Preliminary results, obtained by using a proton beam, for an active ...

Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015, pp. 581∼589

Preliminary Results, Obtained by Using a Proton Beam, for an Active Scanning System to Installed on the KHIMA Chang Hyeuk Kim, Hwa-Ryun Lee, Sea Duk Jang, Hyunyong Kim, Garam Hahn, Jeong Hwan Kim, Hong Suk Jang, Dong Wook Park, Won Taek Hwang and Tae-Keun Yang∗ Korea Institute of Radiological and Medical Science, Seoul 139-706, Korea (Received 16 March 2015, in final form 3 June 2015) The active scanning technique is a pencil beam delivery method in particle therapy. The active scanning beam delivery system consists of a beam scanner, beam monitor, energy modulator, and related programs, such as the irradiation control and planning programs. A proposed prototype active scanning system was designed and installed on MC-50 at the Korea Institute of Radiological and Medical Science (KIRAMS) with a 45-MeV proton beam. The laminated magnetic yoke of the scanning magnet supported fast ramping. The beam intensity and the beam profile monitors were designed for measuring the beam’s properties. Both the range shifter and the ridge filter modulate the incoming beam energy. The LabVIEW-based beam-irradiation-control program operates the system in a sequential operation manner for use with the MC-50 cyclotron. In addition, an in-housecoded irradiation-planning program generates an optimal irradiation path. A scanning experiment was successfully completed to print the logo of the Korea Heavy Ion Medical Accelerator (KHIMA) on GaF film. Moreover, the beam’s position accuracy was measured as 0.62 mm in the x-direction and as 0.83 mm in the y-direction. PACS numbers: 87.53.Qc, 87.56.-v, 87.52.-g, 87.53.Mr Keywords: Particle therapy, Active scanning, Scanning magnet, Beam monitor, Ridge filter, Irradiation control system DOI: 10.3938/jkps.67.581

I. INTRODUCTION

by the National Cancer Center (NCC) since 2007 [7]. At the Korea Institute of Radiological and Medical Science (KIRAMS), the Korea Heavy Ion Medical Accelerator (KHIMA), which is a carbon-ion cancer-treatment system, has been under development since 2010. The active scanning beam delivery technique will be the primary treatment method used at KHIMA. However, this technique has not been previously developed or utilized in Korea. In this study, a prototype active scanning beam delivery system is proposed for verifying the system design and for acquiring specific practical knowledge during treatment operations. This prototype system was tested by using the 45-MeV proton beam of the MC50 cyclotron at the Korea Institute of Radiological and Medical Science (KIRAMS).

Particle therapy is a method for treating cancer with an externally generated energetic ion beam [1,2]. Worldwide, many patients have been treated with ion beam therapy. Statistics show that the proton is the most commonly used particle in particle therapy treatment while the carbon ion is the next most commonly used ion [3]. Carbon-ion beams have several advantages compared with X-ray and proton beams. The carbon-ion beam delivers a high linear energy transfer (LET) radiation dose to the target medium [4] and causes a high relative biological effectiveness (RBE) [5], demonstrating a higher cell-killing effect than other particles. In addition, the carbon-ion beam can be generated by using a narrow Bragg peak, and the resulting pencil beam can be focused through variable depth. Therefore, the carbonion beam can penetrate the target medium or the patient with little lateral scattering or longitudinal straggling [6]. Based on these physical and biological advantages, the carbon ion is an appropriate particle for use in an active scanning beam delivery system. In Korea, a proton therapy facility has been operated ∗ E-mail:

II. MATERIALS AND METHODS 1. Overview of the Prototype Active Scanning System

As with other scanning systems [8, 9], the prototype active scanning system design consists of beam steering

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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015 Table 1. Scanning magnet design specifications.

