Vol.7 No.4 2015

Research paper : International standardization of four dimensional radiotherapy system (Y. HIRATA et al.)−233−Synthesiology - English edition Vol.7 No.4 (2015) conventional radiotherapy (5 mm or less).Specifically, as shown in Fig. 5, when the tumor moving due to respiration is treated with 4DRT, the irradiation area can be narrowed to 5 mm or less compared to the conventional radiotherapy.4.3 Items to be considered for 4DRT and its safety requirementsIn order to achieve the accuracy discussed in subchapter 4.2, we narrowed down the important 4DRT keywords that were not included in the existing IEC standards to “latency,” “prediction model,” “baseline shift,” “dynamic phantom,” and “4DCT.”“Latency” is the time interval between the recognition of a tumor and the actual irradiation by the therapeutic rays on the patient’s body. If latency increases, the irradiated position of the therapeutic rays may deviate from the tumor position as shown in Table 1, and therefore, the prediction of tumor position is performed using a reliable prediction model.For latency, we must consider the latency of the overall system that includes the X-IGRT equipment and the external beam equipment to ensure the accuracy of irradiation of therapeutic rays. For example, the position deviation D in 4DRT for the tumor moving with mean velocity V can be considered as follows. Latency T of the overall system is the sum of the following times: the interval between the recognition of tumor position by a tumor-position-measuring equipment such as the X-IGRT equipment and the transmission of irradiation instructing signal to the therapeutic-ray-irradiating equipment (T1); the interval between the reception of irradiation instructing signal and the irradiation of therapeutic rays, change in beam direction, or other instructed action (T2); and the time required for communication (T3). The deviation of recognition of position by the IGRT equipment D0 is added to the final position deviation D that is represented by following formula:D = V (T1 + T2 + T3)+ D0(1)That is, D represents the irradiation accuracy of the 4DRT systems, and it is dependent on a number of factors such as the latency of various equipment, the time of communication, and the deviation of recognition of position by the X-IGRT equipment. To assure the safety of overall 4DRT systems, we need not only the international standards for the various types of equipment (T1, T2, T3, and D0), but also the international standards for the system at the level superior to the equipment (T and D).“Baseline shift” is the change of the patient’s respiration state that cannot be predicted by the prediction model. If the baseline shift is ignored, the tumor moving in a way that cannot be foreseen by the prediction model may be irradiated according to the model, and in such a case, the normal cells may be irradiated with the therapeutic rays. In cases in which the baseline shift occurs, a mechanism to definitely terminate the irradiation of therapeutic rays is necessary in the 4DRT systems.Fig. 5 Comparison between the irradiation field of 4DRT and that of conventional radiotherapyFig. 6 Photograph of the dynamic phantomThe dynamic phantom is used to evaluate the geometrical deviations of the X-IGRT equipment in 4DRT.Table 1. Maximum position deviation due to latencyCalculated values of deviations between the tumor position and the irradiated position in cases where the tumor moves along the 2 cm distance in 3 sec cycle sinusoidal waveIt can be seen that the latency of about 250 msec causes a position deviation of 5 mm between the tumor position and the actual irradiated position.Irradiation field can be narrowed to 5 mm or smallerIrradiation field of 4DRTIrradiation field of ordinary radiotherapyMovement due to respiration etc.TumorTumor5.22504.22003.11502.11001.050Maximum position deviation due to latency [mm]Latency [msec]


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