Detector R&D center
The detector R&D center has many years of experiences in experimental high energy physics and works closely with industry, academic and research institutes and hospitals on biomedical related detectors R&D. Currently our focus is on proton therapy related detector R&D. Projects executed include “Proton therapy related detector R&D” (MoEA), “Patient dose simulation and measurement” (CMRP), currently “Applying Particle Physics Technology to develop On-line Monitoring and Control System for Hadron Therapy” (AS Thematic project) and “Relative dose calibration with photon and electron beams on a patent new large area IC detector” (FJUH project). We hosted projects at RCNP, Osaka University on basic properties and performance test of detectors with proton beams, with assistant from KEK detector center. National Standard of Ionization Radiation and isotope group of INER are our long-term partner, they help us on detector calibration and basic properties test. There are patents awarded and technology transfer to companies, including National Innovation Award (2015) on large area XY strip detector and its application. The later product has been sold and are in hospital daily practice at Chang-Gung Memorial Hospitals (Linkou & Kaohsiung), Veteran General Hospital Taipei and National Cancer Center (Kashiwa) Japan. Its outstanding performance is highly praised.
In viewing advantages of modern particle therapy is bringing to radiotherapy, MoHW will adjust previous restriction on limitation of particle therapy facilities and is encouraging hospitals to deploy it. This action matches progress of AS Thematic project. The palm-size PET image system developed by AS HEP group consists of 8 modules (Figure 1). Each module is with 5cm x 10cm size, 512 channels of readout, with ~1ns time resolution and 5% energy resolution. Its readout speed is > 1 MHz and is expected to reach 10 MHz in near future. Such system is suitable for in-vivo range verification and dose monitoring, and will improve accuracy of treatment, especially for Pediatric or H&N tumors. The prototype is expected to be tested at CGMH-Linkou by end of 2021, and will be available for clinical test soon. The same detector can also detect prompt gamma, which is generated upon proton interacts with nucleus of patient. Different species of nucleus generates prompt gamma with different energy. Detection of such gamma provides information of range of proton beam, also species of atom proton is interacting, for example, Oxygen or Carbon. This method is expected to be developed as a functional medical image (PGI, Prompt Gamma Image) in the future. For this purpose, more accurate nuclear reaction information is necessary. Information are collected from published paper and data banks and more reliably from our own data. Precise measurements of cross sections are carried out at INER with collaboration of Isotope group with their 30 MeV proton beam. Through large quantity of simulation, detector design and related algorithm can be developed and optimized. One example is demonstrated in Table 1, detection efficiency of a 511KeV photon is 23% with present configuration and analysis algorithm, it can be improved as 48%, factor of two improvement, if 40 mm long crystals are used and simple cluster algorithm is deployed. This implies a factor of 4 improvement in PET image where crystals from opposite sides are matched for coincidence. Similar methodology can be applied to prompt gamma detection and prompt gamma image resoncstruction.
Neutron background is annoyed and unavoidable during photon detection. This problem can’t be handled by simulation due to lack of information on high energy neutron. Predictions on high energy neutron yield could be different by a factor of 10 between different simulations, yet no reliable justification can be made. The reason lies on no detector can perform direct measurement of high energy neutrons. Recently we realize that RPC (Resistive Plate Chamber) used in high energy physics experiments can be deployed for such purpose. As shown in Figure 2, direct measurement of energy and direction of high energy neutron can be achieved easily with PE converter and layers of RPC. Measurements of high energy neutrons produced at proton therapy will be conducted at hospitals in the future. At the same time, production cross sections of neutron from proton interacts with different materials will be measured as well.
The joint project with Fu-Jen University Hospital is development of QA system on photon radiotherapy. Specific workflow and analysis software will be developed based on large area 2D XY strip detector with comprehensive QA items to promote treatment quality and to reduce time required for QA. This shall bring significant impact to radiotherapy.
Figure 1 Palm-size PET image system & module
Table 1 Detection efficiencies for different photon energy, crystal size and algorithm
Figure 2 A method of measuring energy and direction of high energy neutron