Journal of the Korean Physical Society, Vol. 63, No. 7, October 2013, pp. 1341∼1346
Current Design Status of High Energy Beam Transfer Line for KHIMA Project Heejoong Yim, Dong Hyun An, Kun Uk Kang, Ga Ram Hahn, Bong Hwan Hong, Seong Seok Hong, Won Taek Hwang, Hong Suk Jang, In Su Jung, Joonsun Kang, Chang Hyeuk Kim, Min Yong Lee, Yeun-Soo Park, Tae-Keun Yang and Geun-Beom Kim∗ Division of Heavy Ion Accelerator, Korea Institute of Radiological & Medical Sciences, Seoul 139-706, Korea (Received 14 December 2012, in final form 2 April 2013) The Korea heavy ion medical accelerator (KHIMA) project is developing and constructing the heavy ion medical accelerator for the first time in Korea since 2010. This paper describes the status of high energy beam transfer line (HEBT), which connects the superconducting cyclotron to the treatment rooms. The HEBT provides a patient with 145 - 430 MeV/u carbon (12C6+) beam to cure cancer. The HEBT is consisted of 6 beam lines (4 horizontal, 1 vertical and 1 oblique). The size of the design HEBT is 41 m in length, 11 m in width and 17 m in height. Total number of magnets is 53 for focusing magnets and 22 for bending magnets. In the description, the current design status of the HEBT is described. PACS numbers: 87.56.-v, 29.20.Hm, 29.27.Eg, 29.27.Fh Keywords: KIRAMS, KHIMA, Hadron therapy, Carbon therapy, Cyclotron, Beam line, Beam optics DOI: 10.3938/jkps.63.1341
I. INTRODUCTION Carbon therapy is increasingly recognized as a superb alternative to conventional radiation therapy method like photon therapy. Carbons have a well-defined penetration range in tissues. The particles deposit most of energy in the end of penetration range in contrast to photons and electrons. Beyond the penetration range, there is almost no dose. This makes the so-called Bragg peak in the dose distribution and provides dose localization in tissues. The dose localization of carbons is even better than protons because carbons are heavier than protons. Therefore carbon therapy can reduce side effect during the treatment. In the radiation therapy linear energy transfer (LET) of radiation is important factor for radiation treatment. High LET radiation is more efficient for cancer therapy than low LET radiation because double strand structure of DNA can be easily broken with high LET radiation [1]. Carbons are high LET radiation compared to low LET radiation like photons and protons. Therefore, carbons beam therapy is more efficient than typical radiation therapy for refractory tumor that has radiation resistance. Since first carbon therapy at Lawrence Berkeley Laboratory (LBL) [2], more than 6000 patients were treated by carbon therapy. Most of the treatments were per∗ E-mail:
[email protected]; Fax: +82-2-970-1332
formed at Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan, which is world first carbon therapy facility, and their excellent outcome of carbon therapy makes the carbon therapy is spreading [3]. After success of HIMAC, many carbon therapy facilities like HIT in Germany [4], CNAO in Italy [5], MedAustron in Austria [6], GHMC in Japan [7], and SAGA HIMAT in Japan [8] are constructed or in development. The Korea heavy ion medical accelerator (KHIMA) project is developing and constructing the heavy ion medical accelerator for the first time in Korea since 2010. The term of KHIMA project is from April 2010 to March 2016. By 2016, we plan to build the carbon treatment facility in Busan, and start the treatment. The facility will deliver 145 - 430 MeV/u carbon beam to the patients for cancer treatment. The KHIMA is consisted of ECR ion source, Low Energy Beam Transfer line (LEBT), Superconducting Cyclotron, Energy Selection System (ESS), High Energy Beam Transfer line (HEBT), Beam Delivery System (BDS). The 3D layout of KHIMA is shown in Fig. 1. A role of the HEBT is beam transfer to target point (iso-center) of each patient treatment room from the ESS minimizing beam loss. To be used for medical purpose, the HEBT should satisfy medical requirements to reduce possible side effect of carbon ion therapy. In addition scale of the HEBT dominates whole scale of KHIMA. Therefore, the HEBT considerably influence a structure of building layout and is one of costly part. In the paper we describe the current status of HEBT development
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magnet. The magnetic rigidity is given by 1 p 2 + 2E m , Bρ = = EK K q q
Fig. 1. (Color online) Layout of the High Energy Beam Transfer line (HEBT) with the superconducting cyclotron; Carbon ion beam is generated at ECR ion source then the beam is transfer to the Superconducting Cyclotron which accelerate the beam to the 430 MeV/u. Extracted beam from the cyclotron is degraded by Energy Selection System (ESS) to adjust the beam energy. Then the beam is transported to each treatment room and experiment room by the HEBT. Three treatment rooms have different beam port configuration to satisfy various treatment conditions; a room for horizontal beam, a room for horizontal & vertical beam, and a room for horizontal & 45◦ oblique beam.
