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Simple dose verification system for radiotherapy radiation
Radiation Measurements 43 (2008) 954 – 958 www.elsevier.com/locate/radmeas
Simple dose verification system for radiotherapy radiation J.H. Lee a,b , C.Y. Yeh c , S.M. Hsu d , M.Y. Shi a , W.L. Chen d , C.F. Wang b,∗ a Health Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-10, Longtan 325, Taiwan b Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu 300, Taiwan c Radiation Oncology Department, Chang Gung Memorial Hospital, Linkou 333, Taiwan d Department of Biomedical Imaging and Radiological Science, National Yang Ming University, Beitou 112, Taipei, Taiwan
Abstract The aim of this paper is to investigate an accurate and convenient quality assurance programme that should be included in the dosimetry system of the radiotherapy level radiation. We designed a mailed solid phantom and used TLD-100 chips and a Rexon UL320 reader for the purpose of dosimetry quality assurance in Taiwanese radiotherapy centers. After being assembled, the solid polystyrene phantom weighted only 375 g which was suitable for mailing. The Monte Carlo BEAMnrc code was applied in calculations of the dose conversion factor of water and polystyrene phantom: the dose conversion factor measurements were obtained by switching the TLDs at the same calibration depth of water and the solid phantom to measure the absorbed dose and verify the accuracy of the theoretical calculation results. The experimental results showed that the dose conversion factors from TLD measurements and the calculation values from the BEAMnrc were in good agreement with a difference within 0.5%. Ten radiotherapy centers were instructed to deliver to the TLDs on central beam axis absorbed dose of 2 Gy. The measured doses were compared with the planned ones. A total of 21 beams were checked. The dose verification differences under reference conditions for 60 Co, high energy X-rays of 6, 10 and 15 MV were truly within 4% and that proved the feasibility of applying the method suggested in this work in radiotherapy dose verification. © 2008 Elsevier Ltd. All rights reserved. Keywords: Quality assurance; Radiotherapy; Polystyrene phantom; Monte Carlo BEAMnrc code; Acceptance level
1. Introduction According to the 2000–2005 statistics of the National Health Administration in Taiwan, the equipment of linear accelerators operating in hospital radiation oncology departments increased from 63 to 97 units, and the yearly number of radiotherapy treatments for oncology patients also grew from 0.62 to 1.15 million. It is generally accepted that ±5% uncertainty in dose delivery to the target volume can be considered as a safe limit causing no severe radiotherapy treatment consequences (Izewska et al., 1995; Kroutilíková et al., 2003). With the annual increase of radiotherapy equipment and the number of patients receiving the treatment, an accurate and convenient ∗ Corresponding author at: Health Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-10, Longtan 325, Taiwan. Tel.: +886 3 471 1400x7623; fax: +886 3 471 1171. E-mail address: [email protected] (C.F. Wang).
