Four-dimensional Dosimetric Evaluation Under Respiratory Exercise Based On Stereotactic Radiotherapy For Non-small Cell Lung Cancer

This study analyzed patients with non-small cell lung cancer treated with SBRT,when the respiratory movement existed and the positioning was consistent with that under treatment,the dosimetric differences between 4D dosimetry distribution under respiratory exercise and 3D dosimetry distribution on average density projection were compared,and the deficiency of dosimetry evaluation based on average density projection was discussed.In addition,considering that the amplitude of respiratory may differ between positioning and treatment,under the big gating window technology of respiratory gating,4D dosimetry assessment method based on the change of the patient’s respiratory amplitude under actual treatment status is proposed,SBRT technology is used for treatment more accurate dosimetry assessments for patients with non-small cell lung cancer.which can help clinicians make more accurate assessments of the prognosis of radiotherapy.20 patients with non-small cell lung cancer treated with SBRT technology were selected,the respiratory waveforms were recorded by optical surface tracking technology,and the 10 respiratory phases located by the 4DCT scan were obtained,and then the average density projection was reconstructed according to the 10 respiratory phases.The dose distribution under the average intensity projection is fused and mapped to each respiratory phase through the Raystation?v4.7.5.4 treatment planning system.First of all,for the dose distribution,the dose is re-accumulated according to the 10%weight of each phase,and the 4D dose under the average weight of the respiratory phase is obtained,which is compared and evaluated with the 3D planned dose on the average intensity projection.The next,5 respiratory cycles were be selected according to the patient’s CT simulation of the positioning breathing signal,and the average peaks and troughs of respiratory signals in 5cycles were calculated respectively.The mean value of the peaks and troughs obtained was used as the upper and lower bounds of the position of the gating window under respiratory gating.After the plan designs were carried out on the average density projection,the respiratory signals of patients at the same position during the treatment were collected at the same time.By means of the difference in amplitude changes in adjacent time phases,sequentially record the amplitude changes in two adjacent time phases of 0%,10%...90%,and then in the treatment According to the change in amplitude,ten time phases of 0%,10%...90%were redistributed according to the changes in the amplitude of the respiratory signal,and the proportion of the duration of each phase was obtained.Then,according to the time proportion of each phase in the treatment process,the corresponding weighted dose proportion of each respiratory phase is given,and the difference between the weighted 4D dose of the respiratory signal and the 3D planned dose on the average intensity projection was evaluated.The results show that all dosimetric parameters D_X,CI,D_2cm of PTV,D_max of spinal cord,the D_max,D₃ and D₃₀ of chest wall and the V₁₂₅₀,V₁₃₅₀of lung were significantly different from those of 3D dose in this study（P<0.05）.Among them,in averagely weighted 4D dose,except for the significant increase in spinal cord D_max and CI,the other dosimetry parameters decreased significantly.The D_x of PTV were decreased,and the doses of V₁₀₀₀,R_50%of the other lungs and D_mean of the heart were not statistically different（P>0.05）.The difference of CI has the largest range of variation,with a floating range of-4.49%to 3.57%,and an average deviation of 1.14%.The difference of heart D_mean is the smallest,with a floating range of-0.1%to 0.02%,with an average deviation of-0.03%.Except for the relatively large deviation of CI average dose,the average dose deviations of other target areas and organs at risk were within 0.5%.Secondly,comparison of 4D dose weighted by respiratory signal with 3D dose:only D_max,D₅₀,D₂ and D₁ of PTV,the D_3cc,D_max of chest wall and D_2cm were significantly different between the two groups（P<0.05）,and shows a decreasing trend.The proportions of other dosimetry parameters are inconsistent in each phase due to differences in individual respiratory signals,which leads to non-statistical significance.The variation range of CI and the deviation of the average value are both the largest,the floating range is-4.49%～2.38%,the deviation of the average value is 0.46%,the difference of D_2cm has the small change,the floating range is-0.39%～0.34%,and the deviation of the average value is-0.11%.The deviation of the average value of other dosimetry parameters is generally small,all within 0.5%.The deviation of the dose of PTV has a floating range of-3.92%～1.34%.The results indicate that the patient’s breathing movement may cause the dose at the tumor site to be about 1.5%less than the planned dose,it is suggested that the clinician’s prescription should be set with an estimate of about minus 1.5%,so as to avoid the situation that the actual dose in the target area is insufficient due to respiratory movement.The dose of chest wall and lung will be reduced due to breathing movement.For planned cases where the dose to the organ-at-risk is at a critical threshold,more accurate 4D dosimetry assessments can be used to obtain more accurate predictions of radiotherapy complications.In addition,we found that using the respiration gating method under the gating window could avoid the decrease of the treatment accuracy caused by the deviation of the target area caused by the excessive or small respiration amplitude,and it is compared with the traditional free breathing lower breathing gating can improve the efficiency of treatment and is suitable for lung cancer patients who cannot perform breath-holding.A method is proposed to redistribute respiratory weights in each phase according to the amplitude of the expiratory curve under treatment,and a more accurate personalized evaluation can be obtained according to individual differences.