7"
Mello e l 01 (1983). b N ~ r m a ne l a1 (1991). ' Matsudaira c l 01 (1980). Dawson et a1 (1987). a
'
the scattered radiation is not very different in energy distribution from the primary beam. Figure 4 shows the regression lines through two sets of measurements of the DEF, one at 140 kV (Norman et a1 1991) and the other at 250 kV (Dawson et a1 1987). Two additional points, diamond shapes in the latter plot, are from Mello et a1 (1983). The DEF at 5 mg ml-' iodine concentration were calculated from the regression lines. Table 1 shows a comparison of calculated values of DEF with measured values at the indicated kV and 5 mg ml-' iodine. 4. Discussion
Our basic hypothesis is that the increased damage shown by cells exposed to x-rays when suspended in C M is due entirely to the increased absorption of iodine as compared to water and the subsequent release from iodine of photoelectrons, Auger electrons and characteristic x-rays. The good agreement between the calculated and measured values of the DEF for various x-ray spectra certainly provides strong suppon for the hypothesis. Further support comes from the linear dependence of the measured DEF on the iodine concentration as illustrated in figure 4.
442
T D Solberg
et
a/
The calculated values are somewhat higher than the measured ones; this was expected since the calculations were based on the assumption that the iodine was uniformly distributed in the tumour whereas measured values of DEF were obtained from cells that are generally assumed to exclude the CM. However, the better than expected agreement of measured and calculated values may indicate that the CM in fact d o enter the cells; indeed recent experiments have demonstrated CM within cells after they have been suspended in C M (Nordby et a/ 1989), and electrons released within the cells should be highly effective in sterilizing them (Fairchild et a/ ,1982). The electrons released as the result of photoelectric absorption have a lower average energy and thus a higher relative biological effect( R B E ) than the electrons released by Compton scattering in water. This factor may compensate for the smaller energy absorption by the cells as the result of a decreased CM concentration. The gadolinium at equal weights in the tumour to the iodine produced a slightly smaller DEF with the 140 kV spectrum. Although gadolinium is higher in atomic number than iodine, it has a lower absorption cross section for the low-energy photons in the CT spectrum and thus has a smaller DEF than iodine. At higher energies of the x-ray beam gadolinium may be a more effective agent than iodine for dose enhancement therapy. The calculations were carried out primarily at 5 mg ml-' concentration of iodine. This is a typical concentration found in our work using the CTX to treat spontaneous brain tumours in dogs. It corresponds to an increase in CT number over the tumour of about 90 (from 18 to 106) as the result of the CM uptake. The correspoiidiiig DBF is calculated as 1.8 and measured as 1.7. It is possible to increase the DEF by lowering the energy of the x-ray beam; and high DEF can be achieved by using monochromatic beams with energies near the K-edge of iodine or gadolinium. Such beams can be produced by filtering the x-ray beams from the CTX and lowering the kV but at the expense of a lower x-ray intensity and decreased beam penetration. It is certainly also possible to increase the DEF by simply injecting more iodine, and this may be feasible with new CM. With available CM it appears to be practical to achieve a DEF of about 2 with a CTX operated at 140 kV, and that makes the CTX a useful tool for treating selected brain tumours. RCsumC Calcul des fadcurs d'acroissement de dose pour la radiotherapie par accraisrement de dose des tumeurs ckrkbrales. Quand des tumeum cCrtbrales sont chargees avec des milieux de contraste iodes (CM) et expostes aux rayons I, les photatlectrans, les tlectrons Auger et les rayons x de fluorescence produitr dass Piode accroisent la dose absorbte dam la tumeur. On peuf utiliser un scanneur modifre, le CTX, pour localiser la tumeur et pour deliver le traitement avec accroissement de dose. Les auteurs prCsentent des calculs par la mithode de Monte Carlo de la dose dClivrCe sur l'axe central dam un cerveau contenant une tumeur chargee B V ~ Cune concentration en iode de 5 mg ml-', et irradite avec le CTX fonctionnant sous difrerentes difftrences de potentiel. Le facteur d'accraisscment de dose (DEF) a aussi Ctt calculi pour differentes dimensions de champ et pour une concentration de 5 mg ml-' de gadolinium dam la tumeur, le CTX fonctionnant sous 140 kV. Les valeurs calculCes pour le D E F sont praches des rtsultats expirimcntaux publids.
