- Email: [email protected]
Secondary neutron ambient dose equivalent measurement of the wobbling system of a proton beam radiotherapy facility
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Secondary neutron ambient dose equivalent measurement of the wobbling system of a proton beam radiotherapy facility Ying-Lan Liaoa,b,c, Hsien-Hsin Chena,d,e, Hsin-Yu Chena,b, Hsiao-Chieh Huanga,d,e, Chien⁎ Yi Yehd, Hui-Yu Tsaia,b,f, a
Department of Medical Imaging and Radiological Sciences, College of Medicine, Chang Gung University, Taoyuan, Taiwan Medical Physics Research Center, Institute for Radiological Research, Chang Gung University / Chang Gung Memorial Hospital, Linkou, Taoyuan, Taiwan c Health Physics Division, Institute of Nuclear Energy Research, Taoyuan, Taiwan d Department of Radiation Oncology, Linkou Chang Gung Memorial Hospital, Taoyuan, Taiwan e Graduate of Institute of Clinical Medical Sciences, College of Medicine, Chang Gung University, Taoyuan, Taiwan f Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 300, Taiwan b
A R T I C L E I N F O
A BS T RAC T
Keywords: Neutron ambient dose equivalent Wobbling nozzle Proton beam radiotherapy WENDI-II
Purpose: The purpose of this study was to assess the neutron ambient dose equivalent (H*(10)) per prescribed proton dose (D), H*(10)/D, for a wobbling system of the first proton beam facility in Taiwan and compare the H*(10)/D values with those of other facilities, including two wobbling system, two double scattering system, and one uniform scanning proton facilities. Materials and methods: A wide energy neutron detector, WENDI-II, was used to measure the H*(10) at a distance, d, of 50–225 cm between the center of WENDI-II and the isocenter. The effects of proton beam range and SOBP width were assessed for a middle wobbling diameter. Thereafter, the H*(10)/D values were compared with those of the uniform scanning proton beam facility. The H*(10)/D values were measured at a d of 50– 225 cm by using beam energy of 230 and 150 MeV as well as an SOBP width of 6 and 10 cm. The H*(10)/D values of the maximum energy and 150 MeV of the proton beam facilities were compared. Results: The measured H*(10)/D values were a function of a proton beam range for SOBP widths of 5 and 10 cm. The H*(10)/D values ranged from 0.2 to 2.3 mSv/Gy at a range of 50–269 mm for 5-cm SOBP beams and from 0.2 to 2.9 mSv/Gy for 10-cm SOBP beams. The H*(10)/D values for 230-MeV proton beams increased by 20.4% when the width of SOBP was increased from 6 to 10 cm. These values for 230 MeV at a d of 50 cm were four times those of 150 MeV. The H*(10)/D values increased with the beam scanning area. Moreover, the H*(10)/D values of the medium and large wobbling diameters were respectively 2.1 and 3.3 times those of the small wobbling diameter. Conclusion: We assessed the H*(10)/D values of the wobbling system of the first proton beam radiotherapy facility in Taiwan. The H*(10)/D values increased as the wobbling size increased, and these values decreased with increasing distance between the measurement position and isocenter. The H*(10)/D values of the wobbling nozzle proton beam system were higher than those of the uniform scanning system because of the interaction between proton beams and the internal components of the nozzle. The discrepancies in H*(10)/D values between facilities depend on the design of a proton beam line and the operational setup of the irradiated condition. Our experiment can serve as a reference for comparing secondary neutron radiation in wobbling system of the proton beam system.
1. Introduction The objective of radiotherapy is to deliver a prescribed radiation dose to a well-defined target while minimizing the radiation dose to the surrounding normal tissue and critical organs. Conventional radio-
therapy uses photon beams to deliver a radiation dose to patients through three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), or volumetric modulated arc therapy (VMAT). Both techniques can provide effectively distributed dose conformity to a tumor and preserve normal tissue. However, the
⁎ Correspondence to: Department of Medical Imaging and Radiological Sciences, College of Medicine, Chang Gung University, 259, Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan. E-mail address: [email protected] (H.-Y. Tsai).
http://dx.doi.org/10.1016/j.radphyschem.2017.01.030 Received 2 November 2015; Received in revised form 6 January 2017; Accepted 26 January 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Liao, Y.-L., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.01.030
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al.
2.2. Neutron ambient dose equivalent measurement
integrated dose delivered to normal tissue is still problematic because of the physical properties involved when a photon beam interacts with a patient. These integrated doses may expose the patient to secondary radiation. Another source of secondary radiation is the interaction between the photon beams and the components of the treatment head, such as the flattening filter, multileaf collimator (MLC), or block. The risk of radiation-induced secondary cancer for cancer survivors who are exposed to radiation at an early age or live longer is an increasing concern (Xu et al., 2008). Particle radiotherapy was introduced to be an alternative treatment technique. The physical characteristic of Bragg peak is of the advantage of heavy charged particles and is superior to photon beams. The physical properties of a proton beam may both reduce the radiation dose to normal tissue and increase the conformal dose distribution to the tumor volume. The number of particle beam radiotherapy facilities, including proton beam and carbon-ion beam facilities, has increased by 60% worldwide in the past 5 years (Jermann, 2015). A total of 71 particle beam radiotherapy facilities (proton beam: 61; carbon-ion beam: 10) are in operation, and 44 facilities (proton beam: 41; carbon-ion beam: 3) are under construction until November 2016. Two methods of producing a proton beam are beam spreading (scattering) mode and active scanning mode. Beam spreading mode could be achieved by using double scattering or beam wobbling with scatters or magnets to form a uniform field (Yonai et al., 2008). Pencil beam scanning utilizes scanning magnets to controlling the beam delivery method (Schneider et al., 2002; Zheng et al., 2012). Highenergy neutrons above 10 MeV are produced by (p, n) reactions when protons strike the nozzle components (Tayama et al., 2006; Yan et al., 2002); secondary neutron radiation is then generated. The secondary neutron radiation of scanning proton beam therapy is less than that of passive scattering proton beam (Xu et al., 2008). Although using scanning proton beam therapy increases, the beam spreading mode still needs to be kept for some disease treatment. As the first proton beam radiotherapy facility in Taiwan, it is necessary to evaluate the induced-secondary neutron in the wobbling system. Secondary neutron radiation can be detected and measured using an active neutron dosimeter. The wide energy neutron detector (WENDI-II) is a rem meter with an energy-response that is extended to 5-GeV neutrons and is suitable for measuring the neutron ambient dose equivalent, H*(10), outside the radiation treatment field. Detailed information regarding the physical construction of WENDI-II is contained in the study by Olsher et al. (2000). The purpose of this study was to measure the neutron ambient dose equivalent per prescribed therapeutic dose (H*(10)/D) of the wobbling system of the first proton beam therapy facility in Taiwan. The H*(10)/ D values were compared with those of other proton beam facilities with similar experimental conditions, including two wobbling system, two double scattering system, and one uniform scanning proton facilities.
