Development of evaluation methods of organ dose in image-guided radiation therapy

Abstract

Image-guided radiation therapy (IGRT), such as a linear accelerator, cyclotron, and synchrotron for x-ray, proton and carbon, are combined with imaging technology that allow the latest patient’s imaging before/after or during the treatment to track changes through all stages of treatment in order to more precisely target radiation at the tumor and avoid healthy surrounding tissue. It has greatly improved the precision of radiation delivery by providing exact position of the tumor and control of the organ motions. Kilovoltage (kV)-CBCT is a widely used IGRT technique owing to its rich image information and faster and more robust image acquisition process. High-precision radiotherapy is critical when the surrounding tissues are highly radiosensitive. It may, however, increase health risks associated with imaging radiation. Unlike general diagnostic imaging and image-guided surgery, IGRT adds an imaging dose to the therapeutic radiation dose, which is already high. This could create a complex distribution of the dose and increase the risk of the development of secondary malignancies, which has given rise to the need for ways of evaluating and minimizing the imaging dose. Accurate evaluation of the dose delivered to specific organs is, therefore, very important to assessing the risk of complications due to kV-CBCT scans. Furthermore, the efforts to develop techniques for reducing the imaging dose are also important. On the other hand, proton therapy is also important in radiation therapy which is a sort of charged particle therapy gaining a wide interest in clinics, enables precise dose delivery onto a tumor by use of the Bragg peak. However, uncertainties in determination of Bragg peak range can lead to suboptimal treatment efficiency. Therefore, image-guided verification is desirable and can contribute to improving the therapeutic outcomes and securing the patient safety. In-beam PET imaging system is a PET system specifically tailored to verifying the range of charged particles and to monitoring the dose distribution. In this study, we simulated the kV-CBCT system used in the image-guided procedures, and calculated organ doses using Monte Carlo (MC) simulation for adult and child. Also, we have developed an attenuation？based tube current modulation (TCM) method and framework for kV-CBCT imaging using the prior information of planning CT image of a patient to reduce the organ dose. Then, it has successfully demonstrated the feasibility and dosimetric merit of the TCM method for kV-CBCT by simulation and experimental study. Finally, we have developed a real time in-beam PET image reconstruction algorithm (SPIRA), thus, error of the beam can be minimized by monitoring organ dose during particle beam treatment. Then, we have shown the potential of concurrent imaging by the SPIRA for in-situ monitoring using in-beam TOF-PET system. We believe that these results would be useful to reduce the risk of radiation and helpful to maximize the use of IGRT.