Fig. 1. (Color online) Prototype design for the active scanning beam line.

components, beam monitoring components, and energy modulation components. However, the prototype system was operated and tested by using an in-house coded beam-irradiation control system and the irradiationplanning program used for the MC-50 cyclotron. These systems will be discussed in detail in the following sections. The following initial parameters were used for the prototype active scanning system. The active scanning area was 25 × 25 cm2 at the iso-center. The target scanning speed was 20 m/s. The position accuracy of the scanning beam was ±0.5 mm in the transverse direction. The dose uniformity of the spread-out Bragg peak (SOBP) was ±2.5%. The initial beam line distance was assumed to be about 7 m from the Faraday cup to the iso-center. Based on this assumption, the deflection angle of the 45MeV proton beam was simulated by varying the magnetic field using the Particle and Heavy Ion Transport code System (PHITS) [10]. Furthermore, vacuum pipes with various diameters were placed along most of the beam’s path to reduce the probability of scattering from air. The determined layout and the distance between major components are shown in Fig. 1.

Item SMx SMy Magnet length [mm] 400 600 Magnet gap width [mm] 40 80 Pole width [mm] 90 160 Number of coil turn [#] 12 15 Current ramp rate [A/sec] 400 k 200 k Effective length [mm] 431.70 685.43 Field uniformity [%] 0.15 (±20 mm) 0.15 (±35 mm) Coil inductance [mH] 1.03 1.97 Coil resistance [mOhm] 6.20 13.78 246.36 222.20 Max eddy current [A/cm2 ] Max field strength [T] 0.31 0.22 Max current [A] 410 460 Max voltage [V] 413.85 393.20

Fig. 2. Modification of the scanning magnetic field by using a trapezoidal shim.

2. Beam Steering Components

The scanning magnet is a core component of the active scanning system. The magnetic field is changed by varying the coil current. If the requirement for a 20 m/sec scan speed is to be satisfied, a high ramping rate, at least 200 kA/sec, is needed. During high-speed ramping at the magnet, an Eddy current is produced, causing an energy loss. A 0.35 mm laminated magnet yoke was considered to reduce the Eddy current. The pole width and the gap width of the scanning magnet should be determined by considering the beam path and the position of each scanning magnet, X (SMx) and Y (SMy). In the beam line, SMx is located before SMy. Therefore, SMy has a larger pole and gap width than

Fig. 3. (Color online) Manufactured scanning magnets (Left: SMx, right: SMy).

SMx. Based on the beam tracking simulation, the pole and the gap widths of each scanning magnet were determined. Furthermore, an additional trapezoidal shim was attached at the magnet pole to increase the homogeneity of the magnetic field. The magnetic field uniformity is shown in Fig. 2. The finalized scanning magnet design specification is shown in Table 1, and the manufactured

Preliminary Results, Obtained by Using a Proton Beam · · · – Chang Hyeuk Kim et al.

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Table 2. Scanning magnet power supply design specifications. Item Ramping rate [kA/s] Minimum current step [A] Maximum current step [A] Output current range [A] Current precision [ppm] Steady-state time [ms] Converter switching frequency [kHz]

Value 220 1.2 80 −80 − +80 ±100 0.05 − 100 80

Fig. 4. (Color online) Magnetic field difference on SMx and SMy.

Table 3. Comparison between the design and the measured scanning magnet parameters. SMx SMy Design Measured Design Measured Effective length [mm] 431.7 424.7 685.4 665.9 Uniformity [%] 0.15 0.13 0.15 0.32 Max. field strength [T] 0.308 0.307 0.216 0.216 Coil inductance 1.03 0.94, 0.88 1.97 1.75, 1.62 (100 Hz, 1 kHz) [mH] Coil resistance [mOhm] 6.20 6.63 13.78 14.27

scanning magnets are shown in Fig. 3. The scanning magnet’s power supply (SMPS) controls the input current of the scanning magnet and the steering of the beam. Therefore, the beam stability and the beam position accuracy are related to the output current stability of the SMPS. In this study, the targeted output current stability was 100 ppm. For high-speed scanning, the current ramping speed must be 220 kA/s. Moreover, for covering of the active area of 25 × 25 cm2 , a wide current range was necessary. The SMPS specifications are shown in Table 2. The major components of the SMPS are the power generation and control components. The power generation component consists of the ramping booster and the converter [11]. When the current change is greater then 2.5 A, the ramping speed is considered to be high. The ramping booster is operated for fast ramping. For current changes that are less than 2.5 A, the converter is operated for fine current control. Furthermore, a high-speed control algorithm ensured 100-ppm precision of the output current stability during massive current changes by using a high-resolution analog-to-digital converter (ADC) and a digital signal processor (DSP). The major magnetic parameters such as the field’s uniformity and effective length and the maximum field strength were measured, and the results are shown in Table 3. Most of the parameters agreed with the designed specifications with the exception of the magnetic