WENDI-II (FHT 762, Thermo Scientific, MA, USA), was used to measure the neutron ambient dose equivalent, H*(10). The WENDI-II was calibrated at the National Standard Laboratory in Taiwan using a neutron source (bare 252Cf). The detector is composed of a He3 counter tube, which is sensitive to the range from thermal to slow neutrons. The tube is enveloped in a cylindrical polyethylene (PE) moderator, which is embedded in a tungsten powder shell. The PE moderator and tungsten powder improve the detector energy response, which extends up to 5 GeV. Therefore, WENDI-II was designed to cover a wide range of neutron energy levels from thermal to 5 GeV, and it has an output response function for neutrons in this energy range. The output response function of WENDI-II is similar to and higher than the fluence-to-ambient dose equivalent conversion function of International Commission on Radiological Protection Report 74 (Olsher et al., 2000). Therefore, the H*(10) could be directly obtained immediately after each irradiation. The WENDI-II overestimates the H*(10) values as well. The prescribed proton dose, D, was measured using an ionization chamber (Farmer-Type Chamber, PTW30013, 0.6 cm3; PTW, Freiburg, Germany) at the center of the SOBP of the isocenter on the phantom (high density polyethylene, ρ of 0.952 g/cm3). The phantom is used for daily QA measurements. The measured dose values were calculated according to the international dosimetry protocol of the International Atomic Energy Agency TRS-398 guidelines (IAEA, 2000). The H*(10)/ D values were obtained by dividing the total neutron ambient dose equivalent, H*(10), by the prescribed proton dose, D. The reproducibility of WENDI-II and the ionization chamber in the same irradiation condition was about 0.2%. 3. Experimental setup Fig. 2 displays a diagram of the experimental setup in our study. A phantom was placed at the isocenter to simulate a patient because the center of the phantom coincided with the isocenter. The phantom was sufficiently large to enable a primary beam to enter and sufficiently long to stop the primary protons placed at the isocenter. The center of WENDI-II was set at the isocenter height. The measured positions were at the distance, d, of 50, 100, 150, and 225 cm between the center of WENDI-II and the isocenter (Fig. 2). Table 1 provides the beam line parameters used in this study. A 90×90 mm brass MLC and a 50×50 mm brass patient-specific collimator were employed in all the measurements. Diameters of the laterally uniform irradiation field were set at the middle size of the wobbling diameter that was typically used in radiotherapy. Distances between the final collimator and the isocenter were set at 300 and 400 mm for regular use conditions. The H*(10)/D values affected by a series of proton beam ranges of fixed SOBP widths under daily QA conditions were assessed and compared with those of the uniform scanning proton beam facility (Zheng et al., 2012). Additionally, the H*(10)/D values were measured at the d from 50 to 225 cm by using high-energy proton beams and SOBP widths. The secondary neutrons increase as the incident particle energy increases; thus, investigating the neutron dose for the maximum beam energy at each beam line is necessary. Another proton beam energy of 150 MeV was selected to enable the settings in this study to be comparable with those in previously published articles. Because the lateral uniform irradiation diameter can be selected from three wobbling diameter sizes (i.e., small (62–68 mm), medium (116–127 mm), and large (150–165 mm)), the contribution of the irradiation field size was investigated.
2. Materials and methods 2.1. Proton beam therapy facilities The experimental data were obtained from the wobbling system (Sumitomo Heavy Industry, Japan) of the proton beam therapy facility. The wobbling nozzle of our facility is composed of a scatterer, ridge filter, fine degrader, dose monitor, flatness monitor, MLC, compensator, and patient aperture to form a sufficiently large uniform irradiation field (Fig. 1). Table 1 displays the major properties of the beam line and parameter settings in our measurements. The experimental data were compared with those in the study by Yonai et al. (2008b), who compared the H*(10)/D values of two wobbling system, namely the Hyogo Ion Beam Medical Center (HIBMC), two double scattering system, National Cancer Center Hospital East (NCCHE), Sizuoka Cancer Center (SCC), and Proton Medical Research Center (PMRC) at Tukuba University, and one uniform scanning proton facility, the ProCure Proton Therapy Center.