1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.12.029
quality assurance programme included in the dosimetry system of the radiotherapy radiation is strongly required. In 1969 the International Atomic Energy Agency (IAEA), together with the World Health Organization (WHO), established the IAEA/WHO TLD postal programme to verify the calibration of radiotherapy dosimetry in developing countries (Izewska and Andreo, 2000; Izewska et al., 2002). Different from the methodology of the IAEA/WHO TLD programme, in this work we tried to design a simple system using the mailed solid phantom and TLD-100 chips for the purpose of dose verification tests in Taiwanese radiotherapy centers to help hospitals save the time for setting up TLD audit system and undertake performance evaluation for the TLD chips employed in the absorbed dose verification. The Monte Carlo BEAMnrc code was applied in calculations of the dose conversion factor of water and solid phantom under the same calibration depth condition. The dose conversion factor measurements were also obtained by the TLDs to verify the accuracy of the theoretical
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
calculation results. In the dose verification experiments performed for the 10 radiotherapy centers, the planned dose for the TLD located on the central beam axis is 2 Gy. The dose verification differences under the reference conditions of 60 Co, 6, 10 and 15 MV of high energy X-rays will prove the feasibility of applying our research in clinical dose verification for radiotherapy. 2. Materials and methods The TLD chips used in this dose verification experiment are LiF:Mg, Ti, type TLD-100 (Harshaw Co.). The chips are 3.2 mm in width and length, with thickness of 0.89 mm, sealed in polyethylene (approximately 0.004 inch thickness) and a unique identification number is associated with each chip. Before each measurement, the TLDs were annealed at 400 ◦ C for 1 h, followed by fast cooling and subsequent annealing at 100 ◦ C for 2 h. After irradiation, the TLD should be annealed at 100 ◦ C for 10 min. The TLD would not be read until its temperature went down to room temperature. TLD manual Rexon reader model UL320 is used for readout. The polystyrene phantom measured 60 mm × 60 mm × 55 mm. At the center of the phantom, a cross groove could be placed with five TLDs simultaneously. For calibration depths of different photon energies, the TLDs would be covered with polystyrene materials of 5, 10, 20 and 30 mm thick. After being assembled, the phantom weighted only about 375 g which was suitable for mailing. The detailed design of the phantom is shown in Fig. 1. In this work, the Monte Carlo BEAMnrc code (Nelson et al., 1985) was applied in calculations of high energy radiotherapy machines dose verifications under the same calibration depth condition to get the absorbed dose of the 30 × 30 × 30 cm3 water phantom, Dw, and the absorbed dose of the 6×6×h cm3 polystyrene phantom, Dp as well as determining their dose
955
conversion factor (C.F.) as the following equation: C.F. =
Dw . DP
Fig. 2 is the calculation indication diagram of the dose conversion factor. In Fig. 2, the height h of the polystyrene phantom equaled to the 5 cm of TLD position added up with the polystyrene buildup thickness. BEAMnrc is composed of three programs which have respective functions (Nelson et al., 1985): BEAM plays as the main program and is used to simulate the head structures and the beam production for the 60 Co unit and the linear accelerator; BEAMDP (BEAM Data Processor) analyzes the phase-space files for spectrum, fluence and energy fluence; DOSXYZ simulates the migration of photons and electrons inside the phantom and records energy deposition of voxel. In this work, program input was done in Windows operation system and loaded to Linux platform for execution. Parallel algorithm was applied to shorten the running time. BEAM code uses the Bremsstrahlung splitting method (Nelson et al., 1985) and splits the photons produced at each Bremsstrahlung into as many as the number N ; the weighting of each photon would be 1/N . In this work, the N value of the BEAM program was properly adjusted to between 20 and 100. The cutoff energies of electron and photon were set for 0.7 and 0.01 MeV. The particles’ history would be terminated for those having energies lower than the cutoff energy. In DOSXYZ code, the cutoff energy of electron was down to 0.521 MeV to improve the accuracy of dose calculations. In addition to using the BEAMnrc code for the dose conversion factor calculations, the dose conversion factor measurements were obtained by switching the TLDs at the same calibration depth of water and polystyrene phantom to measure the absorbed dose and verify the accuracy of the theoretical calculation results.
Fig. 1. Structure of polystyrene phantom design. (a) Lateral view; (b) Top view.
956
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
120
120
100
100
80
80
Isodose (%)
Isodose (%)
Fig. 2. Indication of dose conversion factor calculation.
60 40
40
20
20 0
4
2
6 Position (cm)
8
12
10
120
120
100
100
80
80
Isodose (%)
Isodose (%)
60
60
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20
20 2
4
6 8 Position (cm)
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6
10 8 Position (cm)
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8 10 Position (cm)
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0
Fig. 3. Comparison of TLD measurements and IMRT treatment planning for the dose verification (—IMRT treatment planning, TLD). (a) Line 1; (b) Line 2; (c) Line 3; (d) Line 4.