Fig. 5. (Color online) Scanning magnet ramping test result.

field uniformity of magnet SMy. For both SMx and SMy, the field stabilities were measured by using currents between −80 and 80 A. The magnetic field differences are shown in Fig. 4. The maximum magnetic field differences appeared at 2.5 G for SMx and 0.6 G for SMy. The fast ramping test was completed based on a 200kA/s ramping speed between 0 and 80 A. The magnetic field increase occurred 12 microseconds after the current increase. A comparison between the ramping current and the measured magnetic field is shown in Fig. 5.

3. Beam Monitoring Components

The beam monitoring system is another key component of the active beam irradiation system. In this study, two different monitors were proposed. The first monitor is an ion chamber, which measures the incoming particle intensity and its two-dimensional profile. The second beam monitor is a scintillation monitor, which measures the center of the incoming beam before the scanning magnets. To monitor the appropriate beam parameters during irradiation, we used a set of ion chambers monitored the

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Fig. 6. (Color online) Schematic diagram for the ion chambers.

Fig. 8. Output currents at the focusing cup and intensity monitor.

Fig. 7. (Color online) Manufactured ion chambers.

intensity and profile of the beam. With the exception of the cathode, the designed structures for the intensity and the profile monitors were very similar. Both chambers used flame-retardant composition 4 (FR-4) as the substrate, and Cu-Au was printed as the cathode. Air filled the 5-mm electrode gap, and polyethylene terephthalate (PET) was placed as anode. A schematic diagram of the ion chamber is shown in Fig. 6, and the manufactured ion chamber is shown in Fig. 7. For the profile monitor, Cu-Au was printed in a 1.5-mm-wide strip with a 0.2mm gap between strips. Each profile monitor has 144 strips, which are placed orthogonal to the beam direction to measure the x and the y-profiles of the beam. For the intensity monitor, a whole printed sheet of CuAu was used. Also, two intensity monitors are placed in the beam line for safety. When the output signal detected at one intensity monitor is more than 5% different from the signal detected at the other monitor, an interlock signal will be generated, and the monitoring system will be considered to have failed [12]. In the fast scanning condition, the electronics were designed with a dual-switched integrator for the ion chambers to support a 100-kHz readout rate. Three measured current range options (300, 600, and 1200 nA) were provided to prevent current saturation between readouts.

Fig. 9. Beam profiles from the (a) beam profile monitor and (b) HD-810 GaF film.

Fig. 10. (Color online) Designed scintillation monitor: (a) CAD drawing and (b) control program view.

The intensity monitor was tested with a 45-MeV proton beam for currents between 1 nA and 10 nA. The Faraday cup and the intensity monitor output are shown in Fig. 8. The output from the profile monitor was compared with the response of the GaF chromatic film by using simultaneous beam irradiation. The comparison of the 8-mm beam size (represented as sigma of the full width at half maximum) is shown in Fig. 9. The purpose of the scintillation monitor was to detect the center and the symmetry of the incoming beam be-

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Fig. 11. (Color online) Range Shifter: (a) designed view and (b) manufactured RSF.

Fig. 13. (Color online) LabVIEW project view of the beam-irradiation control system. Fig. 12. (Color online) Spread-out Bragg peak generation and manufactured RGF.

fore it reached the active scanning system. The major parts of this monitor were a scintailator and a chargecoupled device (CCD) camera. The scintillator used in this system is ZnS:Ag (P11), which has a 3-ms decay time and a 450-nm output wavelength. For fast analysis of the beam shape, the CCD camera supported 71 frames/s. The beam center was calculated 10 − 15 times/s, depending on the selected image size. The scintillation monitor housing was designed by using a reflecting mirror with a 22.5◦ mirror angle to protect the electrical components of the CCD. The design is shown in Fig. 10.

located at the end of the prototype beam line. In a passive beam delivery system, the RGF is used to generate a SOBP at the target [13]. In an active scanning system, a small ridge filter is used to generate a mini peak [12]. Based on the 45-MeV proton beam, the range in waterequivalent matter is about 16 mm. In this study, the ridge filter for the proton beam was designed to generate a 10-mm depth SOBP, which had a uniformity of 1.62%, by using 21 Bragg peaks with different weighting factors [14]. The simulation results of the generated SOBP and fabricated RGF are shown in Fig. 12.