with emphasis on the beam optics calculation.
II. REQUIREMENTS For cancer therapy, the HEBT is required to satisfy several conditions that are mainly medical constraints and geometrical constraints. The beam optical constraints are related to clinical requirements for penetration depth of the beam, beam transverse size variation with the energy spread and the beam passage [8]. The geometrical constraints are related to construction cost.
1. Beam optical constraints
The general specification of the HEBT is limited by beam optical requirements to minimize side effects of the therapy. In the design, 3 constraints were applied. To treat tumor in the body, energy of delivered carbon beam is 145 - 430 MeV/u that correspond to 5 - 30 cm in water equivalent length (WEL). With a range shifter which can modulate beam energy, any depth of human body can be treated. The required beam energy determines the magnetic beam rigidity, Bρ , where B is the magnetic field flux, ρ is the bending radius of beam in a
(1)
where p is the particle momentum, q is the charge state of carbon ion, EK is the kinetic energy of the beam, and m is the mass energy of the carbon ion. The Bρ is 6.62 Tm and 3.60 Tm for 430 MeV/u and 145 MeV/u. Then we estimated the required magnetic field of the HEBT magnets during energy modulation through the ESS. A beam cannot be perfectly monochromatic due to its statistical characteristic. Especially, the extracted beam of the cyclotron system has relatively large energy spread compared to synchrotron case due to the extraction system of the cyclotron and the energy degrading system of the ESS. In the momentum dispersion region, the energy spread reflects on the increase of beam transverse size. Additionally, in case of beam scanning, the beam transverse size depends on a beam position in the scanning domain. To minimize such dependence of energy spread, doubly achromatic condition should be required. The doubly achromatic condition means momentum dispersion and their derivative is zero. The relation at exit of bending magnet is given by ∆x = (D + D s)
∆p , p
(2)
where ∆x is the beam transverse beam spread for the momentum spread, D is the momentum dispersion at exit of bending magnet, D is the derivative of momentum dispersion, s is longitudinal position from the end of bending magnet, and ∆p/p is the relative momentum spread. If either D or D is not zero, there will be momentum dependence in the transverse beam size along the beam path. The D and D can be controlled by magnetic dipole field and quadrupole field. The ∆p/p is derived by ∆p EK + m ∆EK = , p EK + 2m EK
(3)
where EK is the kinetic energy, m is the mass energy of the carbon ion, and ∆EK is the energy spread. The ∆EK /EK is inversely proportion to the EK right after the ESS system due to multiple scattering with degrader block. The ∆EK /EK can be adjusted by using collimation slit system at the ESS and the smaller ∆EK /EK is better, but there is a limitation. The maximum ∆EK /EK at the end of ESS is defined as 2% for 145 MeV/u. Then ∆p/p maximum is defined as 1.2% by above relation. The beam envelope at the iso-center should form a waist. Around the envelope waist, the cross-sectional variation of the beam along the beam path in the body is minimized. Then, the treatment planning which calculate dose distribution of carbon beam in the human body will be simple. This reduces calculation time and
Current Design Status of High Energy Beam Transfer Line for KHIMA Project – Heejoong Yim et al.
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Table 1. Initial beam conditions at the end of ESS. Horizontal Vertical
β 10 m 15 m
α 0 0
D 0 0
D 0 0
Table 2. Final beam conditions at iso-center. Horizontal Vertical
β 3.6 m 3.6 m
α 0 0
D 0 0
D 0 0
enhances predictability of real dose distribution. As part of the same, the beam should be symmetric in its crosssectional shape to enhance the flexibility of treatment planning.