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
When the TLDs and the phantom arrived at the hospital by mail, the physicist would align the center of the phantom with the central beam axis and then decided to use SSD or SAD of 100 cm setup with 10 cm×10 cm field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup to determine the buildup depth (6 MV: 1.5 cm; 10 MV: 2.5 cm; 15 MV: 3.5 cm) and calculated the monitor unit (MU) corresponding to the 2 Gy irradiated absorbed dose. After irradiation, the phantom and TLDs would be posted back to the Institute of Nuclear Energy Research (INER) for reading. 3. Results and discussion The TLDs selected and used in dose verification should have been tested for reproducibility and dose response. Referring to the paper of Kron et al. (1998), TLD-100 chips have almost the same reaction for the photon energy higher than 1 MeV. Thus, the energy dependence is negligible. The 3–5% standard deviations of TLD-100 reproducibility response to delivered doses ranging from 1 to 5 Gy were attained in the research of Harris et al. (1997). After each TLD in the same batch was irradiated with an absorbed dose to water of 2 Gy from the 60 Co source Table 1 Dose conversion factors obtained by Monte Carlo calculations and TLD measurements
60 Co
6 MV 10 MV 15 MV
Dose conversion factor Monte Carlo simulation
TLD measurement
1.055 1.063 1.040 1.050
1.060 1.068 1.039 1.053
for five times, the TLD-100 chips whose reproducibility within 3% and sensitivity within 10% were used in this experiment. In the dose response test, the selected TLDs were exposed to the absorbed doses to water in 60 Co ranging from 0.5 to 3 Gy to set up the TLDs calibration curve. In addition, for verifying feasibility of TLD-100 chips used in clinical dosimetry verification, 78 TLD-100s were selected for IMRT dose evaluation. The TLD chips were placed inside the phantom to form four lines which crossed with each other at the intersection point. Some procedures that patients usually undergo during the radiotherapy process can be modeled in this verification, including the sequence from CT data acquisition to treatment planning and then finally the phantom irradiation in accordance with the calculated plan. Then the TLD readouts were measured by means of calibration curve and then converted to absolute doses to compare with the dose distribution produced by the computer treatment planning system. Figs. 3(a)–(d) showed that the dose evaluation differences between the TLD measurement and the IMRT computer treatment planning were less than 5%, which were lower than the total uncertainty of radiotherapy predicted dose. Hence, the TLD-100 chips in this study are usable in clinical dose verification. Table 1 showed the conversion factors in AECL Theratron 780C 60 Co unit and 6, 10 and 15 MV beam qualities of Varian 21EX accelerator using BEAMnrc code. Besides, Table 1 listed the experimental values of conversion factors determined by the TLD-100 chips which were placed at the same calibration depths in water and polystyrene phantom. The results showed that the dose conversion factors from TLD measurements and BEAMnrc calculations were in good agreement with a difference within 0.5%. By multiplying the absorbed dose measured by the TLDs in the polystyrene phantom with the dose conversion factor calculated from Monte Carlo code or obtained from experiments, the absorbed dose to water at the calibration depth can be obtained.
1.10
6 MV 10 MV 15 MV Co-60
1.08 1.06 1.04 DTLD/Dstat
Beam quality
957
+3.5%
1.02 1.00 0.98 -3.5%
0.96 0.94 0.92 0.90
A
B
C
D
E F G Participant code
H
I
J
Fig. 4. Results of TLD postal dose verification in Taiwanese radiotherapy centers.