5. Beam Irradiation Control System 4. Energy Modulation Components

For the MC-50 cyclotron, the proton extraction energy is fixed at 45 MeV. Therefore, a beam energy modulation system, such as a range shifter (RSF), is required to change the longitudinal location of the Bragg peak for three-dimensional scans. A plate-type RSF was introduced to cover the 25 × 25-cm2 active area. The minimum thickness of the RSF was 0.5 mm of polymethyl methacrylate (PMMA). Additional RSFs had thickness up to 6 mm in steps of 1 mm. For the moving test, a 150mm thick RSF was also included. A pneumatic system was applied to the RSF plate to reach a moving speed of 300 m/s. The manufactured RSF system is shown in Fig. 11. The ridge filter (RGF), which is similar to the RSF, is

The beam irradiation control system is the core component for operating this prototype active scanning system. When a synchrotron is used as a particle accelerator, the beam irradiation control system is connected to the main timing system to synchronize the treatment sequence and operation, which includes the chopper and the dump to stop the beam [15]. However, when a cyclotron such as the MC-50 is used, a continuous beam is provided. Therefore, the beam irradiation should not be stopped during treatment. This makes possible the use of a simple control system and a fully sequential operation between target voxels and layers. The control system was coded in a National Instruments LabVIEW environment. The project was operated at the main PC, which displayed only the user interface. The PXI programs for each triggering operation were loaded at

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Fig. 15. Timing chart for the major signals.

Fig. 14. Beam irradiation control flowchart.

each PXI. The project and sub-programs are shown in Fig. 13. The sub-programs can be classified into four functions: data converting, checking, sending, and triggering. The programs are organized as follows: DataConversion.vi converts the data to acceptable SMPS and IC formats; SMPSdataCheck.vi inspects the SM input current from the transferred data; SMdataSend.vi transfers the SM input current to the SMPS controller; and SMcontrolPXI.vi transfers the layer start signal to the SMPS controller. The workflow of the beam irradiation control system is shown in Fig. 14. Each stage is processed sequentially based on the flow chart by using the triggers at the end of each stage. The major trigger signals are IC ready, layer start, coil OK, fluence OK, and layer end. This work process was controlled by using the SMcontrolPXI module. The signal timing chart is shown in Fig. 15.

6. Irradiation Planning Program

An irradiation plan was needed to test the manufactured prototype active scanning system. Therefore, an in-house irradiation-planning program was manufactured based on a MATLAB (Mathworks) script.

Fig. 16. Flow chart for the irradiation-planning program.

Fig. 17. (Color online) Irradiation planning results: (a) target image, (b) generated intensity map, and (c) position map.

The irradiation program was used to generate a twodimensional irradiation plan for an arbitrary image based on the properties of the prototype scanning system. The irradiation-planning program consists of several subprograms for image import, image conversion, scanning path optimization, intensity calibration, and final image prediction. The workflow for the irradiation-planning program is shown in Fig. 16. The first example image used in the irradiationplanning program is shown in Fig. 17(a). The resulting intensity map and position map are shown in Figs. 17(b)

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Table 4. Comparison between the planned and the measured scanning results.