2. Geometrical constraints
The KHIMA have three treatment rooms and one research room. Among the treatment rooms, first room is equipped with horizontal beam port (H), second room is equipped with horizontal beam port and vertical beam port (H + V), and the third room is equipped with horizontal beam port and 45◦ oblique beam port (H + O) to satisfy various angle request for the treatment. Depending on the tumor shape and site, the patients will be treated with most suitable beam port configuration (H only, H + V, and H + O). The research room is equipped with a horizontal beam port to be used for biological research and development of delivery system. For the beam delivery system, 9 m long beam line is attached to insure enough space at the end of last magnet. On the 9 m long line, scanning magnets and beam monitoring system, and beam shaping devices will be installed and there will be enough space between the scanning magnet and iso-center for quasi-parallel beam scanning which is required to minimize dose concentration at the skin rather than deep site. Overall scale of the HEBT with 4 rooms and 6 beam ports dominates other parts of the KHIMA as shown in Fig. 1. Therefore it is necessary to be compact to reduce construction and production cost.
III. BEAM LINE OPTICS In the followings, different beam lines of the HEBT are discussed in detail. The specific considerations of each line are described and the Twiss functions and layouts are shown.
Fig. 2. Beam phase space distribution for initial (upper figure) and final (bottom figure) HEBT line positions; to satisfy symmetrical beam shape at final position, phase space distribution is set to identical. The results were obtained with beam optics code WinAgile [11].
Each beam line has one common part that is used for matching between the exit of ESS and the entrance of HEBT. The beam optics is calculated for predefined beam conditions. The initial beam conditions at the entrance of HEBT and the final beam conditions at the end of HEBT are listed in Table 1 and Table 2 respectively. Phase space particle distribution of initial and final beam is shown in Fig. 2. The figures show the transformation of beam distribution between the entrance of HEBT and the exit of HEBT. The beam at the exit of ESS has no dispersion (D = 0 and D = 0) in order to reduce dependence on momentum. Otherwise beam size increase and distortion of beam track can be occurred. At the end of HEBT (iso-center), the beam should satisfy the medical constraints that are no momentum dispersion, symmetric beam shape, and envelope waist. With these conditions, we tried to reduce scale and number of magnet component for the beam optics design. In order to satisfy all those requirements for the HEBT, beam optics should be calculated by using computer code like MAD-X [10]. Before the calculation, determine what kind of magnet components we were. Then we defined length and maximum field strength of the components as listed in Table 3. The optics calculation result of the HEBT is described as follows.
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Fig. 3. (Color online) Beam optics of horizontal beam line; horizontal betatron function (black), vertical betatron function (red), horizontal dispersion function (green). All the beam optics calculation performed with beam optics code MAD-X [10].
Fig. 4. (Color online) Beam optics of 45◦ oblique beam line; horizontal betatron function (black), vertical betatron function (red), horizontal dispersion function (green), vertical dispersion function (blue).
1. Twiss function
Twiss functions explain beam behavior in the beam lines and are calculated by using well known optics calculation code MAD-X [10]. Figure 3 shows the Twiss function of 3rd room as a typical beam optics result for horizontal beam line. First two quadrupole magnets are used for matching between the exit of ESS and the entrance of HEBT. First three bending magnets work as switchers of beam direction. Those 1st , 2nd and 3rd bending magnets switch a beam into 1st room, vertical-oblique beam line, 2nd room and 3rd room, respectively. For reference, 4th room is connected with a straight beam line from the exit of ESS. These bending magnets are switched off for the 3rd room. A function of 3rd to 6th quadrupole magnets are for extension of the beam line to the room. Last two bending magnets is used for deflecting a beam into the 3rd room by 45◦ . The three quadrupole magnets in-between the last two bend-
Fig. 5. (Color online) Beam optics of vertical beam line; horizontal betatron function (black), vertical betatron function (red), horizontal dispersion function (green), vertical dispersion function (blue).
