| Purpose:Respiratory gating technique and real-time tumor tracking radiotherapy technique are effective methods to manage the target dose deviation caused by respiratory movement.The optical surface system can be used for real-time respiratory tracking and provide a gating signal for gating treatment.The tracking accuracy of respiratory movement will affect the accuracy of gating treatment.There is a time delay between the respiratory signal entering or leaving the gated window and the beam on or beam off of the accelerator.Time delay is an important factor to limit the accuracy of respiratory gating technology.The application of optical surface system in real-time tumor tracking radiotherapy is affected by the correlation between the changes of body surface contour and tumor motion.The main purpose of this study is to explore the accuracy of real-time motion tracking of the optical surface system,and to test the time delay when the optical surface system guides the accelerator to perform respiratory gating therapy,and propose a new method based on film dose profiles fitting to measure the time delay of the system,so as to provide guidance for clinical treatment.Furthermore,the correlation between the movement of body surface and tumor movement in lung cancer patients are analyzed and to explore the feasibility of chest contour changes in predicting lung cancer tumor motion.Methods:(1)The test of tracking accuracy:According to the mathematical fitting formula of respiratory movement,10 respiratory curves with amplitude unchanged and periods range from 1 s to 10 s with 1 s interval were generated,and 7 respiratory curves with period unchanged and amplitude range from 3mm to 15mm with 2mm interval were generated.The QUASAR programmable respiratory motion phantom is used to simulate respiratory movement and the movement of the phantom is monitored by the optical surface system.The respiratory curves recorded by the system are fitted according to the formula,the accuracy of real-time motion tracking of the system is analyzed.The influence of periodic and amplitude changes of respiratory curves on the tracking accuracy is explored.Meanwhile,the tracking accuracy of clinical respiratory curves by optical surface system was further explored.The respiratory curves of 30patients were input into the software,and the actual movement curves of the phantom was regarded as the reference curves.The correlation between the respiratory curves recorded by the system and the reference curve were analyzed,and the monitoring error of the system was also analyzed.Finally,the influence of the spectral characteristics of the respiratory curves on the tracking accuracy of the optical surface system was analyzed.The reference curve and the respiratory curves recorded by the system were perform Fourier transformed,respectively.And the curves in the time domain were converted to the frequency domain,the tracking errors of each frequency component were analyzed.(2)The test of time delay:The respiratory movement of the patients were simulated by the phantom,and the movement of the phantom was monitored by Catalyst system as a gating signal to guide Elekta Versa-HD and Varian Edge accelerators to perform respiratory gating therapy.In respiratory gating treatment,the phantom was used to drive the films to perform periodic motion,and by analyzing the film,the corresponding dose profiles was extracted.Then the Matlab software was used to simulate the pulsed beam process of the linac,and the distribution of the dose profiles for each pulsed beam was calculated,the cumulative dose of each pixel was calculated to get the corresponding dose profiles.By comparing the dose curve with different respiratory gated delay time with the actual dose curve of film,the least square method is used to find the optimal solution to obtain the corresponding gated delay time.In order to explore the influence of beam hold time on gating delay time,the gating time delay of the system was tested respectively when the beam hold time was range from409 ms to 4264 ms.In the experiment,the gating signal was controlled to enter the gated window at a specific time by selecting the gated window.(3)The test of correlation:In this study,5 patients with isolated lesions of lung cancer were selected,all of whom underwent 4-dimensional computed tomography scan(4D-CT).During the CT scan,the patient was fixed in position with a vacuum pad.Meanwhile,the respiratory curve which was generate from the chest contour changes was obtained by using the optical surface system(Sentinel),and the average respiratory curve of the patient was obtained by using Matlab software.The Python software was used to extract the position of the tumor in ten respiratory phases of the 4D-CT images,and the average motion curve of the tumor was obtained.The correlation between the mean motion curve of the tumor and the mean respiratory curve of the patient was analyzed.Furthermore,the consistency between the movement in each respiratory cycle and the average respiratory curve was analyzed.The respiratory curve in each respiratory cycle was divided into ten phases,and the standard deviation of the amplitude change in each respiratory phase was calculated to obtain the stability of the patient's respiratory movement amplitude.Meanwhile,the stability of the patient's respiratory movement cycle was analyzed.Results:When the period and amplitude of the respiratory curve were changed,the range of curve fitting parameters was 0.9976?0.9996(A?R~2),0.1102?0.2412(RMSE),0.9968?0.9994(A?R~2)and 0.0590?0.1750(RMSE),respectively.The results show that the optical surface system has a high accuracy in real-time motion tracking,and the changes of respiratory curve period and amplitude have no obvious influence on the system tracking accuracy.In clinical,the respiratory curves recorded by the system have a high consistency with the reference curves,and the correlation coefficient is higher than 0.99,which is significantly correlated.The average value of monitoring error is 0.24±0.04 mm,which decreases while the respiratory signal frequency increases,mainly between 0 and 0.4 Hz.The fitted film dose profiles are in good agreement with the actual film dose profiles.In gating radiotherapy,the time delay of the system when the gating signal enters the gating window is much greater than the time delay of the system when the gating signal leaves the gating window.For the Elekta Versa-HD accelerator,the time delay for beam on was 1663±72 ms and the time delay for beam off was 21±29 ms.For the Varian Edge accelerator,the time delay for beam on was 303±45ms while the time delay for beam off was 34±25ms.The time delay of Edge accelerator in respiratory gating therapy has good stability,and the time delay is basically consistent in 10 measurements.The gated delay time for beam on is about 252±20ms,and the delay time for beam off is about 10±7ms.The tumor motion curves extracted from the 4D-CT images were significantly correlated with the surface respiratory data,and the correlation coefficients greater than 0.9.The mean error and standard deviation were 0.82mm and 0.53mm,respectively,and the maximum fitting error range from 0.44 mm to 2.38mm.The respiratory curves of patients in each respiratory cycle have a high consistency with the average respiratory curve.The amplitude of respiratory is relatively stable,and in all respiratory phases,the average standard deviation of amplitude was 0.96±0.45 mm.The period of respiratory movement is also stable,and the maximum deviation is less than 1 s.The average respiratory curve of the patient can be used as an approximation of the real-time respiratory movement.Conclusion:The real-time motion tracking accuracy of the optical surface system is high,which can be used for the respiratory motion tracking for patients in clinical.In the application of respiratory gating treatment,monitoring error introduced by the respiratory monitoring system should be considered.The method based on film dose fitting can measure the time delay in respiratory gating accurately.For any gating devices,the time delay should be measured before clinical use.For patients with isolated lesions of lung cancer,changes in chest contour can be used to predict tumor motion in vivo.In radiotherapy,the body surface monitoring system can be used in real time track radiotherapy. |
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