958
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
In the dose verification experiments performed for the 10 radiotherapy centers with the Theratron 780C 60 Co unit and the Varian 21EX accelerator, the planned dose for the points containing TLD on the central beam axis is 2 Gy. The irradiation of the phantom with TLDs has to be made according to the calculation plan. Consequently, it is possible to compare the planned dose for the TLD points with the doses measured by TLD afterwards. The combined relative standard uncertainty uc (1SD) of the TLD system was estimated to be about 1.05% for 60 Co and about 1.75% for high energy X-rays. Based on these uncertainties and a compromise for photon beams, an acceptance level of 3.5% has been chosen for both 60 Co and X-ray comparisons (2SD). The TLD measured doses (DTLD ) compared with the doses stated by the radiotherapy center (Dstat ) were shown in Fig. 4. In Taiwan, the absorbed dose to water calibrations in medical accelerator photon beams are traceable to the INER 60 Co standards following the recommendations given in the AAPM TG-21 and TG-51 dosimetry protocols. INER calibrated the TLDs in terms of the absorbed dose to water in this experiment. It should be noticed in Fig. 4 that the same dosimetry code of practice is used at a hospital or INER to determine the dose given to the TLD, some of its contribution in uc can be canceled out and its DTLD /Dstat ratio would be kept within tighter limits. For dose verifications in 95% beam qualities, the differences under reference conditions for 60 Co, high energy X-rays of 6, 10 and 15 MV were within ±3.5% acceptance level and it proved the feasibility of applying this method in clinical dose verification for radiotherapy. The acceptance level of ±3.5% in this study is evidently stricter than the levels used within the IAEA/WHO TLD postal programme (±5%). This is justified by the better capability of the small national network to repeat the audit promptly or to investigate the situation thoroughly if necessary. 4. Conclusions In this work, the postal polystyrene phantom was used instead of the traditional water phantom. The BEAMnrc code was used to calculate the dose conversion factor at the same calibration depth of the polystyrene and water phantom. The conversion factor can convert the dose to polystyrene into the dose to water to verify the clinical dosimetry at radiotherapy centers. The postal TLD verification has shown that the high energy photon beam dosimetry in most centers remains within
acceptable limits. Out of 21 beam qualities checks of 60 Co and high energy X-rays, in 20 cases the deviation of absorbed dose stated by participants compared with the values measured by the INER did not exceed the acceptance level of ±3.5%. The comparison pilot study has made the simple dose verification system possible to test the consistency of high energy photon beam dosimetry in regional radiotherapy centers in Taiwan. It is expected that the advanced mode of the TLD audit will be used as a strict tool for purposes of state supervision in Taiwan in the future. At present it is operated as a pilot study and that gives the radiotherapy centers the opportunity to become familiar with the method and to be prepared for a new situation. Following this, we will conduct as soon as possible additional photon and electron dose verifications for the other centers and hope to execute a detailed and complete analysis for the acceptance level. As to the radiotherapy centers whose dose deviations were larger than the acceptance level, a follow-up programme would be established to resolve the discrepancies. Acknowledgments The authors would like to thank the staff members at the 10 radiotherapy centers in Taiwan for their assistance in the clinical dose verification programme. We also express our thanks to Mr. Shi-Hwa Su for his useful comments and discussions. References Harris, C.K., Elson, H.R., Lamba, M.A.S., Foster, A.E., 1997. A comparison of the effectiveness of thermoluminescent crystals LiF:Mg, Ti and LiF:Mg, Cu, P for clinical dosimtery. Med. Phys. 24 (9), 1527–1529. Izewska, J., Andreo, P., 2000. The IAEA/WHO TLD postal programme for radiotherapy hospitals. Radiother. Oncol. 54, 65–72. Izewska, J., Gajewski, R., Gwiazdowska, B., Kania, M., Rostkowska, J., 1995. TLD postal dose intercomparison for megavoltage units in Poland. Radiother. Oncol. 36, 143–152. Izewska, J., Bera, P., Vatnitsky, S., 2002. IAEA/WHO TLD postal audit service and high precision measurements for radiotherapy level dosimetry. Radiat. Prot. Dosim. 101, 387–392. Kron, T., Duggan, L., Smith, T., Rosenfeld, A., Butson, M., Kaplan, G., Howlett, S., Hyodo, K., 1998. Dose response of various radiation detectors to synchrotron radiation. Phys. Med. Biol. 43, 3235–3259. Kroutilíková, D., Novotný, J., Judas, L., 2003. Thermoluminescent dosimeters (TLD) quality assurance network in the Czech Republic. Radiother. Oncol. 66, 235–244. Nelson, W.R., Hirayama, H., Rogers, D.W., 1985. The EGS4 Code System. Stanford Linear Accelerator Center, Report SLAC-265.