4. Results and discussion Fig. 3 displays the measured H*(10)/D values at 50 cm between the center of the WENDI-II and the isocenter as a function of the proton 2
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al. MLC ridge filter
dose monitor/ fine flatness degrader monitor
scanning magnets scatterer
aperture tumor
compensator
beam direction
Fig. 1. Schematic diagram of the structure of the wobbling system.
proton beams, respectively. Increasing proton beam energy results in the production of more secondary neutrons. The H*(10)/D values of a given SOBP width of 230 MeV are four times those of 150 MeV. Fig. 5 illustrates the measured H*(10)/D values under three wobbling diameter sizes at (a) the maximum energy of 230-MeV and (b) 150-MeV proton beams. The H*(10)/D values increased when the beam scanning area was increased. The measured H*(10)/D values of the 230-MeV incident proton energy levels ranged from 1.17 to 0.15 mSv/Gy, 2.46 to 0.55 mSv/Gy, and 3.82 to 1.11 mSv/Gy for small, medium, and large wobbling diameters, respectively, at a d from 50 to 225 cm. The H*(10)/D values of medium and large wobbling diameters are 2.1 and 3.3 times those of a small wobbling diameter. A similar trend was observed in conditions in which the proton beam energy was 150 MeV. Three wobbling diameter sizes can be selected for various irradiation field sizes. When a patient's treatment area is larger than 130 mm in diameter, a large wobbling diameter should be selected for the beam delivery setup parameter. The H*(10)/D values at a d of 50 cm for medium and small wobbling diameters were similar to those of the NCCHE and HIBMC, respectively. Furthermore, the H*(10)/D values at a d of 50 cm for the NCCHE were higher than those of the medium wobbling diameter. The discrepancies in H*(10)/D values between the proton beam facilities may be derived from the proton beam energy, proton beam nozzle design, operational beam settings, materials and locations of the beam line devices, or experimental setup (phantom material and measurement position). The proton beam energy of the NCCHE was 235 MeV and exceeded that of the 230MeV proton beams in our measurement. The H*(10)/D values at other d positions were similar to one another because of the increasing distance. Furthermore, the H*(10)/D values of the PMRC and SCC were higher than those in our measurements for the small wobbling diameter because of the proton beam energy and irradiation field.
beam range with a SOBP width of 5 and 10 cm, respectively. The measured H*(10)/D values for both proton beam systems depended on the proton range (energy) and SOBP width. The H*(10)/D values increased with the proton range from approximately 0.2 to 2.3 mSv/Gy in the range of 50–269 mm for 5-cm SOBP beams and from 0.2 to 2.9 mSv/Gy for 10-cm SOBP beams. Higher-energy proton beams result in a deep range. Therefore, a large amount of secondary neutron radiation is produced when these beams are stopped in the nozzle collimation system or the phantom. Our observation was similar to that in the study by Zheng et al. (2012). In that study, the H*(10)/D values were assessed under uniform scanning proton beams, which were composed of the a first scatterer, a range modulator wheel, two scanning magnets, variable collimators, monitor unit ionization chambers, a snout, an aperture, and a range compensator. In a passive scattering proton beam system, the treatment nozzle is composed of a scatterer, ridge filter, fine degrader, dose monitor, flatness monitor, MLC, compensator, and patient aperture. In this system, the proton beams have a greater opportunity to interact with these components, resulting in higher secondary neutron radiation. Thus, the H*(10)/D values of the active beam scanning system were lower than those of the passive scattering beam system (Xu et al., 2008). Fig. 4 provides comparisons of H*(10)/D values at a d of 50– 225 cm by using 230- and 150-MeV proton beams and 6- and 10-cm SOBP widths. The medium-sized wobbling diameter was selected. The secondary neutron dose in the particle therapy depended on the incident particle energy and SOBP width. The H*(10)/D values using 230 MeV of 6-cm SOBP width were reduced from 3.8 to 1.1 mSv/Gy at a d from 50 to 225 cm. Moreover, the H*(10)/D values using 150 MeV of 6-cm SOBP width were reduced from 1.0 to 0.3 mSv/Gy at a d from 50 to 225 cm. Increasing the SOBP width from 6 to 10 cm increased the neutron dose equivalent by 20.4% and 14.1% for 230- and 150-MeV Table 1 Operation parameters in the beam line used in this study. Facility
Beam energy (MeV)
Beam line type (G/H)a
Method for producing a lateral uniform irradiation fieldb
Diameter of uniform irradiation field, (mm)
SOBP width, (cm)
Precollimatorc (mm2)
Distance between the final collimator and the isocenter (mm)
Final collimatord (mm2)
CGMH
230, 150
G
W
5, 6, 10
90×90 (MLC, brass)
300, 400
50×50 (PC, brass)
HIBMC
210, 150
H
W
S (124–139), M (232–254), L (300–330) 160
6
–
400
NCCHE PMRC SCC
235, 150 250, 155 220, 160
G G G
DS DS W
283 200 190
6 6 6
90×90 (FLC, brass) 110.2×100 (MLC, brass) –
300, 500 300 400
WPE
230
H
US
180
100
100×100
300
52.5×50 (MLC, iron) 50×50 (PC, brass) 50×50 (PC, brass) 52.5×50 (MLC, iron) 100 × 100 (brass)
a b c d
G: gantry line; H: fixed horizontal line. W: wobbling; DS: double scattering; US: uniform scanning. MLC: multileaf collimator; RC: ring collimator; FLC: four-leaf collimator. PC: patient-specific collimator.
3
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al.
Fig. 2. Schematic diagram of the experimental setup for H*(10)/D measurements. A neutron detector was located 50, 100, 150, and 225 cm from the isocenter.
Fig. 3. Measured H*(10)/D values as a function of the proton range for proton beams of 2-, 5-, 10-, and 15-cm SOBP widths. WENDI-II was placed at a d of 50 cm from the isocenter. The wobbling nozzle proton beam in our facility and the uniform scanning beam in the study by Zheng et al. (2012) were compared.
Fig. 5. Measured H*(10)/D values as a function of the d from the isocenter under (a) the maximum proton beam energy and (b) 150 MeV. Our measured H*(10)/D values were compared with those of other proton beam therapy facilities (HIBMC, NCCHE, PMRC, and SCC).
Fig. 4. Measured H*(10)/D values as a function of the d from the isocenter under two proton beam energy levels (230 and 150 MeV) and two SOBP widths (6 and 10 cm).