ing magnets are to make achromatic beam line. Otherwise momentum dispersion is generated from the deflecting magnets. Last three quadrupole magnets and last 7th and 8th quadrupole magnets are used for beam size control at the iso-center. Our beam delivery system use passive scattering method for beam scanning, but we also consider active beam scanning method. Therefore, we considered redundant quadrupole magnets that can be used for beam size adjustment. 4th room has no deflection part and the length of the line for 4th room is 44 m. Vertical and oblique beam line have 90◦ bending upward and downward parts by configuring four 22.5◦ bending magnet. To minimize height of vertical line, one quadrupole magnet in 90◦ deflection part was used to make achromatic condition as shown in Figs. 4 and 5. First two 90◦ deflecting part is used for changing height of beam center, and first one 45◦ deflecting part of vertical beam line is used for directing the beam to treatment room. Last 90◦ deflecting part will be used for directing the beam of up-stair beam line to the 3rd room that is placed at down-stair. Overall beam optics of vertical line is shown in Fig. 5. As a result of calculations, the height of the line from the iso-center position is 17 m. For oblique beam line, 45◦ downward deflecting part is used with one quadrupole magnet in-between two 22.5◦ bending magnets to make achromatic condition and to reduce length of the HEBT as shown in Fig. 4. As shown in Figs. 3-5, momentum dispersion (D) is confined only within all the deflecting parts and overall scale of betatron functions is controlled less than 60 m in both of horizontal and vertical direction.
2. Beam envelope and ray tracing
In order to determine beam line aperture, we need to estimate beam envelope. The beam envelope is esti-
Current Design Status of High Energy Beam Transfer Line for KHIMA Project – Heejoong Yim et al.
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Table 3. Magnet parameters for the HEBT. Magnet type Bending 1 Bending 2 Focusing
Quantity 17 5 53
Bending θ / Max. k 22.5◦ 22.5◦ 2.64 m−2
Length 1.7 m 1.7 m 0.4 m
Fig. 6. (Color online) Beam envelopes for vertical beam line with momentum spread; solid line is for betatron envelope and dashed line is for momentum envelope; momentum spread dependence of the envelope is evaluated for ∆p/p = 0.02 (blue), 0.01 (red), 0.005 (green), and 0.001 (purple). Beam envelope contains 99% of transverse beam distribution. The envelope shows that momentum spread (∆p/p) of up to 0.02 is acceptable.
mated with the following relation xenv = 2.236 εβ + (D
∆p 2 ) , p
(4)
where xenv is half size of beam envelope, ε is the beam emittance which is determined at the exit of the ESS, β is the betatron function which is determined by optics calculation, D is the momentum dispersion, and ∆p/p is the relative momentum spread. The estimated beam envelope is shown in Fig. 6. The envelope indicated maximum beam size, which is in quadrupole magnets, is about 8 cm. The ray tracing study is used for beam collimation study as shown in Fig. 7.
Max. field / gradient 1.54 T 1.54 T 17.5 T/m
Fig. 7. (Color online) Tracking result of horizontal (upper) and vertical (bottom) beam trajectory for vertical beam line; beam starting position is 0.01, 0.0, and –0.01 m for both horizontal and vertical direction; at the each position beam with slope of 1.0, 0.0, and –1.0 mrad is tracked.
IV. CONCLUSION The beam optics and structure of HEBT line for KHIMA is calculated by using computer code while satisfying the given conditions that are medical and geometrical constrains. As a result, scale of the designed HEBT is 41, 11, and 17 m for total length, width, and height each respectively. Despite the adverse conditions such as very poor beam emittance at the exit of ESS, the beam envelopes could be controlled within 8 cm. With the envelope, we can determine magnet gap size that affects operational cost of the HEBT line, but additional information such as maximum allowed trajectory excursion. This will be estimated by considering error factor and multi-pole effect of magnet components.
ACKNOWLEDGMENTS This work was supported by the Ministry of Educational Sciences and Technology (MEST), Korean Gov-
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ernment.
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[5] E. Borloni et al., Proceedings of NIRS-CNAO Joint Symposium on Carbon Ion Radiotherapy (2006), p. 1. [6] E. Griesmayer et al., Nucl. Inst. Meth. Phys. Res. B 258, 134 (2007). [7] K. Noda et al., J. Radiat. Res. 48, A34 (2007). [8] K. Noda et al., J. Korean Phys. Soc. 59, 528 (2011). [9] W. Chu et al., LBL Report No. 33749, 1993. [10] F. Schmidt et al., Methodical Accelerator Design (MAD) (1997). [11] P. Bryant, Proceedings of EPAC (2000).