Simple dose verification system for radiotherapy radiation J.H. Lee a,b , C.Y. Yeh c , S.M. Hsu d , M.Y. Shi a , W.L. Chen d , C.F. Wang b,∗ a Health Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-10, Longtan 325, Taiwan b Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu 300, Taiwan c Radiation Oncology Department, Chang Gung Memorial Hospital, Linkou 333, Taiwan d Department of Biomedical Imaging and Radiological Science, National Yang Ming University, Beitou 112, Taipei, Taiwan
Abstract The aim of this paper is to investigate an accurate and convenient quality assurance programme that should be included in the dosimetry system of the radiotherapy level radiation. We designed a mailed solid phantom and used TLD-100 chips and a Rexon UL320 reader for the purpose of dosimetry quality assurance in Taiwanese radiotherapy centers. After being assembled, the solid polystyrene phantom weighted only 375 g which was suitable for mailing. The Monte Carlo BEAMnrc code was applied in calculations of the dose conversion factor of water and polystyrene phantom: the dose conversion factor measurements were obtained by switching the TLDs at the same calibration depth of water and the solid phantom to measure the absorbed dose and verify the accuracy of the theoretical calculation results. The experimental results showed that the dose conversion factors from TLD measurements and the calculation values from the BEAMnrc were in good agreement with a difference within 0.5%. Ten radiotherapy centers were instructed to deliver to the TLDs on central beam axis absorbed dose of 2 Gy. The measured doses were compared with the planned ones. A total of 21 beams were checked. The dose verification differences under reference conditions for 60 Co, high energy X-rays of 6, 10 and 15 MV were truly within 4% and that proved the feasibility of applying the method suggested in this work in radiotherapy dose verification. © 2008 Elsevier Ltd. All rights reserved. Keywords: Quality assurance; Radiotherapy; Polystyrene phantom; Monte Carlo BEAMnrc code; Acceptance level
1. Introduction According to the 2000–2005 statistics of the National Health Administration in Taiwan, the equipment of linear accelerators operating in hospital radiation oncology departments increased from 63 to 97 units, and the yearly number of radiotherapy treatments for oncology patients also grew from 0.62 to 1.15 million. It is generally accepted that ±5% uncertainty in dose delivery to the target volume can be considered as a safe limit causing no severe radiotherapy treatment consequences (Izewska et al., 1995; Kroutilíková et al., 2003). With the annual increase of radiotherapy equipment and the number of patients receiving the treatment, an accurate and convenient ∗ Corresponding author at: Health Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-10, Longtan 325, Taiwan. Tel.: +886 3 471 1400x7623; fax: +886 3 471 1171. E-mail address: [email protected] (C.F. Wang).
1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.12.029
quality assurance programme included in the dosimetry system of the radiotherapy radiation is strongly required. In 1969 the International Atomic Energy Agency (IAEA), together with the World Health Organization (WHO), established the IAEA/WHO TLD postal programme to verify the calibration of radiotherapy dosimetry in developing countries (Izewska and Andreo, 2000; Izewska et al., 2002). Different from the methodology of the IAEA/WHO TLD programme, in this work we tried to design a simple system using the mailed solid phantom and TLD-100 chips for the purpose of dose verification tests in Taiwanese radiotherapy centers to help hospitals save the time for setting up TLD audit system and undertake performance evaluation for the TLD chips employed in the absorbed dose verification. The Monte Carlo BEAMnrc code was applied in calculations of the dose conversion factor of water and solid phantom under the same calibration depth condition. The dose conversion factor measurements were also obtained by the TLDs to verify the accuracy of the theoretical
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