(Moyers et al., 2008; Tayama et al., 2006). The use of scanning beam technique of proton beam therapy can result in less neutron dose radiation because the proton beams was focused by magnetic dipoles to form a pencil beam to scanning the treatment target. Working group of the European Radiation Dosimetry Group (EURADOS WG9—Radiation protection in medicine) carried out a large measurement schedule to qualifying the stray neutron radiation dose in scanning technique of proton therapy utilized active dosimetry systems (Farah et al., 2015). Under the sophisticated dosimetry systems, they determined the neutron spectra at specific
Higher proton beam energy and the diameter of the irradiation field for the PMRC and SCC resulted in a large H*(10)/D value contribution. The diameters of the PMRC and SCC irradiation fields were 200 and 190 mm and were larger than the small wobbling diameter in our measurement. The HIBMC and SCC used the beam wobbling method for producing irradiation fields, whereas the NCCHE and PMRC used the double scattering method. However, the further design of the components in each beam line differed. The components and materials of the nozzle can produce various levels of secondary neutron radiation
4
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al.
References
treatment condition with extended-range Bonner sphere spectrometry (BSS) and detected the H*(10) values with several rem-counters and tissue equivalent proportional counters (TEPCs). Their experiments indicated that the extended-BSS, TEPCs, and WENDI-II enable accurate measurements of stray neutrons. WENDI-II can be a used to estimate the H*(10) values for clinical therapy facilities because the simple use and reasonable response to higher-energy range of neutrons.
Farah, J., Mares, V., Romero-Exposito, M., Trinkl, S., Domingo, C., Dufek, V., Klodowska, M., Kubančák, J., Knezevic, Z., Liszka, M., Majer, M., Miljanić, S., Ploc, O., Schinner, K., Stolarczyk, L., Trompier, F., Wielunski, M., Olko, P., Harrison, R.M., 2015. Measurement of stray radiation within a scanning proton therapy facility: EURADOS WG9 intercomparison exercise of active dosimetry systems. Med. Phys. 42, 2572–2584. http://dx.doi.org/10.1118/1.4916667. IAEA, T., 2000. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water, TRS 398 report. Vienna Int. At. Energy Agency 28, 110–116. http:// dx.doi.org/10.1007/s11604-009-0393-5. Jermann, M., 2015. Particle therapy statistics in 2014. Int. J. Part. Ther. 2, 50–54. http://dx.doi.org/10.14338/IJPT-15-00013. Moyers, M.F., Benton, E.R., Ghebremedhin, A., Coutrakon, G., 2008. Leakage and scatter radiation from a double scattering based proton beamline. Med. Phys. 35, 128–144. http://dx.doi.org/10.1118/1.2805086. Olsher, R.H., Hsu, H.H., Beverding, A., Kleck, J.H., Casson, W.H., Vasilik, D.G., Devine, R.T., 2000. WENDI: an improved neutron rem meter. Health Phys. 79, 170–181. Schneider, U., Agosteo, S., Pedroni, E., Besserer, J., 2002. Secondary neutron dose during proton therapy using spot scanning. Int. J. Radiat. Oncol. Biol. Phys. 53, 244–251. http://dx.doi.org/10.1016/S0360-3016(01)02826-7. Tayama, R., Fujita, Y., Tadokoro, M., Fujimaki, H., Sakae, T., Terunuma, T., 2006. Measurement of neutron dose distribution for a passive scattering nozzle at the Proton Medical Research Center (PMRC). Nucl. Instrum. Methods Phys. Res. Sect. A: Accel., Spectrometers, Detect. Assoc. Equip. 564, 532–536. http://dx.doi.org/ 10.1016/j.nima.2006.04.028. Xu, X.G., Bednarz, B., Paganetti, H., 2008. A review of dosimetry studies on externalbeam radiation treatment with respect to second cancer induction. Phys. Med. Biol. 53, R193–R241. http://dx.doi.org/10.1088/0031-9155/53/13/R01. Yan, X., Titt, U., Koehler, A.M., Newhauser, W.D., 2002. Measurement of neutron dose equivalent to proton therapy patients outside of the proton radiation field. Nucl. Instrum. Methods Phys. Res. Sect. A: Accel., Spectrometers, Detect. Assoc. Equip. 476, 429–434. http://dx.doi.org/10.1016/S0168-9002(01)01483-8. Yonai, S., Kanematsu, N., Komori, M., Kanai, T., Takei, Y., Takahashi, O., Isobe, Y., Tashiro, M., Koikegami, H., Tomita, H., 2008. Evaluation of beam wobbling methods for heavy-ion radiotherapy. Med. Phys. 35, 927. http://dx.doi.org/10.1118/ 1.2836953. Zheng, Y., Liu, Y., Zeidan, O., Schreuder, A.N., Keole, S., 2012. Measurements of neutron dose equivalent for a proton therapy center using uniform scanning proton beams. Med. Phys. 39, 3484–3492. http://dx.doi.org/10.1118/1.4718685.
5. Conclusion We assessed the H*(10)/D values of the wobbling system with three wobbling sizes of the first proton beam radiotherapy facility in Taiwan. The H*(10)/D values increased as the depth of the proton beam range increased. The H*(10)/D values decreased as the distance between the measurement position and isocenter increased. Higher proton beam energy and greater SOBP width resulted in a higher contribution of secondary neutron radiation. The distribution of the H*(10)/D values of our wobbling system proton beam therapy facility was similar to that of other facilities. The discrepancies in H*(10)/D values among facilities depends on the design of each proton beam line device and the operational setup of each experimental condition. Our experimental results can be a reference to compare secondary neutron radiation in proton beam systems with wobbling spread beam mode. Acknowledgments This study was supported by grants from Chang Gung Memorial Hospital (CMRPD1C0681, CMRPD1C0682, CIRPD1C0053). The authors express their gratitude to the Dose Assessment Core Laboratory of the Institute for Radiological Research, Chang Gung University/Chang Gung Memorial Hospital at Linkou for assistance with dose assessments.
