Vol.7 No.4 2015
Research paper : International standardization of four dimensional radiotherapy system (Y. HIRATA et al.)−230−Synthesiology - English edition Vol.7 No.4 (2015) changes of anatomy during the imaging, planning, and delivery of the radiotherapy.” That is, 4DRT is a highly accurate radiotherapy that improves the dose concentration on tumors and minimizes damages to normal tissues, by taking into account the time variation of the position of the tumor against the irradiation timing, in addition to the three-dimensional position of tumors that has been considered in conventional radiotherapy. Recently, the applications of 4DRT have rapidly spread to tumors that move due to patient’s respiration and others.Among the 4DRT methods, the Hokkaido University has been developing the gating radiotherapy (real-time tumor-tracking radiotherapy). In the gating radiotherapy, as it will be explained below, the irradiation position of the therapeutic rays is fixed, and the irradiation is performed in synch with the moving target. Here, accuracy of the irradiation timing of the therapeutic rays is important. Figure 2 shows the conceptual diagram of the gating radiotherapy, the therapy apparatus at the Hokkaido University, and the gold marker. In this method, the gold marker is implanted near the tumor position inside the patient’s body, and the x-ray image-guided radiation therapy (X-IGRT) equipment detects the gold marker that points to the position of the tumor that moves due to respiration. In the actual treatment, the external beam equipment irradiates the tumor with x-rays, protons, or others when the gold marker enters the region of the gating window. In cases in which this method was not used, the irradiation region had to be expanded to the entire region in which the tumor moved, and the surrounding normal tissues were irradiated with the same quantity as for the tumor position. The gating radiotherapy allowed the region of irradiation to be narrowed.In contrast to the gating radiotherapy, the dynamic tracking radiotherapy places importance on the accuracy of the irradiation position of therapeutic rays in 4DRT. As shown in Fig. 3, in the tracking radiotherapy, X-IGRT equipment tracks the position of the moving tumor using the markers, and the irradiation position of the therapeutic rays is controlled to ensure that the tumor is continuously irradiated with the therapeutic rays. The Kyoto University conducts the R&D for tracking radiotherapy using the external x-ray beam equipment that includes the ultra-compact LINAC developed by the Mitsubishi Heavy Industries, Ltd. The characteristic of the external x-ray beam equipment is that the ultra-compact LINAC is mounted inside the gantry along with the multi-leaf collimator (MLC) on the rotation mechanism called the gimbals mechanism, and this enables the irradiation direction of the therapeutic rays to be changed freely. At the Kyoto University, the tracking radiotherapy is achieved by continuously tracking the tumor position inside the patient’s body and irradiating the tumor with therapeutic rays during treatment.Fig. 1 Transition of the number of patients who were newly diagnosed with cancer (number of individuals affected with cancer) in one year, and the number of patients who were treated with radiation therapy in the yearDue to the penetration of the LINAC equipment, the number of patients who were treated with radiation therapy increased almost threefold in the last 20 years, and this number is expected to continue to increase.Application rate of radiation therapy / %5040302010020102005200019951990198519801975Year80706050403020100Number of patients / 10,000Application rate of radiation therapyNumber of patients treated with radiation therapyNumber of individuals affected with cancerExternal beam equipment (LINAC)X-ray for imagingX-IGRT equipmentSource:http://rad.med.hokudai.ac.jp/rad_research/motion_tracking/Gold markerGating radiotherapy system TumorIrradiation of therapeutic rayGating windowGold markerMovement due to respiration etc.TumorFig. 2 Conceptual diagram of the gating radiotherapy (left), and photographs of the gating radiotherapy system and gold marker (right)Left photograph is cited from the website of the Hokkaido University Hospital.