calculation results. In the dose verification experiments performed for the 10 radiotherapy centers, the planned dose for the TLD located on the central beam axis is 2 Gy. The dose verification differences under the reference conditions of 60 Co, 6, 10 and 15 MV of high energy X-rays will prove the feasibility of applying our research in clinical dose verification for radiotherapy. 2. Materials and methods The TLD chips used in this dose verification experiment are LiF:Mg, Ti, type TLD-100 (Harshaw Co.). The chips are 3.2 mm in width and length, with thickness of 0.89 mm, sealed in polyethylene (approximately 0.004 inch thickness) and a unique identification number is associated with each chip. Before each measurement, the TLDs were annealed at 400 ◦ C for 1 h, followed by fast cooling and subsequent annealing at 100 ◦ C for 2 h. After irradiation, the TLD should be annealed at 100 ◦ C for 10 min. The TLD would not be read until its temperature went down to room temperature. TLD manual Rexon reader model UL320 is used for readout. The polystyrene phantom measured 60 mm × 60 mm × 55 mm. At the center of the phantom, a cross groove could be placed with five TLDs simultaneously. For calibration depths of different photon energies, the TLDs would be covered with polystyrene materials of 5, 10, 20 and 30 mm thick. After being assembled, the phantom weighted only about 375 g which was suitable for mailing. The detailed design of the phantom is shown in Fig. 1. In this work, the Monte Carlo BEAMnrc code (Nelson et al., 1985) was applied in calculations of high energy radiotherapy machines dose verifications under the same calibration depth condition to get the absorbed dose of the 30 × 30 × 30 cm3 water phantom, Dw, and the absorbed dose of the 6×6×h cm3 polystyrene phantom, Dp as well as determining their dose
955
conversion factor (C.F.) as the following equation: C.F. =
Dw . DP
Fig. 2 is the calculation indication diagram of the dose conversion factor. In Fig. 2, the height h of the polystyrene phantom equaled to the 5 cm of TLD position added up with the polystyrene buildup thickness. BEAMnrc is composed of three programs which have respective functions (Nelson et al., 1985): BEAM plays as the main program and is used to simulate the head structures and the beam production for the 60 Co unit and the linear accelerator; BEAMDP (BEAM Data Processor) analyzes the phase-space files for spectrum, fluence and energy fluence; DOSXYZ simulates the migration of photons and electrons inside the phantom and records energy deposition of voxel. In this work, program input was done in Windows operation system and loaded to Linux platform for execution. Parallel algorithm was applied to shorten the running time. BEAM code uses the Bremsstrahlung splitting method (Nelson et al., 1985) and splits the photons produced at each Bremsstrahlung into as many as the number N ; the weighting of each photon would be 1/N . In this work, the N value of the BEAM program was properly adjusted to between 20 and 100. The cutoff energies of electron and photon were set for 0.7 and 0.01 MeV. The particles’ history would be terminated for those having energies lower than the cutoff energy. In DOSXYZ code, the cutoff energy of electron was down to 0.521 MeV to improve the accuracy of dose calculations. In addition to using the BEAMnrc code for the dose conversion factor calculations, the dose conversion factor measurements were obtained by switching the TLDs at the same calibration depth of water and polystyrene phantom to measure the absorbed dose and verify the accuracy of the theoretical calculation results.
Fig. 1. Structure of polystyrene phantom design. (a) Lateral view; (b) Top view.
956
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
120
120
100
100
80
80
Isodose (%)
Isodose (%)
Fig. 2. Indication of dose conversion factor calculation.
60 40
40
20
20 0
4
2
6 Position (cm)
8
12
10
120
120
100
100
80
80
Isodose (%)
Isodose (%)
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60
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20
20 2
4
6 8 Position (cm)
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10 8 Position (cm)
12
14
16
0
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4
6
8 10 Position (cm)
12
14
16
60
40
0
0
Fig. 3. Comparison of TLD measurements and IMRT treatment planning for the dose verification (—IMRT treatment planning, TLD). (a) Line 1; (b) Line 2; (c) Line 3; (d) Line 4.