5
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Secondary neutron ambient dose equivalent measurement of the wobbling system of a proton beam radiotherapy facility Ying-Lan Liaoa,b,c, Hsien-Hsin Chena,d,e, Hsin-Yu Chena,b, Hsiao-Chieh Huanga,d,e, Chien⁎ Yi Yehd, Hui-Yu Tsaia,b,f, a
Department of Medical Imaging and Radiological Sciences, College of Medicine, Chang Gung University, Taoyuan, Taiwan Medical Physics Research Center, Institute for Radiological Research, Chang Gung University / Chang Gung Memorial Hospital, Linkou, Taoyuan, Taiwan c Health Physics Division, Institute of Nuclear Energy Research, Taoyuan, Taiwan d Department of Radiation Oncology, Linkou Chang Gung Memorial Hospital, Taoyuan, Taiwan e Graduate of Institute of Clinical Medical Sciences, College of Medicine, Chang Gung University, Taoyuan, Taiwan f Institute of Nuclear Engineering and Science, National Tsing Hua University, Hsinchu 300, Taiwan b
A R T I C L E I N F O
A BS T RAC T
Keywords: Neutron ambient dose equivalent Wobbling nozzle Proton beam radiotherapy WENDI-II
Purpose: The purpose of this study was to assess the neutron ambient dose equivalent (H*(10)) per prescribed proton dose (D), H*(10)/D, for a wobbling system of the first proton beam facility in Taiwan and compare the H*(10)/D values with those of other facilities, including two wobbling system, two double scattering system, and one uniform scanning proton facilities. Materials and methods: A wide energy neutron detector, WENDI-II, was used to measure the H*(10) at a distance, d, of 50–225 cm between the center of WENDI-II and the isocenter. The effects of proton beam range and SOBP width were assessed for a middle wobbling diameter. Thereafter, the H*(10)/D values were compared with those of the uniform scanning proton beam facility. The H*(10)/D values were measured at a d of 50– 225 cm by using beam energy of 230 and 150 MeV as well as an SOBP width of 6 and 10 cm. The H*(10)/D values of the maximum energy and 150 MeV of the proton beam facilities were compared. Results: The measured H*(10)/D values were a function of a proton beam range for SOBP widths of 5 and 10 cm. The H*(10)/D values ranged from 0.2 to 2.3 mSv/Gy at a range of 50–269 mm for 5-cm SOBP beams and from 0.2 to 2.9 mSv/Gy for 10-cm SOBP beams. The H*(10)/D values for 230-MeV proton beams increased by 20.4% when the width of SOBP was increased from 6 to 10 cm. These values for 230 MeV at a d of 50 cm were four times those of 150 MeV. The H*(10)/D values increased with the beam scanning area. Moreover, the H*(10)/D values of the medium and large wobbling diameters were respectively 2.1 and 3.3 times those of the small wobbling diameter. Conclusion: We assessed the H*(10)/D values of the wobbling system of the first proton beam radiotherapy facility in Taiwan. The H*(10)/D values increased as the wobbling size increased, and these values decreased with increasing distance between the measurement position and isocenter. The H*(10)/D values of the wobbling nozzle proton beam system were higher than those of the uniform scanning system because of the interaction between proton beams and the internal components of the nozzle. The discrepancies in H*(10)/D values between facilities depend on the design of a proton beam line and the operational setup of the irradiated condition. Our experiment can serve as a reference for comparing secondary neutron radiation in wobbling system of the proton beam system.
1. Introduction The objective of radiotherapy is to deliver a prescribed radiation dose to a well-defined target while minimizing the radiation dose to the surrounding normal tissue and critical organs. Conventional radio-
therapy uses photon beams to deliver a radiation dose to patients through three-dimensional conformal radiotherapy (3D-CRT), intensity-modulated radiotherapy (IMRT), or volumetric modulated arc therapy (VMAT). Both techniques can provide effectively distributed dose conformity to a tumor and preserve normal tissue. However, the
⁎ Correspondence to: Department of Medical Imaging and Radiological Sciences, College of Medicine, Chang Gung University, 259, Wen-Hwa 1st Rd., Kwei-Shan, Taoyuan 333, Taiwan. E-mail address: [email protected] (H.-Y. Tsai).
http://dx.doi.org/10.1016/j.radphyschem.2017.01.030 Received 2 November 2015; Received in revised form 6 January 2017; Accepted 26 January 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Liao, Y.-L., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.01.030
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al.
2.2. Neutron ambient dose equivalent measurement
integrated dose delivered to normal tissue is still problematic because of the physical properties involved when a photon beam interacts with a patient. These integrated doses may expose the patient to secondary radiation. Another source of secondary radiation is the interaction between the photon beams and the components of the treatment head, such as the flattening filter, multileaf collimator (MLC), or block. The risk of radiation-induced secondary cancer for cancer survivors who are exposed to radiation at an early age or live longer is an increasing concern (Xu et al., 2008). Particle radiotherapy was introduced to be an alternative treatment technique. The physical characteristic of Bragg peak is of the advantage of heavy charged particles and is superior to photon beams. The physical properties of a proton beam may both reduce the radiation dose to normal tissue and increase the conformal dose distribution to the tumor volume. The number of particle beam radiotherapy facilities, including proton beam and carbon-ion beam facilities, has increased by 60% worldwide in the past 5 years (Jermann, 2015). A total of 71 particle beam radiotherapy facilities (proton beam: 61; carbon-ion beam: 10) are in operation, and 44 facilities (proton beam: 41; carbon-ion beam: 3) are under construction until November 2016. Two methods of producing a proton beam are beam spreading (scattering) mode and active scanning mode. Beam spreading mode could be achieved by using double scattering or beam wobbling with scatters or magnets to form a uniform field (Yonai et al., 2008). Pencil beam scanning utilizes scanning magnets to controlling the beam delivery method (Schneider et al., 2002; Zheng et al., 2012). Highenergy neutrons above 10 MeV are produced by (p, n) reactions when protons strike the nozzle components (Tayama et al., 2006; Yan et al., 2002); secondary neutron radiation is then generated. The secondary neutron radiation of scanning proton beam therapy is less than that of passive scattering proton beam (Xu et al., 2008). Although using scanning proton beam therapy increases, the beam spreading mode still needs to be kept for some disease treatment. As the first proton beam radiotherapy facility in Taiwan, it is necessary to evaluate the induced-secondary neutron in the wobbling system. Secondary neutron radiation can be detected and measured using an active neutron dosimeter. The wide energy neutron detector (WENDI-II) is a rem meter with an energy-response that is extended to 5-GeV neutrons and is suitable for measuring the neutron ambient dose equivalent, H*(10), outside the radiation treatment field. Detailed information regarding the physical construction of WENDI-II is contained in the study by Olsher et al. (2000). The purpose of this study was to measure the neutron ambient dose equivalent per prescribed therapeutic dose (H*(10)/D) of the wobbling system of the first proton beam therapy facility in Taiwan. The H*(10)/ D values were compared with those of other proton beam facilities with similar experimental conditions, including two wobbling system, two double scattering system, and one uniform scanning proton facilities.
