Beam-Specific Intensity Modulated Radiotherapy For Lung Cancer And Cyberknife Aperture Optimization

Margins are employed in radiotherapy treatment planning to mitigate the dosimetric effects of geometric uncertainties for the clinical target volume（CTV）.Here,we proposed a margin concept that takes into consideration the beam direction,thereby generating a beam-specific planning target volume（BSPTV）on a beam entrance view.The total merged BSPTV was considered a target for optimization.We investigated the impact of this novel approach for lung intensity-modulated radiotherapy（IMRT）treatment,and compared the treatment plans generated using BSPTV with general PTV.We generated the BSPTV by expanding the CTV perpendicularly to the incident beam direction using the 2D version of van Herk’s margin concept.The BSPTV and general PTV margin were analyzed using digital phantom simulation.Fifteen lung cancer patients were used in the planning study.First,all patient targets were performed with the CTV projection area analysis to select the suitable beam angles.Then,BSPTV was generated according to the selected beam angles.IMRT plans were optimized with the general PTV and BSPTV as the target volumes,respectively.The dosimetry metrics were calculated and evaluated between these two plans.The plan robustness of both plans for setup uncertainties was evaluated using worst-case analysis.Both general PTV and BSPTV plans satisfied the CTV coverage.In addition,the BSPTV plans improved the sparing of high doses to target-surrounding lung tissues compared to the general PTV plans.The V₂₀,V_5,and mean lung dose of the affected lung were significant lower in BSPTV plans（16.20%,28.75%and 8.93 Gy）compared to general PTV plans（16.69%,29.22%and 9.18 Gy）.In uncertainty scenarios,about 80%of target coverage was achieved for both general PTV and BSPTV plans.The results suggested that plan robustness can be guaranteed in both the BSPTV and general PTV plans.However,the BSPTV plan spared normal tissues,such as the lungs,significantly better compared to the general PTV plans.To analyze the influence of Tracking error of Xsight lung tracking system caused by the cardiac pulse.Patients with lung tumors close to the cardiac were enrolled into this study.The tumor trajectories were recorded by the Xsight lung tracking system in the treatment log files during the Cyberknife treatment process.The curves were analyzed by filtering and separating the respiratory frequency which is below 1 Hz and the cardiac pulse which is above 1 Hz.According to the filtering results,the patient treatment tracking data is divided into two groups:accurately establishing the cardiac pulse model which exists cardiac pulse frequency peak and not accurately establishing the cardiac pulse model which do not exist cardiac pulse frequency peak.Compare the tracking errors between the two groups based on the X-ray image data collected by the Xsight lung tracking system during the treatment.For the fractionation with cardiac pulse information,the tracking errors of the patient’s correalation models are 1.45±0.99mm,0.46±0.21mm,0.70±0.54mm,in the left-right,superior-inferior,anterior-posterior direction,respectively.For the fractionation without cardiac pulse information,the tracking errors of the patient’s related models are 1.52±1.17mm、0.63±0.37mm、1.07±0.62mm,in the left-right,superior-inferior,anterior-posterior direction,respectively.The tracking errors of the group which accurately tracking the cardiac pulse was 28.34%,34.86%less than the group not accurately tracking the cardiac pulse in superior-inferior,anterior-posterior direction（P<0.05）.The tracking accuracy of the Xsight lung tracking system will be improved if the cardiac pulse model was accurately established.We conducted this study to construct a target coverage-volume histogram（CVH）and leakage-volume histogram（LVH）metrics and optimization strategy for the selection of the Iris collimator in Cyberknife Xsight lung tracking treatment through a retrospective analysis of target structures and clinical data.CVH and LVH were the same as dose-volume histogram（DVH）,but with a coverage and leakage replacing dose.CVH and LVH metrics were retrospectively analyzed for 37 lung cancer patients.For each patient,Iris collimator was optimized and selected based on CVH and LVH metrics.The planning target volume（PTV）C95 and C90 coverage,absolute mean leakage value,leakage/coverage ratio,selected collimator diameter（Φ）,Φ/length of the long axis of PTV（Amax）,andΦ/length of the short axis（Amin）of PTV were compared.The correlation of the absolute mean leakage value,leakage/coverage ratio,Φ/Amin andΦ/Amax were evaluated.For each patient,the PTV C95 coverage（70.45 vs 63.19）and C90 coverage（77.25 vs 69.96）were higher in the C95 coverage threshold group compared to the C90 coverage threshold group.The leakage/coverage ratio（0.56 vs 0.69）and absolute mean leakage value（0.56 vs0.61）was lower in C90 coverage threshold group than in C95 coverage threshold group.The Spearmen correlation test showed theΦ/Amin were significantly correlated with leakage/coverage ratio and absolute mean leakage value.Upon analysis of the selected collimator diameters,the mean value ofΦ/Amin of the optimized collimator diameters was found to be 1.10.The CVH and LVH analysis is able to quantitatively evaluate the trade-off between target coverage and normal tissue sparing.