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
When the TLDs and the phantom arrived at the hospital by mail, the physicist would align the center of the phantom with the central beam axis and then decided to use SSD or SAD of 100 cm setup with 10 cm×10 cm field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup to determine the buildup depth (6 MV: 1.5 cm; 10 MV: 2.5 cm; 15 MV: 3.5 cm) and calculated the monitor unit (MU) corresponding to the 2 Gy irradiated absorbed dose. After irradiation, the phantom and TLDs would be posted back to the Institute of Nuclear Energy Research (INER) for reading. 3. Results and discussion The TLDs selected and used in dose verification should have been tested for reproducibility and dose response. Referring to the paper of Kron et al. (1998), TLD-100 chips have almost the same reaction for the photon energy higher than 1 MeV. Thus, the energy dependence is negligible. The 3–5% standard deviations of TLD-100 reproducibility response to delivered doses ranging from 1 to 5 Gy were attained in the research of Harris et al. (1997). After each TLD in the same batch was irradiated with an absorbed dose to water of 2 Gy from the 60 Co source Table 1 Dose conversion factors obtained by Monte Carlo calculations and TLD measurements
60 Co
6 MV 10 MV 15 MV
Dose conversion factor Monte Carlo simulation
TLD measurement
1.055 1.063 1.040 1.050
1.060 1.068 1.039 1.053
for five times, the TLD-100 chips whose reproducibility within 3% and sensitivity within 10% were used in this experiment. In the dose response test, the selected TLDs were exposed to the absorbed doses to water in 60 Co ranging from 0.5 to 3 Gy to set up the TLDs calibration curve. In addition, for verifying feasibility of TLD-100 chips used in clinical dosimetry verification, 78 TLD-100s were selected for IMRT dose evaluation. The TLD chips were placed inside the phantom to form four lines which crossed with each other at the intersection point. Some procedures that patients usually undergo during the radiotherapy process can be modeled in this verification, including the sequence from CT data acquisition to treatment planning and then finally the phantom irradiation in accordance with the calculated plan. Then the TLD readouts were measured by means of calibration curve and then converted to absolute doses to compare with the dose distribution produced by the computer treatment planning system. Figs. 3(a)–(d) showed that the dose evaluation differences between the TLD measurement and the IMRT computer treatment planning were less than 5%, which were lower than the total uncertainty of radiotherapy predicted dose. Hence, the TLD-100 chips in this study are usable in clinical dose verification. Table 1 showed the conversion factors in AECL Theratron 780C 60 Co unit and 6, 10 and 15 MV beam qualities of Varian 21EX accelerator using BEAMnrc code. Besides, Table 1 listed the experimental values of conversion factors determined by the TLD-100 chips which were placed at the same calibration depths in water and polystyrene phantom. The results showed that the dose conversion factors from TLD measurements and BEAMnrc calculations were in good agreement with a difference within 0.5%. By multiplying the absorbed dose measured by the TLDs in the polystyrene phantom with the dose conversion factor calculated from Monte Carlo code or obtained from experiments, the absorbed dose to water at the calibration depth can be obtained.
1.10
6 MV 10 MV 15 MV Co-60
1.08 1.06 1.04 DTLD/Dstat
Beam quality
957
+3.5%
1.02 1.00 0.98 -3.5%
0.96 0.94 0.92 0.90
A
B
C
D
E F G Participant code
H
I
J
Fig. 4. Results of TLD postal dose verification in Taiwanese radiotherapy centers.