WENDI-II (FHT 762, Thermo Scientific, MA, USA), was used to measure the neutron ambient dose equivalent, H*(10). The WENDI-II was calibrated at the National Standard Laboratory in Taiwan using a neutron source (bare 252Cf). The detector is composed of a He3 counter tube, which is sensitive to the range from thermal to slow neutrons. The tube is enveloped in a cylindrical polyethylene (PE) moderator, which is embedded in a tungsten powder shell. The PE moderator and tungsten powder improve the detector energy response, which extends up to 5 GeV. Therefore, WENDI-II was designed to cover a wide range of neutron energy levels from thermal to 5 GeV, and it has an output response function for neutrons in this energy range. The output response function of WENDI-II is similar to and higher than the fluence-to-ambient dose equivalent conversion function of International Commission on Radiological Protection Report 74 (Olsher et al., 2000). Therefore, the H*(10) could be directly obtained immediately after each irradiation. The WENDI-II overestimates the H*(10) values as well. The prescribed proton dose, D, was measured using an ionization chamber (Farmer-Type Chamber, PTW30013, 0.6 cm3; PTW, Freiburg, Germany) at the center of the SOBP of the isocenter on the phantom (high density polyethylene, ρ of 0.952 g/cm3). The phantom is used for daily QA measurements. The measured dose values were calculated according to the international dosimetry protocol of the International Atomic Energy Agency TRS-398 guidelines (IAEA, 2000). The H*(10)/ D values were obtained by dividing the total neutron ambient dose equivalent, H*(10), by the prescribed proton dose, D. The reproducibility of WENDI-II and the ionization chamber in the same irradiation condition was about 0.2%. 3. Experimental setup Fig. 2 displays a diagram of the experimental setup in our study. A phantom was placed at the isocenter to simulate a patient because the center of the phantom coincided with the isocenter. The phantom was sufficiently large to enable a primary beam to enter and sufficiently long to stop the primary protons placed at the isocenter. The center of WENDI-II was set at the isocenter height. The measured positions were at the distance, d, of 50, 100, 150, and 225 cm between the center of WENDI-II and the isocenter (Fig. 2). Table 1 provides the beam line parameters used in this study. A 90×90 mm brass MLC and a 50×50 mm brass patient-specific collimator were employed in all the measurements. Diameters of the laterally uniform irradiation field were set at the middle size of the wobbling diameter that was typically used in radiotherapy. Distances between the final collimator and the isocenter were set at 300 and 400 mm for regular use conditions. The H*(10)/D values affected by a series of proton beam ranges of fixed SOBP widths under daily QA conditions were assessed and compared with those of the uniform scanning proton beam facility (Zheng et al., 2012). Additionally, the H*(10)/D values were measured at the d from 50 to 225 cm by using high-energy proton beams and SOBP widths. The secondary neutrons increase as the incident particle energy increases; thus, investigating the neutron dose for the maximum beam energy at each beam line is necessary. Another proton beam energy of 150 MeV was selected to enable the settings in this study to be comparable with those in previously published articles. Because the lateral uniform irradiation diameter can be selected from three wobbling diameter sizes (i.e., small (62–68 mm), medium (116–127 mm), and large (150–165 mm)), the contribution of the irradiation field size was investigated.
2. Materials and methods 2.1. Proton beam therapy facilities The experimental data were obtained from the wobbling system (Sumitomo Heavy Industry, Japan) of the proton beam therapy facility. The wobbling nozzle of our facility is composed of a scatterer, ridge filter, fine degrader, dose monitor, flatness monitor, MLC, compensator, and patient aperture to form a sufficiently large uniform irradiation field (Fig. 1). Table 1 displays the major properties of the beam line and parameter settings in our measurements. The experimental data were compared with those in the study by Yonai et al. (2008b), who compared the H*(10)/D values of two wobbling system, namely the Hyogo Ion Beam Medical Center (HIBMC), two double scattering system, National Cancer Center Hospital East (NCCHE), Sizuoka Cancer Center (SCC), and Proton Medical Research Center (PMRC) at Tukuba University, and one uniform scanning proton facility, the ProCure Proton Therapy Center.