Zusammenfassung Berechnung der DosisventPrkungsTaktorenfiir die Therapie von Hirntumaren. Wenn Hirntumore unter Venvendung lod-haltiger Kontrastmittel RBntgenstrahlen ausgesetz sind, so erhoht
Radiation dose enhancement
443
sich die Strahlendosis im Tumors durch Photoelektronen, Augerelektronen und Fluares2enz-R~ntgensfrahlung des radioaktiven Jod. Ein madifiriener CT Scanner (CTX) kann verwendet werden zur Lokalisierung des Tumors und zur Anwendung der Dosisversterkungstherapie. Es wurden Monte Carla Berechnungen der Tiefendosis auf der Zentralachse durchgefiihrt f i r ein Gehirn, das einen Tumor angereichert mit einer Jad-Kanzentration yon 5 mg mi-' hat und mit dem CTX bei venchiedenen kV bestrahlt wird. Der Dosisversterkungsfaktar (DEF) wird berechnet fiir verschiedene FeldgrGBen, sowie fiir 5 mg ml-' gadolinium im Tumor und einer CTX Spannung von 140 kV. Die bereehneten Werte des DEF stimmen gut iiberein mit den veraffentlichten experimentellen Wenen.
References I
Adams F H, Norman A, Mello R S and Bass D 1977 Effect of radiation and contrast media on chromosomes Radiology 124 823-6
Berger M J and Seltzer S M 1968 ETRAN Monte Carlo code system for electron and photon transport through extended media Radiation Shielding Informotion Center, Compufer Code Collection Publieotion ccc-107 Dawson P, Penhaligan M, Smith E and Saunders J Iodinated contrast agents as 'radiosensitizers Br. J. Rodiol. 60 201-3 Fairchild R G,Brill A B and Ettinger K V 1982 Radiation enhancement with iodinated deoxyuridine Inoe$f. Rndiol. 17 407-16
Hableib J A and Vandebender W H 1916 Computer code abstract Nucl. Sci. Eng. 57 288-9 lwamoto K S, and Cnchran S T, Winter J, Holbert E, Higashida R T and Norman A 1987 Radiation dose enhancement therapy with iodine in rabbit W-2 brain tumours Rndiother. Oncol. 8 161-70 lwamoto K S, Norman A, Kagan A R, Wollin M, Olch A, Bellotti J, lngram M and Skillen R G 1990 T h e CT scanner as a therapy machine Rodiofher. Oncol. 19 337-43 Johns H E and Cunningham J R, 1983 The Physics ofRodiology 4th edn (Springfield, I L Thomas) h le Physics ojRodiation Therapy (Baltimore, MD: Williams and Wilkins) Kahn F M 1984 n Matscheko G and Carlsson G A 1989 Measurement of absolute energy spectra from a clinical CT machine under working conditions using a Compton spectrometer Phys. Med. B i d 34 209-22 Matsudaira H, Ueno A M and Furuno I 1980 Iodine contrast medium Sensitizes cultured mammalian cells to x-rays but not to y-rays Radial. Res. 84 144-8 Mello S R, Callisen H, Winter J, Kagan A R and Norman A 1983 Radiation dose enhancement in tumors with iodine Med. Phys. 10 75-8 Nordby A, Tvvedt, K E, Halgunset J, Kopstad and Haugen 0 A 1989 Incorporation of Contrast media in cultured cells h e s t . Rndiol. 24 703-10 Norman A, lwamoto K S a n d Cochran S T 1991 Iodinated contrast agents for brain tumor localization and radiation dose enhancement Inuesf. Rodiol. at press
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My IOPscience
Calculation of radiation dose enhancement factors for dose enhancement therapy of brain tumours
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1992 Phys. Med. Biol. 37 439 (http://iopscience.iop.org/0031-9155/37/2/010) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 171.67.34.69 The article was downloaded on 29/06/2012 at 10:51
Please note that terms and conditions apply.
