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Research paper : International standardization of four dimensional radiotherapy system (Y. HIRATA et al.)−234−Synthesiology - English edition Vol.7 No.4 (2015) “Dynamic phantom” is a model for simulating the tumor motions inside the patient’s body. This model is used to examine the performances of the 4DRT systems in terms of “latency,” “prediction model,” and “baseline shift.”Finally, “4DCT” is a method for reconstructing the moving CT images by obtaining the x-ray CT images during, for example, five respirations and then using the markers placed on the surface of patient’s body. Since the information for tumor motion can be obtained using the 4DCT and the treatment plan for 4DRT can be created based on this information, the accuracy of 4DCT often becomes a problem in the 4DRT systems.The above keywords are items that must be at least considered in the clinical practices of 4DRT, and they must be defined in the IEC standards.4.4 Introduction of the new concepts to the existing standardsThe above keywords play important roles in assuring the safety of 4DRT. Therefore, we set the required items that should be standardized based on the keywords, and proposed the safety standards for 4DRT in the IEC TC62/SC62C WG1 held in Germany on September 2011.However, at that time, the basic concept of standardization of the 4DRT systems (refer to subchapter 4.1) was not clear, and we could not obtain the consensus to develop a completely new standard in the IEC TC62/SC62C WG1 to assure safety of 4DRT. Therefore, we reached the consensus to add the requirements related to 4DRT in the existing standard of the X-IGRT equipment (IEC 60601-2-68) that was being discussed in the IEC TC62/SC62C WG1, and we obtained results on the latency and the baseline shift as follows.4.4.1 LatencyRegarding latency, the requirements of latency of the X-IGRT equipment (the time interval between the acquisition of images that includes the tumor position information and the outputting signal to external beam equipment (T1)) were added to the IEC 60601-2-68. This was because the importance of latency was recognized for the safety of the X-IGRT equipment.4.4.2 Baseline ShiftRegarding the baseline shift that is expected to occur frequently during 4DRT, the requirements of the baseline shift were added to the standard of the X-IGRT equipment (IEC 60601-2-68), since we obtained consensus on the necessity of requirements by which the irradiation of therapeutic X-rays can be interrupted and settings of the equipment can be modified to continuously administer safe and smooth treatment.4.4.3 Correspondence to the existing standardsIn addition, we continued discussions in the IEC TC62/SC62C WG1 and obtained the following results corresponding to the existing standards (IEC 60601-2-68).We found that the offline X-IGRT, online X-IGRT, and real-time X-IGRT 4DRT, which were defined in the IEC 60601-2-68, were all related to 4DRT. Therefore, we organized the above relationships and were able to add examples of the 4D versions of the offline X-IGRT, online X-IGRT, and real-time X-IGRT to the new annex of IEC 60601-2-68.4.4.4 Updating the standard for the external x-ray beam equipmentThe concept of latency is important for the external x-ray beam equipment that was the other component for realizing the 4DRT, and the above results for latency became a trigger for updating the safety standard of the external x-ray beam equipment (IEC 60601-2-1). In the update (62C/574/RR) of the standard for external x-ray beam equipment (IEC 60601-2-1), the item for motion management in 4DRT will almost certainly be added to the IEC 60601-2-1. In the item for motion management, gating and tracking irradiations that are representative methods of 4DRT as well as latency were listed. Particularly, the Japanese national commission was requested from the start to create the proposal for latency requirements at the IEC TC62/SC62C WG1. Since the standard for external x-ray beam equipment (IEC 60601-2-1) was the most basic and most important standard for assuring the safety of equipment for the radiotherapy equipment manufacturers, it was highly significant that the Japanese IEC experts could participate from the start in updating the standard.4.4.5 Items other than the latency and baseline shiftAmong the five items discussed in subchapter 4.3, we decided to fully develop three items (a prediction model, a dynamic phantom, and 4DCT) other than latency and baseline shift in the new work item proposal (NP) that will be proposed by Japan at the IEC, as they seemed to have high novelty.5 Future directionTo further contribute internationally in the standardization for the safety of treatment equipment in the future, it is necessary to engage in the international standardization activities as follows.Internationally, there are examples where a large-scale company that specializes in radiotherapy systems independently takes a proactive stance for the international standardization of radiotherapy equipment. In addition to the activities that we have been conducting, it is essential to engage in vigorous international standardization activities through industry-academia-government collaboration. The companies developing

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