4. Results and discussion Fig. 3 displays the measured H*(10)/D values at 50 cm between the center of the WENDI-II and the isocenter as a function of the proton 2
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al. MLC ridge filter
dose monitor/ fine flatness degrader monitor
scanning magnets scatterer
aperture tumor
compensator
beam direction
Fig. 1. Schematic diagram of the structure of the wobbling system.
proton beams, respectively. Increasing proton beam energy results in the production of more secondary neutrons. The H*(10)/D values of a given SOBP width of 230 MeV are four times those of 150 MeV. Fig. 5 illustrates the measured H*(10)/D values under three wobbling diameter sizes at (a) the maximum energy of 230-MeV and (b) 150-MeV proton beams. The H*(10)/D values increased when the beam scanning area was increased. The measured H*(10)/D values of the 230-MeV incident proton energy levels ranged from 1.17 to 0.15 mSv/Gy, 2.46 to 0.55 mSv/Gy, and 3.82 to 1.11 mSv/Gy for small, medium, and large wobbling diameters, respectively, at a d from 50 to 225 cm. The H*(10)/D values of medium and large wobbling diameters are 2.1 and 3.3 times those of a small wobbling diameter. A similar trend was observed in conditions in which the proton beam energy was 150 MeV. Three wobbling diameter sizes can be selected for various irradiation field sizes. When a patient's treatment area is larger than 130 mm in diameter, a large wobbling diameter should be selected for the beam delivery setup parameter. The H*(10)/D values at a d of 50 cm for medium and small wobbling diameters were similar to those of the NCCHE and HIBMC, respectively. Furthermore, the H*(10)/D values at a d of 50 cm for the NCCHE were higher than those of the medium wobbling diameter. The discrepancies in H*(10)/D values between the proton beam facilities may be derived from the proton beam energy, proton beam nozzle design, operational beam settings, materials and locations of the beam line devices, or experimental setup (phantom material and measurement position). The proton beam energy of the NCCHE was 235 MeV and exceeded that of the 230MeV proton beams in our measurement. The H*(10)/D values at other d positions were similar to one another because of the increasing distance. Furthermore, the H*(10)/D values of the PMRC and SCC were higher than those in our measurements for the small wobbling diameter because of the proton beam energy and irradiation field.
beam range with a SOBP width of 5 and 10 cm, respectively. The measured H*(10)/D values for both proton beam systems depended on the proton range (energy) and SOBP width. The H*(10)/D values increased with the proton range from approximately 0.2 to 2.3 mSv/Gy in the range of 50–269 mm for 5-cm SOBP beams and from 0.2 to 2.9 mSv/Gy for 10-cm SOBP beams. Higher-energy proton beams result in a deep range. Therefore, a large amount of secondary neutron radiation is produced when these beams are stopped in the nozzle collimation system or the phantom. Our observation was similar to that in the study by Zheng et al. (2012). In that study, the H*(10)/D values were assessed under uniform scanning proton beams, which were composed of the a first scatterer, a range modulator wheel, two scanning magnets, variable collimators, monitor unit ionization chambers, a snout, an aperture, and a range compensator. In a passive scattering proton beam system, the treatment nozzle is composed of a scatterer, ridge filter, fine degrader, dose monitor, flatness monitor, MLC, compensator, and patient aperture. In this system, the proton beams have a greater opportunity to interact with these components, resulting in higher secondary neutron radiation. Thus, the H*(10)/D values of the active beam scanning system were lower than those of the passive scattering beam system (Xu et al., 2008). Fig. 4 provides comparisons of H*(10)/D values at a d of 50– 225 cm by using 230- and 150-MeV proton beams and 6- and 10-cm SOBP widths. The medium-sized wobbling diameter was selected. The secondary neutron dose in the particle therapy depended on the incident particle energy and SOBP width. The H*(10)/D values using 230 MeV of 6-cm SOBP width were reduced from 3.8 to 1.1 mSv/Gy at a d from 50 to 225 cm. Moreover, the H*(10)/D values using 150 MeV of 6-cm SOBP width were reduced from 1.0 to 0.3 mSv/Gy at a d from 50 to 225 cm. Increasing the SOBP width from 6 to 10 cm increased the neutron dose equivalent by 20.4% and 14.1% for 230- and 150-MeV Table 1 Operation parameters in the beam line used in this study. Facility
Beam energy (MeV)
Beam line type (G/H)a
Method for producing a lateral uniform irradiation fieldb
Diameter of uniform irradiation field, (mm)
SOBP width, (cm)
Precollimatorc (mm2)
Distance between the final collimator and the isocenter (mm)
Final collimatord (mm2)
CGMH
230, 150
G
W
5, 6, 10
90×90 (MLC, brass)
300, 400
50×50 (PC, brass)
HIBMC
210, 150
H
W
S (124–139), M (232–254), L (300–330) 160
6
–
400
NCCHE PMRC SCC
235, 150 250, 155 220, 160
G G G
DS DS W
283 200 190
6 6 6
90×90 (FLC, brass) 110.2×100 (MLC, brass) –
300, 500 300 400
WPE
230
H
US
180
100
100×100
300
52.5×50 (MLC, iron) 50×50 (PC, brass) 50×50 (PC, brass) 52.5×50 (MLC, iron) 100 × 100 (brass)
a b c d
G: gantry line; H: fixed horizontal line. W: wobbling; DS: double scattering; US: uniform scanning. MLC: multileaf collimator; RC: ring collimator; FLC: four-leaf collimator. PC: patient-specific collimator.
3
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al.
Fig. 2. Schematic diagram of the experimental setup for H*(10)/D measurements. A neutron detector was located 50, 100, 150, and 225 cm from the isocenter.
Fig. 3. Measured H*(10)/D values as a function of the proton range for proton beams of 2-, 5-, 10-, and 15-cm SOBP widths. WENDI-II was placed at a d of 50 cm from the isocenter. The wobbling nozzle proton beam in our facility and the uniform scanning beam in the study by Zheng et al. (2012) were compared.
Fig. 5. Measured H*(10)/D values as a function of the d from the isocenter under (a) the maximum proton beam energy and (b) 150 MeV. Our measured H*(10)/D values were compared with those of other proton beam therapy facilities (HIBMC, NCCHE, PMRC, and SCC).
Fig. 4. Measured H*(10)/D values as a function of the d from the isocenter under two proton beam energy levels (230 and 150 MeV) and two SOBP widths (6 and 10 cm).
