The Study Of The Influence Of Respiratory Motion On Target Dose Distribution

Objective To investigate the influence of respiratory motion on target dose distribution in radiotherapy of patients with lung tumors.Methods The Big Bore Brilliance CT with bellows system was used to gain the 4DCT sets of patients and respiratory frequency information.The moving ranges of the tumors in the directions of lateral(LR),anterior-posterior(AP) and cranial-caudal (CC) were got from the center coordinate values of gross tumor volume of ten time-phases CT sets in the treatment planning system.Then a breathing model was used to simulate tumor motion due to respiration.A 4-dimensional motion table was used to mimic the motion of lung tumor in a beam's eye view(BEV) trajector.A 2-dimensional semiconductor beam measurement system was fixed to the table to measure the two dimensional dose distribution of static and dynamic targets by the treatment beams using gantry angle 0°.Finally,the differences of the dose distribution between the static and moving phantom were compared and analyzed with the statistical software R.Results When the amplitude(half of the moving rang)in the CC direction was 1 cm,the passing ratio of relative dose difference＜4%in one beam field was minimal, and there was 58%maximal relative dose absence.The 4%passing ratio of the CC direction were 92.62%,84.71%,67.72%and 32.16%in＜0.5,0.5-＜0.75,0.75-＜1.5 and＞1.5cm amplitude(χ~2=31.21,P=0.000),but had no satatistical meaning in the AP and LR direction.The mean value of the relative dose change in the high dose area was smaller than the low dose area in the 89.9%beam fields.The 4%passing ratio of 3.6s and 8.2s period was 72%and 60%.For most beam field,after five treatmen fractions,the averag dose distribution significantly improved comparing to the single fraction dose distribution.When the dose rate was 200MU/min,the dose distribution variantions due to motion was smaller than when the high dose rate was 400MU/min.Larger the n value standing for the asymmetry of the motion curve was,longer time the tumor transiently stayed at the end expiration position,more influence the motion caused to the dose variations. Conclusions The amplitude in the CC direction is the impact factor of dose distribution of the moving target.When the other respiratory parameters are the same, motion of long period has more influence on the dose than motion of short period. Motion under the high dose rate condition has more influence to the dose distribution than the same motion under low dose rate condition;larger the asymmetry of respiratory motion waveform is,the more dose influence caused by the motion is.Special attention should be paid to the patients with tumor of more than 0.5 cm amplitude in the CC direction.When planning the intensity modulated radiotherapy., proper margin and radiation treatment technique should be chosen.