1. Thickness of CZT detector 110 MeV140 MeV DETECTOR A (1 mm CZT + 5 mm CZT) DETECTOR B (1 mm CZT + 10 mm CZT) DETECTOR C (1 mm CZT + 15 mm CZT) A. Generation of positron emitting radio-nuclides Figure 2 shows the simulated Bragg curve and spatial distribution of the positron emitting nuclei along the depth direction for 110 and 140 MeV proton beams. This distribution shows a nearly flat plateau, dropping to zero in close proximity to the Bragg peak. The systematic distance between the 50% level of the total activity and dose maximum in PMMA was observed as about 6 mm. This results were subsequently used as the source distribution on the PET simulation. A Range Verification Method for Proton Therapy using a Photon Counting Detector Jin Sung Kim 1, Su Jung An 2,3, Yong Hyun Chung 2,3* 1 Department of Radiation Oncology, Samsung Medical Center, Seoul, 135-710, Korea 2 Department of Radiological Science, College of Health Science, Yonsei University, Wonju, 220-710, Korea 3 Institute of Health Science, Yonsei University, Wonju, 220-710, Korea MATERIALS & METHODS DISCUSSION & CONCLUSION In the past few decades, proton radiotherapy has evolved into a widely practiced modality for cancer treatment. The main advantage of proton beam is its capability to irradiate a highly conformal dose distribution in a given target volume, due to its unique interaction and energy deposition. The deposited dose reaches a maximum called the Bragg peak beyond which it sharply falls to zero. Owing to the typically steep dose gradient at the distal edge of each individual proton beam, however, uncertainties in treatment planning and treatment delivery can have a significant influence on the delivered 3D dose distribution. This study presents the results of the vary first multi-modality imaging system for X-ray and gamma-ray coincidence imaging using a single CdZnTe detector to measure proton range verification. The detector system consists of two parallel planes of detectors and an X-ray generator. An X-ray image is acquired using one detector for the verification of 2-dimensional anatomical structure of the patient. The paired gamma rays from the annihilation are imaged with two modules to determine the maximum range of proton penetration. Image registration is intrinsic because the X-ray and gamma ray images are acquired in the same geometry. 110 and 140 MeV proton beams, cylindrical tissue phantom, and two rectangular CdZnTe detectors were modeled, and the imaging performance of this system was evaluated using GATE simulation The results showed the potential benefits of a X-ray/gamma-ray imaging with a single detector for range verification in proton therapy The proton range verification with x-ray imaging using single CZT detector, as described in this paper could be a simple tool for accurate in vivo range verifications in proton therapy. According to our simulations, it is achievable to use two CZT detectors system, which have different thickness for positron emitting radionuclides. Although 1 mm thickness CZT system was enough for x-ray imaging to see anatomical anatomy of phantom, additional 15 mm thickness CZT detector was required to measure proton distribution. Based on this work, we will proceed with investigation of this method for our clinical practice and the next step will be finding a suitable detector and design. This study also presented the potential feasibility about 3D volume measurement for proton dose distribution using PET/CBCT system with single detector system. A.Generation of positron emitting radio-nuclides GATE(Version 6.0) was used to simulate the generation of positron emitting radio-nuclides by proton irradiation in PMMA cylindrical phantom with a height of 20 cm and a diameter of 15 cm. Cylindrical phantom contains 5 balls (Aluminum, Glass, Spinebone, PVC, Water) that were arranged as shown in figure 1(a). Two monoenergetic pencil-like proton beam were used. (energy: 110 and 140 MeV; FWHM: ~10 mm; intensity : 1 x 10 8 p/s) The passage of protons through the phantom was simulated through physical interactions and the distribution of generated positron emitting isotopes was calculated. GATE V6 allows the user to record so-called phase-space files containing the essential properties of the simulated particles at a given geometric level. The properties of C 11, O 15, C 10 particles in the phantom are stored in a phase-space file. This output file can then be used as an input file in subsequent PET simulations. B. Simulation setup for X-ray imaging The detector system consists of an X-ray generator and one CZT detector module for 2-D X-ray imaging. (figure1 (b)) Date acquisition was performed using an 1 mm-thick CZT detector and a PMMA phantom same as in section A. X-ray system has a source to detector distance of 68.5 cm, maximum fan angle of the x-ray beam of 17°, and magnification factor of 1.17. The spectral X-ray system using a photon counting CZT detector consists of 0.5 x 0.5 mm 2 pixel and 400 x 400 array of pixels. X-ray spectrum of 120 kVp tube voltage was generated in SRS-78 program. The x-ray beam quality was 7 mm Al equivalent half value layer(HVL). Figure 1. Schematic diagram of a simulation setup RESULTSRESULTS a. PMMA phantom Figure 2.The total isotope distribution from GATE6 simulation c. Coincidence imaging Thickness of CZT detector DETECTOR A1 mm CZT + 5 mm CZT DETECTOR B1 mm CZT + 10 mm CZT DETECTOR C1 mm CZT + 15 mm CZT Table 1. Thickness of CZT detector used in simulation B. X-ray imaging with CZT photon counting detector Figure 3 shows the spectral x-ray image of the PMMA phantom with CZT photon counting detector. 1 mm-thick CZT detector is usable for x-ray imaging to see anatomical structure of the object. INTRODUCTIONINTRODUCTION C. In-beam PET imaging with parallel-planes of CZT detector The simulated 2D images of positron emitting radionuclides in the phantom were illustrated in Figure 4 for 110 and 140 MeV proton beams. The results demonstrate the better image quality from a thicker detector. Simulation has proven that the proton therapy dose distribution could be imaged by CZT pair detectors. Figure 3. Simulated X-ray image using CZT photon counting detector C. Simulation setup for in-beam PET imaging The detector system consists of two opposing parallel-planes of CZT detectors to measure proton range verification.(figure1(c)) To verify proper detection of annihilation gamma, several thickness of CZT crystals were used. (Table 1) The distance between the detector heads was fixed at 20 cm. Using phase space file as input, e+ sources were emitted from PMMA phantom using particle filter in GATE 6 actor toolkit. The 2D image of positron emitting radionuclide in the phantom was acquired by plotting the coincidence counts per opposing detector pixel pair. b. X-ray imaging a. 110MeV b. 140MeV Figure 4. in-beam PET image using CZT detector PET image PET + radiography imagePET image PET + radiography image