ABSTRACT
PURPOSE: In general, it is possible to generate better leaf sequencing from the ideal fluence map or dose distribution close to the optimized results of the radiation treatment planning (RTP) system, from the filed smaller segment size in Intensity modulated radiation therapy (IMRT). Conversely, an intra-treatment organ motion issue, which prevents the smallest segment size from being chosen, always exists. Furthermore, the question has been raised regarding the proper target margin for IMRT cases with a moving target, as the field itself moves while the target moves, unlike traditional static fields. In this study, the effects of intra-treatment target motion on the segment size have been examined. MATERIALS AND METHODS: Various sizes of rectangular patterns were designed for an IMRT fluence map. A leaf sequence was generated using the step and shoot beam delivery method. The intensity ratios between adjacent segments were 0.2, 0.4 and 0.8. The range of target motion was assumed to be +/-0.3~2.0 cm, in a sinusoidal shape. The dynamic leaf motion that reflected the target motion was calculated to simulate the motion. Film dosimetry was performed to analyze the motion effects. RESULTS: The intensity ratios of the adjacent segments were degraded in all cases. The dose distribution with segment sizes less than half the breathing amplitude showed a significantly degraded intensity map. With a beam irradiation time for a segment greater than two breathing cycles, the dose distribution around the target margin showed a similar tendency as the static fields. CONCLUSION: The minimum size of IMRT segments in the fluence map should be chosen taking the intra-treatment organ motion into consideration. The dose distribution with segment sizes less than half the breathing amplitude was degraded significantly in the intensity map. With a beam irradiation time for a segment greater than two breathing cycles, the target margin can be defined as the same as for a conventional static field