1. Araki F. (f_araki@hs.kumamoto-u.ac.jp) Ikegami T. and Ishidoya T.
Development of a radiophotoluminescent glass plate detector for small field dosimetry
Araki F.
Kumamoto University School of Medical Sciences
Ikegami T. and Ishidoya T.
Asahi Techno Glass Co.
Moribe N., Shimonobou T. and Yamashita Y.
Kumamoto University Hospital
Miyazawa M.
R-Tech Co. 1
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2. BACKGROUND
The small field measurements are generally performed using a small active volume diode detector, a small thermoluminescent dosimeter (TLD) or a film dosimeter.
Recently, a radiophotoluminescent (RPL) glass rod dosimeter (GRD) has also become commercially available.
However, the output factor measurements of small fields are generally tedious and difficult due to the sharp radial dose fall-off, the small size of the dose plateau region and the lack of lateral electron equilibrium. 2
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3. BACKGROUND
We have been developed a radiophotoluminescent (RPL) glass plate dosimeter (GPD) as a new device for small field dosimetry.
The GPD is able to measure both the output factor and the dose distribution simultaneously. This device has an advantage over a film dosimeter and other detectors. 3
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4. PURPOSE
To estimate the usefulness of GPD developed as a new device for the dosimetry of small radiosurgery fields. 4
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5. METHODS
The dose distribution measured with GPD is compared to those of film dosimeters for 2, 5, 9 and 15 mm circular collimators created by a linear accelerator-based radiosurgery system.
The GPD output factors are evaluated by comparing them to various detectors, including a p-type silicon diode detector, a diamond detector, GRD and an ion chamber for small circular collimators.
The results measured with GPD are also compared to those of Monte Carlo simulations. 5
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6. MATERIALS RPL glass dosimeter (r =2.61, Z =12.04)
Glass plate dosimeter:
30 mm square×1 mm thickness
Glass rod dosimeter (GD-301) :
1.5 mm diameter×8.5 mm length
Film dosimeter
Kodac X-Omat V (XV-2)
GAFCROMIC XR type R 6
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7. MATERIALS Hi-pSi stereotactic field detector (SFD)
　 mm diameter×0.06 mm thickness
PTW diamond detector
3.05 mm diameter×0.26 mm thickness
PTW 31002, cm3 ion chamber
　　 5.5 mm diameter×6.5 mm length 7
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8. Monte Carlo modeling for a Varian Clinac 2100C accelerator using EGSnrc code BEAMnrc
Primary collimator
Flattening filter
Monitor chamber
Mirror
Jaws
Circular collimator
Water phantom
Target
Phase
space 1
space 2
Electron beam
Incident electron histories
to target
5 x 5 cm2 field size
4 MV: 5 x 108
10 MV: 1 x 108
Voxel sizes for DOSXYZ
1 mm x 1 mm x 5 mm
Circular collimator sizes
20 mm, 15 mm, 9 mm,
5 mm, 2 mm 8
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9. Calculation parameters for simulating 4 and 10 MV photon beams
AE=ECUT=0.700 MeV
AP=PCUT=0.010 MeV
Photon interaction forcing: off
Rayleigh scattering: off
Variance reduction technique
Electron range rejection:
4 MV target: ESAVE=0.7 MeV
other CMs: ESAVE=1.0 MeV
10 MV target: ESAVE=0.7 MeV
other CMs: ESAVE=2.0 MeV
Selective bremsstrahlung splitting (SBS):
Nmin=40, Nmax=400
Russian roulette of secondary electron: off 9
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10. Incident electron for 4 MV photon beam
Parameters of the electron beam incident on the target for 4 and 10 MV photon beams
Incident electron for 4 MV photon beam
Energy and spread: 4.2 MeV and FWHM of 3%
Radial intensity distribution: FWHM of 1.2 mm
Incident electron for 10 MV photon beam
Energy and spread: 10.3 MeV and FWHM of 3%
Radial intensity distribution: FWHM of 1.5 mm 10
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11. Block diagram of a radiophotoluminescence (RPL) readout system11
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12. Comparison of dose profiles between GPD and film dosimeters and EGSnrc12
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13. Comparison of dose profiles between GPD and film dosimeters and EGSnrc13
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14. Comparison of dose profiles between GPD and film dosimeters and EGSnrc14
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15. Comparison of dose profiles between GPD and film dosimeters and EGSnrc15
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16. Comparison of energy dependence between GPD and other detectors16
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17. Comparison of o output factors between GPD and other detectors
Output factors normalized to a 10 x 10 cm2 field 17
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18. Output factors normalized Output factors normalized
Energy dependence for a SFD diode detector
Output factors normalized
to a 10 x 10 cm2 field
Output factors normalized
to a 20 mm field 18
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19. RESULT
The GRD profiles are found to coincide well with the dose distribution for Kodak XV2 film and GAFCHROMIC XR film dosimeters and Monte Carlo simulations in 4 and 10 MV x-ray beams.
The GPD response shows little energy dependence in photon energies of a 60Co beam, 4 MV (TPR20,10=0.617) and 10 MV (TPR20,10=0.744) x-ray beams.
The output factors measured with GPD are in agreement with the diode detector and the GRD with a small active volume, and Monte Carlo simulations. 19
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20. DISCUSSION
The GPD system has an advantage over a film dosimeter and other detectors, because it is able to measure both the absolute dose and the dose profiles simultaneously.
The GRD is expected as a new device for small field and IMRT dosimetry. 20
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21. CONCLUSIONS
It is found that GPD is a very useful detector for small field dosimetry, in particular, less than 10 mm circular fields.
Another purpose of this work is to apply more wide glass plate detectors to IMRT dosimetry. 21
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22. ACKNOWLEDGMENTS
We would like to thank Varian Oncology Systems for providing detailed treatment head designs to simulate a Varian Clinac 2100C accelerator. 22
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