Phys. Med. Biol., 1992, Vol. 37, No 2, 439-443. Printed in the UK
I
Calculation of radiation dose enhancement factors for dose enhancement therapy of brain tumours T D Solberg, K S Iwamoto and A Norman Departments of Radiation Oncology and Radiological Sciences and Jonsson Comprehensive Cancer Center, University of California, Los Angeles, CA 90024, USA Received 27 August 1991, in final form 18 October 1991 Abstnd. W h e n brain tumoun are loaded with iodinated contrast media (CM)and exposed to x-rays, the photoelenrons, Auger electrons and fluorescent x-rays irom the iodine enhance the radiation dose absorbed by the tumour. A modified CT scanner, the cTX, can be used to localize the tumour and to deliver the dose enhancement therapy. Monte Carla calculations are presented here of the central-axis radiation depth dose in a brain containing a tumour loaded with an iodine concentration of 5 mg ml-' and irradiated with the CTX operated at various kV settings. The dose enhancement factor ( D E F ) is also calculated for various held sizes and lor 5 mg mlr' of gadolinium in the tumour when the CTX is operated at 140 kV. The calculated values of the DEE are close to published experimental results.
1. Introduction
Irradiating cells in iodinated contrast media (CM) enhances the radiation dose absorbed from diagnostic x-rays (Adams et a1 1977, Matsudaira et a1 1980, Mello et al 1983, Dawson et a1 1987). The potential usefulness of this phenomenon for treating brain tumours was shown by the increased survival of rabbits after radiation therapy to a VX-2 brain tumour when irradiation was preceded by an infusion of CM (Iwamoto et al 1987). The CM thus serves two functions: to help localize the tumour and to increase the absorbed radiation dose. The CTX, a CT scanner with an added collimator, provides a practical method of delivering multiarc rotational x-ray therapy and thus of taking advantage of the dose enhancement factor (DEF) provided by the CM (Iwamoto et a1 1990). The CTX is currently receiving an extended trial via treatment of spontaneous brain tumours in dogs; but there remains some question about the DEF because of the lack of calculated values. We present here the first calculations of the DEF for a CT scanner operated at various kV and field sizes, and compare these with experimental data. 2. Methods The Monte Carlo data for this work were generated using the MCNP4 electron/ photon/neutron transport code, a multimaterial, three-dimensional, arbitrary geometry Monte Carlo code developed at the Los Alamos National Laboratory. The electron transport in this code has been adapted from the Integrated Tiger Series (ITS) code which in turn is based on ETRAN (Hableib and Vandevender 1976, Berger and Seltzer 1968). M)31-9155/92/020439+05$04.50 @ 1992 IOP Publishing Ltd
439
440
T D Solberg et al
0.5 rm
6
Figure 1. Geometry for Monte Carlo calculations.
8
10
12
Figure 2. Central-axis depth-dose data calculated, for a 200 kV x-ray spectrum. Shown is the relative depth dose DIDo
The geometry used is shown in figure 1. An isotropic photon source subtends a 0.5" angie on the piana: end of a cyiinder 40 c ~ l lin dizmeter at B distance af 63 cm from the source. This results in a field 1.1 cm in diameter at the surface of the cylinder. The cylinder is subdivided into a 1 cm diameter inner cylinder and an outer annulus. The inner cylinder is in turn divided into 5 mm thick sections in which the deposited energy is tallied. The composition of the section from 5 mm to 10 mm in depth is bone; the absorption cross sections for composite bone were generated from the data of Johns and Cunningham (1983). The sections from SO to 60 mm in depth, representing the tumour, contain 5 mg ml-' of iodine in water. The remainder of the cylinder is water. The cylinder thus models a skull and brain with a tumour containing CM: the first 5 mm (water) represents skin, the next 5 mm, the skull bone, the rest (water) brain with a 1 cm thick tumour containing iodinated CM. The photon energy distributions for a CT scanner operated at 80, 120 and 140 kV were taken from Matscheko and Carlsson (1989); the distribution for 200 kV was taken from Kahn (1984). Published experimental values of DEF were plotted against the iodine concentration, the equation for the regression line for each set of data was obtained, and the m f at 5 mg ml-' was then calculated from the equation.
3. Results Figure 2 shows the results of a typical calculation for the depth dose from a 200 kV x-ray spectrum when the tumour had 5 mg ml-' of iodine in water or only water. From these runs carried out at a field size of 1.5 cm (diameter) the DEF was 1.64. A similar calculation using the 140 kV CT spectrum yielded a DEF of 1.83 for 5 mg ml-' of iodine in water and 1.64 for 5 mg ml-' of gadolinium in water. Figure 3 shows the results obtained at four field sizes for the depth dose from a 140 kV CT x-ray spectrum when the tumour had 3 mg m1-l of iodine in water. The increased depth-dose for increased field sizes reflects the contribution of the scattered radiation. The OEF increased Only from 1.6 for fields of 1.5 and 3.0 cm to 1.7 for 5.0 and 10 cm diameter fields because
Radiation dose enhancement
-
3.0 on diameter field size 5.0 m diameter field sire 10.0m diameter fieid size
4
6
f
0
2
8
10
441
12
10 20 30 .o 50 IODINE CONCENTRATION(mglml)
Depth (cm) Figure 3. Central-aiis depth-dose data for various field sizes and a 140 kV CT x-ray s p e a r " Shown is the relative depth and dose.
