Original paperPreliminary analysis for integration of spot-scanning proton beam therapy and real-time imaging and gating
Introduction
Proton beam therapy (PBT) has the potential to provide better dose distributions to the target being treated and reduce the dose to the target at risk (OAR) than X-ray therapy in many situations [1]. Spot-scanning proton beam therapy (SSPT) is expected to be more suitable to create more complex dose distributions to the target volume and also expected to be safer in terms of reducing the neutron contamination [2]. The SSPT has been considered disadvantageous to the conventional passive scattering PBT because of larger uncertainty in the dose distribution for the moving target due to interplay effects. The existence of the internal motion mainly from respiration is an issue to be discussed when we consider using the SSPT especially to treat lung and liver tumors.
We developed the real-time tumor-tracking radiation therapy (RTRT) system in 1999 and have been using it in the X-ray clinic for over 10 years [3]. The system can gate therapeutic x-rays according to the internal location of a fiducial in or near the tumor. The 3D trajectory of the fiducial marker is observed and calculated 30 times per second. We have started to develop a spot-scanning dedicated PBT system integrated with the RTRT system. The aim of this integration is to accurately treat tumors with SSPT that have respiratory movement. As the internal motion of lung and liver tumors is often different from the external surface motion of the chest or abdominal wall [4], it is insufficient to use surface motion for accurate estimation of target motion. In this study, a real time 3D fiducial location database obtained from the clinical experience using the RTRT system is used for simulation study of SSPT.
Section snippets
Materials and methods
Two orthogonal sets of X-ray fluoroscopes are installed in the gantry of the system to observe internal fiducials or bony structures before and during the beam delivery (Fig. 1). As an SSPT system does not require compensators and collimators, the X-ray images can be acquired simultaneously with proton beam irradiation at any beam angle, and their fields of view will not be narrowed by these field-shaping devices. The system supports the gating function of the therapeutic beam to the real-time
Results
Three factors are considered to be the main problems in developing a SSPT dedicated gating system from our imaging perspective and comparison with our experience of photon treatment through the clinical usage of the RTRT system. They are 1. synchrotron operation cycle, 2. gating window, and 3. precise targeting.
- 1.
Synchrotron operation cycle is a new factor that was not a problem when we developed the RTRT system for x-ray therapy. For the integration of SSPT and RTRT, we realized that the
Discussion
SSPT is considered a promising system that can deliver to the target volume high dose conformity with reducing secondary neutron dose and realizing a reduction of the total facility size [6]. Existence of interplay effects, however, during beam delivery is considered to be one of the important problems to perform SSPT clinically. In the present study, we investigated the relationship between the organ motion and SSPT. The details of the analysis of the internal motion of fiducial markers can be
Conclusion
We are currently integrating the real-time tumor-monitoring system with the spot-scanning dedicated PBT system. Data from the RTRT system and its clinical use enable us to improve the operation and function of the PBT system and to decide the proper parameters. A proton beam therapy system dedicated to spot-scanning can increase accuracy for moving tumors with real-time imaging and gating. The system will be introduced to clinical service in early 2014.
Conflicts of interest
Dr. Shirato has a patent “MOVING BODY PURSUIT IRRADIATING DEVICE AND POSITIONING METHOD USING THIS DEVICE” licensed to Hokkaido University, Japan. The other authors declare that they have no conflicts of interest.
Acknowledgements
This research has been supported in part by a grant from the Ministry of Education, Science, Sports, and Culture, Japan (No. 24591829) and the Japan Society for the Promotion of Science (JSPS) through the ‘‘Funding Program for World-Leading Innovative R&D on Science and Technology’’ (FIRST Program).
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