Development of a radiophotoluminescent glass plate detector for small field dosimetry

1. Development of a radiophotoluminescent glass plate detector for small field dosimetry Araki F. (f_araki@hs.kumamoto-u.ac.jp) 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 KUSM

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 KUSM

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 KUSM

4. PURPOSE To estimate the usefulness of GPD developed as a new device for the dosimetry of small radiosurgery fields. 4 KUSM

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 KUSM

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 KUSM

7. MATERIALS • Hi-pSi stereotactic field detector (SFD) 0.6 mm diameter×0.06 mm thickness • PTW diamond detector 3.05 mm diameter×0.26 mm thickness • PTW 31002, 0.125 cm3 ion chamber 5.5 mm diameter×6.5 mm length 7 KUSM

8. Electron beam Target Primary collimator Flattening filter Monitor chamber Mirror Phase space 1 Jaws Circular collimator Phase space 2 Water phantom Monte Carlo modeling for a Varian Clinac 2100C accelerator using EGSnrc code BEAMnrc • 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 KUSM

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 KUSM

10. 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 KUSM

11. Block diagram of a radiophotoluminescence (RPL) readout system 11 KUSM

12. Comparison of dose profiles between GPD and film dosimeters and EGSnrc 12 KUSM

13. Comparison of dose profiles between GPD and film dosimeters and EGSnrc 13 KUSM

14. Comparison of dose profiles between GPD and film dosimeters and EGSnrc 14 KUSM

15. Comparison of dose profiles between GPD and film dosimeters and EGSnrc 15 KUSM

16. Comparison of energy dependence between GPD and other detectors 16 KUSM

17. Comparison of o output factors between GPD and other detectors Output factors normalized to a 10 x 10 cm2 field 17 KUSM

18. 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 KUSM

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 KUSM

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 KUSM

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 KUSM

22. ACKNOWLEDGMENTS • We would like to thank Varian Oncology Systems for providing detailed treatment head designs to simulate a Varian Clinac 2100C accelerator. 22 KUSM