60
Figure 4. Measured values of D E F for various iodine concentrations. Cells were exposed either to x-rays at the isacentre of a CT scanner operated at 140 kV or to a fixed source of 250 kV x-rays.
Table 1. Calculated versus measured values of DEF at various kV kV
80
100
120
140
200
250
Calculated Measured
1.98 -
1.9'
1.90
1.83 1.7b
1.64
-
1.4'
1.26
1.7"
Mello e l 01 (1983). b N ~ r m a ne l a1 (1991). ' Matsudaira c l 01 (1980). Dawson et a1 (1987). a
'
the scattered radiation is not very different in energy distribution from the primary beam. Figure 4 shows the regression lines through two sets of measurements of the DEF, one at 140 kV (Norman et a1 1991) and the other at 250 kV (Dawson et a1 1987). Two additional points, diamond shapes in the latter plot, are from Mello et a1 (1983). The DEF at 5 mg ml-' iodine concentration were calculated from the regression lines. Table 1 shows a comparison of calculated values of DEF with measured values at the indicated kV and 5 mg ml-' iodine. 4. Discussion
Our basic hypothesis is that the increased damage shown by cells exposed to x-rays when suspended in C M is due entirely to the increased absorption of iodine as compared to water and the subsequent release from iodine of photoelectrons, Auger electrons and characteristic x-rays. The good agreement between the calculated and measured values of the DEF for various x-ray spectra certainly provides strong suppon for the hypothesis. Further support comes from the linear dependence of the measured DEF on the iodine concentration as illustrated in figure 4.
442
T D Solberg
et
a/
The calculated values are somewhat higher than the measured ones; this was expected since the calculations were based on the assumption that the iodine was uniformly distributed in the tumour whereas measured values of DEF were obtained from cells that are generally assumed to exclude the CM. However, the better than expected agreement of measured and calculated values may indicate that the CM in fact d o enter the cells; indeed recent experiments have demonstrated CM within cells after they have been suspended in C M (Nordby et a/ 1989), and electrons released within the cells should be highly effective in sterilizing them (Fairchild et a/ ,1982). The electrons released as the result of photoelectric absorption have a lower average energy and thus a higher relative biological effect( R B E ) than the electrons released by Compton scattering in water. This factor may compensate for the smaller energy absorption by the cells as the result of a decreased CM concentration. The gadolinium at equal weights in the tumour to the iodine produced a slightly smaller DEF with the 140 kV spectrum. Although gadolinium is higher in atomic number than iodine, it has a lower absorption cross section for the low-energy photons in the CT spectrum and thus has a smaller DEF than iodine. At higher energies of the x-ray beam gadolinium may be a more effective agent than iodine for dose enhancement therapy. The calculations were carried out primarily at 5 mg ml-' concentration of iodine. This is a typical concentration found in our work using the CTX to treat spontaneous brain tumours in dogs. It corresponds to an increase in CT number over the tumour of about 90 (from 18 to 106) as the result of the CM uptake. The correspoiidiiig DBF is calculated as 1.8 and measured as 1.7. It is possible to increase the DEF by lowering the energy of the x-ray beam; and high DEF can be achieved by using monochromatic beams with energies near the K-edge of iodine or gadolinium. Such beams can be produced by filtering the x-ray beams from the CTX and lowering the kV but at the expense of a lower x-ray intensity and decreased beam penetration. It is certainly also possible to increase the DEF by simply injecting more iodine, and this may be feasible with new CM. With available CM it appears to be practical to achieve a DEF of about 2 with a CTX operated at 140 kV, and that makes the CTX a useful tool for treating selected brain tumours. RCsumC Calcul des fadcurs d'acroissement de dose pour la radiotherapie par accraisrement de dose des tumeurs ckrkbrales. Quand des tumeum cCrtbrales sont chargees avec des milieux de contraste iodes (CM) et expostes aux rayons I, les photatlectrans, les tlectrons Auger et les rayons x de fluorescence produitr dass Piode accroisent la dose absorbte dam la tumeur. On peuf utiliser un scanneur modifre, le CTX, pour localiser la tumeur et pour deliver le traitement avec accroissement de dose. Les auteurs prCsentent des calculs par la mithode de Monte Carlo de la dose dClivrCe sur l'axe central dam un cerveau contenant une tumeur chargee B V ~ Cune concentration en iode de 5 mg ml-', et irradite avec le CTX fonctionnant sous difrerentes difftrences de potentiel. Le facteur d'accraisscment de dose (DEF) a aussi Ctt calculi pour differentes dimensions de champ et pour une concentration de 5 mg ml-' de gadolinium dam la tumeur, le CTX fonctionnant sous 140 kV. Les valeurs calculCes pour le D E F sont praches des rtsultats expirimcntaux publids.