958
J.H. Lee et al. / Radiation Measurements 43 (2008) 954 – 958
In the dose verification experiments performed for the 10 radiotherapy centers with the Theratron 780C 60 Co unit and the Varian 21EX accelerator, the planned dose for the points containing TLD on the central beam axis is 2 Gy. The irradiation of the phantom with TLDs has to be made according to the calculation plan. Consequently, it is possible to compare the planned dose for the TLD points with the doses measured by TLD afterwards. The combined relative standard uncertainty uc (1SD) of the TLD system was estimated to be about 1.05% for 60 Co and about 1.75% for high energy X-rays. Based on these uncertainties and a compromise for photon beams, an acceptance level of 3.5% has been chosen for both 60 Co and X-ray comparisons (2SD). The TLD measured doses (DTLD ) compared with the doses stated by the radiotherapy center (Dstat ) were shown in Fig. 4. In Taiwan, the absorbed dose to water calibrations in medical accelerator photon beams are traceable to the INER 60 Co standards following the recommendations given in the AAPM TG-21 and TG-51 dosimetry protocols. INER calibrated the TLDs in terms of the absorbed dose to water in this experiment. It should be noticed in Fig. 4 that the same dosimetry code of practice is used at a hospital or INER to determine the dose given to the TLD, some of its contribution in uc can be canceled out and its DTLD /Dstat ratio would be kept within tighter limits. For dose verifications in 95% beam qualities, the differences under reference conditions for 60 Co, high energy X-rays of 6, 10 and 15 MV were within ±3.5% acceptance level and it proved the feasibility of applying this method in clinical dose verification for radiotherapy. The acceptance level of ±3.5% in this study is evidently stricter than the levels used within the IAEA/WHO TLD postal programme (±5%). This is justified by the better capability of the small national network to repeat the audit promptly or to investigate the situation thoroughly if necessary. 4. Conclusions In this work, the postal polystyrene phantom was used instead of the traditional water phantom. The BEAMnrc code was used to calculate the dose conversion factor at the same calibration depth of the polystyrene and water phantom. The conversion factor can convert the dose to polystyrene into the dose to water to verify the clinical dosimetry at radiotherapy centers. The postal TLD verification has shown that the high energy photon beam dosimetry in most centers remains within
acceptable limits. Out of 21 beam qualities checks of 60 Co and high energy X-rays, in 20 cases the deviation of absorbed dose stated by participants compared with the values measured by the INER did not exceed the acceptance level of ±3.5%. The comparison pilot study has made the simple dose verification system possible to test the consistency of high energy photon beam dosimetry in regional radiotherapy centers in Taiwan. It is expected that the advanced mode of the TLD audit will be used as a strict tool for purposes of state supervision in Taiwan in the future. At present it is operated as a pilot study and that gives the radiotherapy centers the opportunity to become familiar with the method and to be prepared for a new situation. Following this, we will conduct as soon as possible additional photon and electron dose verifications for the other centers and hope to execute a detailed and complete analysis for the acceptance level. As to the radiotherapy centers whose dose deviations were larger than the acceptance level, a follow-up programme would be established to resolve the discrepancies. Acknowledgments The authors would like to thank the staff members at the 10 radiotherapy centers in Taiwan for their assistance in the clinical dose verification programme. We also express our thanks to Mr. Shi-Hwa Su for his useful comments and discussions. References Harris, C.K., Elson, H.R., Lamba, M.A.S., Foster, A.E., 1997. A comparison of the effectiveness of thermoluminescent crystals LiF:Mg, Ti and LiF:Mg, Cu, P for clinical dosimtery. Med. Phys. 24 (9), 1527–1529. Izewska, J., Andreo, P., 2000. The IAEA/WHO TLD postal programme for radiotherapy hospitals. Radiother. Oncol. 54, 65–72. Izewska, J., Gajewski, R., Gwiazdowska, B., Kania, M., Rostkowska, J., 1995. TLD postal dose intercomparison for megavoltage units in Poland. Radiother. Oncol. 36, 143–152. Izewska, J., Bera, P., Vatnitsky, S., 2002. IAEA/WHO TLD postal audit service and high precision measurements for radiotherapy level dosimetry. Radiat. Prot. Dosim. 101, 387–392. Kron, T., Duggan, L., Smith, T., Rosenfeld, A., Butson, M., Kaplan, G., Howlett, S., Hyodo, K., 1998. Dose response of various radiation detectors to synchrotron radiation. Phys. Med. Biol. 43, 3235–3259. Kroutilíková, D., Novotný, J., Judas, L., 2003. Thermoluminescent dosimeters (TLD) quality assurance network in the Czech Republic. Radiother. Oncol. 66, 235–244. Nelson, W.R., Hirayama, H., Rogers, D.W., 1985. The EGS4 Code System. Stanford Linear Accelerator Center, Report SLAC-265.