(Moyers et al., 2008; Tayama et al., 2006). The use of scanning beam technique of proton beam therapy can result in less neutron dose radiation because the proton beams was focused by magnetic dipoles to form a pencil beam to scanning the treatment target. Working group of the European Radiation Dosimetry Group (EURADOS WG9—Radiation protection in medicine) carried out a large measurement schedule to qualifying the stray neutron radiation dose in scanning technique of proton therapy utilized active dosimetry systems (Farah et al., 2015). Under the sophisticated dosimetry systems, they determined the neutron spectra at specific
Higher proton beam energy and the diameter of the irradiation field for the PMRC and SCC resulted in a large H*(10)/D value contribution. The diameters of the PMRC and SCC irradiation fields were 200 and 190 mm and were larger than the small wobbling diameter in our measurement. The HIBMC and SCC used the beam wobbling method for producing irradiation fields, whereas the NCCHE and PMRC used the double scattering method. However, the further design of the components in each beam line differed. The components and materials of the nozzle can produce various levels of secondary neutron radiation
4
Radiation Physics and Chemistry (xxxx) xxxx–xxxx
Y.-L. Liao et al.
References
treatment condition with extended-range Bonner sphere spectrometry (BSS) and detected the H*(10) values with several rem-counters and tissue equivalent proportional counters (TEPCs). Their experiments indicated that the extended-BSS, TEPCs, and WENDI-II enable accurate measurements of stray neutrons. WENDI-II can be a used to estimate the H*(10) values for clinical therapy facilities because the simple use and reasonable response to higher-energy range of neutrons.
Farah, J., Mares, V., Romero-Exposito, M., Trinkl, S., Domingo, C., Dufek, V., Klodowska, M., Kubančák, J., Knezevic, Z., Liszka, M., Majer, M., Miljanić, S., Ploc, O., Schinner, K., Stolarczyk, L., Trompier, F., Wielunski, M., Olko, P., Harrison, R.M., 2015. Measurement of stray radiation within a scanning proton therapy facility: EURADOS WG9 intercomparison exercise of active dosimetry systems. Med. Phys. 42, 2572–2584. http://dx.doi.org/10.1118/1.4916667. IAEA, T., 2000. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water, TRS 398 report. Vienna Int. At. Energy Agency 28, 110–116. http:// dx.doi.org/10.1007/s11604-009-0393-5. Jermann, M., 2015. Particle therapy statistics in 2014. Int. J. Part. Ther. 2, 50–54. http://dx.doi.org/10.14338/IJPT-15-00013. Moyers, M.F., Benton, E.R., Ghebremedhin, A., Coutrakon, G., 2008. Leakage and scatter radiation from a double scattering based proton beamline. Med. Phys. 35, 128–144. http://dx.doi.org/10.1118/1.2805086. Olsher, R.H., Hsu, H.H., Beverding, A., Kleck, J.H., Casson, W.H., Vasilik, D.G., Devine, R.T., 2000. WENDI: an improved neutron rem meter. Health Phys. 79, 170–181. Schneider, U., Agosteo, S., Pedroni, E., Besserer, J., 2002. Secondary neutron dose during proton therapy using spot scanning. Int. J. Radiat. Oncol. Biol. Phys. 53, 244–251. http://dx.doi.org/10.1016/S0360-3016(01)02826-7. Tayama, R., Fujita, Y., Tadokoro, M., Fujimaki, H., Sakae, T., Terunuma, T., 2006. Measurement of neutron dose distribution for a passive scattering nozzle at the Proton Medical Research Center (PMRC). Nucl. Instrum. Methods Phys. Res. Sect. A: Accel., Spectrometers, Detect. Assoc. Equip. 564, 532–536. http://dx.doi.org/ 10.1016/j.nima.2006.04.028. Xu, X.G., Bednarz, B., Paganetti, H., 2008. A review of dosimetry studies on externalbeam radiation treatment with respect to second cancer induction. Phys. Med. Biol. 53, R193–R241. http://dx.doi.org/10.1088/0031-9155/53/13/R01. Yan, X., Titt, U., Koehler, A.M., Newhauser, W.D., 2002. Measurement of neutron dose equivalent to proton therapy patients outside of the proton radiation field. Nucl. Instrum. Methods Phys. Res. Sect. A: Accel., Spectrometers, Detect. Assoc. Equip. 476, 429–434. http://dx.doi.org/10.1016/S0168-9002(01)01483-8. Yonai, S., Kanematsu, N., Komori, M., Kanai, T., Takei, Y., Takahashi, O., Isobe, Y., Tashiro, M., Koikegami, H., Tomita, H., 2008. Evaluation of beam wobbling methods for heavy-ion radiotherapy. Med. Phys. 35, 927. http://dx.doi.org/10.1118/ 1.2836953. Zheng, Y., Liu, Y., Zeidan, O., Schreuder, A.N., Keole, S., 2012. Measurements of neutron dose equivalent for a proton therapy center using uniform scanning proton beams. Med. Phys. 39, 3484–3492. http://dx.doi.org/10.1118/1.4718685.
5. Conclusion We assessed the H*(10)/D values of the wobbling system with three wobbling sizes of the first proton beam radiotherapy facility in Taiwan. The H*(10)/D values increased as the depth of the proton beam range increased. The H*(10)/D values decreased as the distance between the measurement position and isocenter increased. Higher proton beam energy and greater SOBP width resulted in a higher contribution of secondary neutron radiation. The distribution of the H*(10)/D values of our wobbling system proton beam therapy facility was similar to that of other facilities. The discrepancies in H*(10)/D values among facilities depends on the design of each proton beam line device and the operational setup of each experimental condition. Our experimental results can be a reference to compare secondary neutron radiation in proton beam systems with wobbling spread beam mode. Acknowledgments This study was supported by grants from Chang Gung Memorial Hospital (CMRPD1C0681, CMRPD1C0682, CIRPD1C0053). The authors express their gratitude to the Dose Assessment Core Laboratory of the Institute for Radiological Research, Chang Gung University/Chang Gung Memorial Hospital at Linkou for assistance with dose assessments.
5