Zusammenfassung Berechnung der DosisventPrkungsTaktorenfiir die Therapie von Hirntumaren. Wenn Hirntumore unter Venvendung lod-haltiger Kontrastmittel RBntgenstrahlen ausgesetz sind, so erhoht
Radiation dose enhancement
443
sich die Strahlendosis im Tumors durch Photoelektronen, Augerelektronen und Fluares2enz-R~ntgensfrahlung des radioaktiven Jod. Ein madifiriener CT Scanner (CTX) kann verwendet werden zur Lokalisierung des Tumors und zur Anwendung der Dosisversterkungstherapie. Es wurden Monte Carla Berechnungen der Tiefendosis auf der Zentralachse durchgefiihrt f i r ein Gehirn, das einen Tumor angereichert mit einer Jad-Kanzentration yon 5 mg mi-' hat und mit dem CTX bei venchiedenen kV bestrahlt wird. Der Dosisversterkungsfaktar (DEF) wird berechnet fiir verschiedene FeldgrGBen, sowie fiir 5 mg ml-' gadolinium im Tumor und einer CTX Spannung von 140 kV. Die bereehneten Werte des DEF stimmen gut iiberein mit den veraffentlichten experimentellen Wenen.
References I
Adams F H, Norman A, Mello R S and Bass D 1977 Effect of radiation and contrast media on chromosomes Radiology 124 823-6
Berger M J and Seltzer S M 1968 ETRAN Monte Carlo code system for electron and photon transport through extended media Radiation Shielding Informotion Center, Compufer Code Collection Publieotion ccc-107 Dawson P, Penhaligan M, Smith E and Saunders J Iodinated contrast agents as 'radiosensitizers Br. J. Rodiol. 60 201-3 Fairchild R G,Brill A B and Ettinger K V 1982 Radiation enhancement with iodinated deoxyuridine Inoe$f. Rndiol. 17 407-16
Hableib J A and Vandebender W H 1916 Computer code abstract Nucl. Sci. Eng. 57 288-9 lwamoto K S, and Cnchran S T, Winter J, Holbert E, Higashida R T and Norman A 1987 Radiation dose enhancement therapy with iodine in rabbit W-2 brain tumours Rndiother. Oncol. 8 161-70 lwamoto K S, Norman A, Kagan A R, Wollin M, Olch A, Bellotti J, lngram M and Skillen R G 1990 T h e CT scanner as a therapy machine Rodiofher. Oncol. 19 337-43 Johns H E and Cunningham J R, 1983 The Physics ofRodiology 4th edn (Springfield, I L Thomas) h le Physics ojRodiation Therapy (Baltimore, MD: Williams and Wilkins) Kahn F M 1984 n Matscheko G and Carlsson G A 1989 Measurement of absolute energy spectra from a clinical CT machine under working conditions using a Compton spectrometer Phys. Med. B i d 34 209-22 Matsudaira H, Ueno A M and Furuno I 1980 Iodine contrast medium Sensitizes cultured mammalian cells to x-rays but not to y-rays Radial. Res. 84 144-8 Mello S R, Callisen H, Winter J, Kagan A R and Norman A 1983 Radiation dose enhancement in tumors with iodine Med. Phys. 10 75-8 Nordby A, Tvvedt, K E, Halgunset J, Kopstad and Haugen 0 A 1989 Incorporation of Contrast media in cultured cells h e s t . Rndiol. 24 703-10 Norman A, lwamoto K S a n d Cochran S T 1991 Iodinated contrast agents for brain tumor localization and radiation dose enhancement Inuesf. Rodiol. at press
