Technical noteOptimization and evaluation of multiple gating beam delivery in a synchrotron-based proton beam scanning system using a real-time imaging technique
Introduction
Increasing numbers of proton therapy centers have adopted the spot-scanning technique. Spot scanning can potentially increase conformity to the target volume and modulates the dose more flexibly than the passive scattering approach. However, in beam scanning, the dose distribution may deteriorate under organ motions, such as respiratory or cardiac motions, during beam delivery [1], [2], [3], [4]. Real-time four-dimensional radiotherapy, which includes beam gating [5], [6], [7] and beam tracking [8], [9], has been realized in photon therapy to mitigate the dosimetric impacts of target motion. Gating [10], [11], [12] and tracking techniques [13], [14] using external surrogates have also been reported in particle therapy. However, internal organ motions and the external surrogate signals of abdominal motion are not necessarily correlated during treatment [15]. Gated irradiation using internal fiducial markers based on real-time imaging is one of the best solutions in particle therapy.
In respiratory gated radiotherapy, reasonable gated irradiation efficiency is expected for clinical application. In carbon ion therapy system at National Institute of Radiological Science (NIRS), fast scanning technique including multi-energy irradiation with extended flat tops [16] and phase-controlled rescanning method [17] were realized to treat the mobile targets. By applying these techniques, reasonable treatment time can be achievable in NIRS carbon ion therapy. On the other hand, in the synchrotron for typical proton therapy system, beam current is relatively small compared with the carbon ion therapy system. In order to improve the gated irradiation efficiency with synchrotron based proton therapy, variable flat top phase synchrotron operation, described in the later section, has been applied. However, the gating signals change irregularly and sporadically under complex tumor motions [18] and, in some cases, poor recognition of the fiducial markers [19], [20]. The resulting fluctuations in the gating signal can significantly reduce the gated irradiation efficiency in variable flat top phase synchrotron operation [21]. To improve the gated irradiation efficiency, we have developed a real-time-image gated proton beam therapy (RGPT) system [22], [23], [24] which has a new function, called the multiple gated-irradiation function [20], [25]. The combination of variable flat top phase operation and multiple gated-irradiation function was realized in the first in RGPT. This technique is the new approach to improve the gated irradiation efficiency.
Since the gated irradiation efficiency depends on the wait time Tw in multiple gated-irradiation function, we need to optimize this setting. As an initial step to use this new control function, we have decided to start RGPT with fixed Tw operation to make the treatment procedure simple. The purpose of this study is to evaluate the efficiency of gated irradiation and to find the optimum Tw which can improve the mean efficiency on the average.
Section snippets
Real-time-image gated proton beam therapy (RGPT) system
Fig. 1 shows the RGPT system using a synchrotron-based accelerator. As indicated in the figure, a proton scanning nozzle, flat panel detectors, and X-ray tubes are installed in a rotating gantry. The real-time imaging system tracks the internal fiducial markers similarly to photon therapy and is based on the same concept [5]. The three-dimensional position of a fiducial marker located near the tumor is calculated from two fluoroscopic images obtained from orthogonal directions. The field of
Results
Fig. 4 shows the irradiation efficiency E(TW) at each TW determined from the 271 log datasets. The mean E(TW) was maximized at 0.52 for TW = 0.2 s and was not improved by further increases in TW. The maximum mean irradiation efficiency was approximately 21% higher than at TW = 0 s, which corresponds to ordinary synchrotron operation. By applying multiple gated irradiation with TW = 0.2 s, the irradiation efficiency was improved in 154 (57%) of the 271 cases. In contrast, the irradiation efficiency was
Discussion
The actual treatment time of the hepatic tumor patient was 250 s, which was shorter than the expected treatment time of 420 s without the multiple gated-irradiation function for the same dose (4.48 GyE). Multiple gated irradiation reduced the treatment time to 59.5% of its nominal value (250 s/420 s). However, the treatment time depends on the treatment planning parameters; that is, the number of spots per layer, number of energy layers, and other parameters. Total irradiation time will be evaluated
Conclusions
In this study, the gated-irradiation efficiency was evaluated at different Tw values in order to find the optimum parameter. The highest mean efficiency of gated irradiation was obtained at TW = 0.2 s. The mean irradiation efficiency was improved by approximately 21% over ordinary synchrotron operation. By applying multiple gated irradiation with TW = 0.2 s, the irradiation efficiency was improved in 154 (57%) of the 271 cases. We suggest that the multiple gated-irradiation function has potential to
Acknowledgments
This research was supported in part by the Sapporo Health Innovation ‘Smart-H’ project and ‘Global Institution for Collaborative Research and Education (GI-CoRE), Hokkaido University’ of the Ministry of Education, Culture, Sports, Science, and Technology of Japan. The RGPT system was developed by Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program), initiated by the Council for Science and Technology Policy (CSTP). One of the co-authors (Shirato, H.) has
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2017, International Journal of Radiation Oncology Biology PhysicsCitation Excerpt :However, online motion tracking and synchronization are technically highly challenging (94, 95). Respiratory gating using tracking of implanted fiducial markers is being investigated (96-99). However, “shadows” created by the markers in the target dose distributions are a matter of concern, which may be mitigated by further development of markers with low atomic numbers.
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2017, International Journal of Radiation Oncology Biology PhysicsCitation Excerpt :The high-speed nature of this system allows highly accurate irradiation of moving tumors. Compared with conventional methods that irradiate the entire area in which the tumor might migrate, this system can reduce the irradiated volume by 50% to 75%, which can represent a significant reduction in the irradiation of normal tissue (55, 56, 72, 73). Combining gating and rescanning, the National Institute of Radiological Sciences in Japan has implemented phase-controlled rescanning for its carbon ion system (74).