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 STD

Planned [pixel] 2130.2 3058.2 3986.2 4914.2 4914.2 3986.2 3058.2 2130.2 2130.2 3058.2 3986.2 4914.2 4914.2 3986.2 3058.2 2130.2

Position X Measured Difference [pixel] [pixel] [mm] 2097.8 32.42 1.26 3040.3 17.92 0.70 3957.6 28.63 1.11 4881.2 33.03 1.28 4900.5 13.73 0.53 3964.6 21.63 0.84 3035.1 23.12 0.90 2100.0 30.22 1.17 2134.8 −4.58 −0.18 3063.3 −5.08 −0.20 3965.8 20.43 0.79 4903.5 10.73 0.42 4926.3 −12.07 −0.47 3992.4 −6.17 −0.24 3062.4 −44.18 −0.16 2135.7 −5.48 −0.21 0.62

Planned [pixel] 3826.2 3826.2 3826.2 3826.2 2898.2 2898.2 2898.2 2898.2 1970.2 1970.2 1970.2 1970.2 1042.2 1042.2 1042.2 1042.2

Position Y Measured Difference [pixel] [pixel] [mm] 1011.8 −45.87 −1.78 1024.3 −30.87 −1.20 1039.3 −5.57 −0.22 1047.3 −0.17 −0.01 1947.0 −30.67 −1.19 1980.2 −16.07 −0.62 1992.4 −6.27 −0.24 1991.6 18.63 0.72 2879.6 −21.38 −0.83 2904.5 −22.18 −0.86 2914.3 −9.98 −0.39 2928.9 23.22 0.90 3826.4 −5.08 −0.20 3831.8 2.92 0.11 3857.1 17.92 0.70 3872.1 30.42 1.18 0.83

Fig. 18. (Color online) Scanning path optimization. Fig. 19. (Color online) Installed prototype active scanning beam line.

and (c), respectively. The irradiation plan for the example image was generated by using a scanning step size of 5 mm. The irradiation path was optimized by using the simulated annealing algorithm [16] as shown in Fig. 18. The final irradiation plan was transferred to the irradiation control system as a text file, which included the layer number, step size, x index, y index, and target intensity at each point. In this plan, a 273-point data set was considered to be one layer. The starting point was shown as a red dot, and the end point was shown as a blue dot.

III. TEST AND RESULTS The manufactured components were installed at the MC-50 cyclotron in KIRAMS (Fig. 19). The designed beam line was placed after the switching magnet in the MC-50 facility. The initial beam size at the beam line varies according to the tunning condition of the MC-50. Therefore, a collimator with four adjustable diaphragms was installed to control the incoming beam size. For the experiment, the full width at half maximum (FWHM) of the collimated beam was about 5 mm at the backside of the ion chamber. The scanning results were verified with GaF chromatic

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Fig. 20. (Color online) The obtained GaF film image obtained by using the prototype active scanning system. Fig. 22. (Color online) Scanning results for 16 grid points.

Fig. 21. (Color online) Experimental view during beam irradiation for (a) 280 s and (b) 880 s.

film (HD-V2). The optical density of the exposed GaF film was 1.6 after 7 s of 45-MeV proton-beam irradiation. Consequently, the scan of the 273 planned spots took 910 sec, about 15 min. The obtained image is shown in Fig. 20, and images captured at 280 s and 880 s are shown in Fig. 21. Sixteen grid points in a 17 × 17 cm2 area were irradiated and measured with GaF film to analyze the beam position accuracy of the prototype active scanning system. The irradiated GaF chromatic film was converted to a digital image by using Epson Expression 10000XL Photo scanner. The obtained scanning result is shown in Fig. 22. Note that the grid lines in Fig. 22 are separated by 2.5 cm. The position of each spot was determined by using the center-of-mass algorithm in each region of interest. The obtained position data were compared with the plan data in Table 4. The results show that the standard deviation of the beam position was measured as 0.62 mm for the x-direction and 0.83 mm for the y-direction.

IV. CONCLUSION The prototype active scanning system was successfully deigned, manufactured, and installed on MC50 at KIRAMS. An irradiation-control program and an irradiation-planning program were also developed in-house. Using the prototype active scanning sys-

tem, whole beam irradiation was performed in a twodimensional scanning experiment. The analyzed scanning position accuracy was shown to be 0.62 mm and 0.83 mm for the x and the y-direction, respectively. These results show the behavior of the scanning beam at the surface of target. Three dimensional active scanning experiments and delivered dose analyses should follow. An active scanning system for a carbon beam can be designed and manufactured with some modifications, such as the magnetic field strength of the scanning magnets, the total length of the beam line, and the relative biological effectiveness applied to the ridge filter design, based on the current prototype active scanning system.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (no. NRF-2014M2C3A1029534).

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