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Objective:To perform testing and clinical application of a volumetric-modulated arc therapy (VMAT) dosimetry verification system based on three-dimensional dose reconstruction of patient anatomical structures.Methods:ArcCheck array calibration was performed. Then, 200 MU was delivered with a 10 cm×10 cm field when the source to center of ArcCheck was 100 cm to calibrate the absolute dose and the dose was simultaneously measured by a FC65-G detector in the center of the ArcCheck. The absolute dose calibration value or the CT value of ArcCheck was adjusted to minimize the differences between the planning and measurement values of FC65-G, reconstructed value by 3DVH and reconstructed percent depth dose by 3DVH. 10 lung and 10 cervical cancer VMAT cases were selected and measured by ArcCheck and FC65-G under the delivery of a TrueBeam LINAC. The three-dimensional doses of all cases were reconstructed by 3DVH and compared with the planning and measurement values.Results:Different array calibration files of ArcCheck exerted different effect upon the two-dimensional dose measured by ArcCheck and three-dimensional dose reconstructed by 3DVH. The optimal reconstructed dose was obtained when self-calibration file was adopted and 249.96 cGy was regarded as the absolute dose calibration value. The deviations of the mean dose (D mean) and D 95% of the target were within ±4.2% and parameters of some organs at risk significantly differed compared with the reconstructed and planning dose for all cases. A negative mean point dose difference was obtained and the reconstructed dose was closer to the measured value. The γ-passing rate of the target for some cases was low, the proportion of regions irradiated by 50% prescription dose was slightly higher and the proportion of other organs was relatively high. Conclusion:The 3DVH model can be accurately established and tested with the acceptance test method in the present study, which can provide detailed information for dose verification.
RESUMO
Objective To analyze the relationship between planning factors of intensity-modulated radiation therapy (IMRT) and gamma index and investigate the effect of each parameter upon the γ passing rate of IMRT.Methods Gamma analysis was performed using 3％/3 mm acceptance criteria for 457 IMRT beams with different planning factors.During multi-factor ANOVA analysis of planning factors and gamma passing rate,the control variables primarily included the minimum segment area,minimum number of monitor unit (MU),number of segment,segment conversation,and the spatial resolution in the measured dose distribution.Results The percentage of pixels with passingγsignificantly differed under different minimum segment area,segment conversation and the spatial resolution in the measured dose distribution (all P＜ 0.05).No significant correlation was observed between the passing rate and the minimum number of MU and the number of segment (P＞ 0.05).Conclusions According to the actual situation of the equipment,the minimum segment area should be determined during IMRT planning.Direct machine parameter optimization should be performed.Appropriate resolution of the plane dose images can be chosen according to the minimum detector interval of dose matrix device,
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Objective To analyze the impact of electronic portal imagingdevice (EPID) position error on three-dimensional dose verification of volumetric modulated arc therapy (VMAT).Metbods Five Suremark SL-20 lead points were fixed on Elekta tray,and EPID images were collected in 0-360° rotation,one image per 5°.The position error relative to the accelerator was analyzed via Matlab.Then the images position error was corrected according to the analysis,and the 3D dose was reconstructed with the corrected images.The dose distributions of double arcs,clockwise arc(arc 1),and counterclockwise arc (arc 2) of 16 nasopharyngeal carcinoma patients' VMAT plan were evaluated by γ analysis,and the results of before and after position error correction were compared.Results Compared to 0° gantry angle,the error of source to the image distance (SID) was maximum (1.20 cm) when the gantry angle was 180°.On account of the SID change,the maximum error along the up-down (y) direction in the iso-center planar was 2.28 mm and the left-right (x) direction error was within ± O.5 mm.The 3D γ analyses of 16 nasopharyngeal carcinoma in VMAT plans were obviously increased after the position error along y was corrected.The double arcs,arc1 and arc 2 were increased by (4.12 ±1.67) ％ (t =-9.86,P＜ 0.05),(3.47±1.64) ％ (t=-8.46,P＜ 0.05) and (5.08±1.30) ％ (t=-15.63,P＜ 0.05) in 5％/3 mm standard,respectively.However,in 3％/3 mm standard,γ value of the double arcs,arc 1 and arc2 were increased by (7.63 ±2.24) ％ (t =-13.63,P＜ 0.05),(6.03 ±2.07) ％ (t =-11.66,P＜ 0.05),(9.17 ±2.23) ％ (t =-16.41,P＜ 0.05),respectively.Since the EPID position error along x was corrected after y,the 3D γ analysis of reconstruction dose indicated that the average of the 5％/3 mm and 3％/3 mm γ value were increased by 0.23％ and 0.24％,respectively.Conclusions EPID motion error along the gantry to table direction of the accelerator can't be ignored.When reconstruct dose based on EPID,a modification should be made for rebuilding more accurate patients' 3D dose distribution.
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Objective To study the dosimetry for copper compensator in IMRT.Methods 10 patients (3 nasopharyngeal carcinoma,4 esophageal cancer,3 colorectal cancer) from IMRT plans made in Oncentra Planning System enrolled in this study.The fluence matrices of each plan were converted into compensator thickness matrices based on attenuation coefficient of copper under 6 MV radiation ray which was measured with ion chamber.While irradiating the homogenous phantom with the intensity modulated plans performed by copper compensator,film was used to measure the planar dose.In the end,γanalysis was carried out between the measured and calculated planar doses with the criteria of 3％ DD/3 mm DTA.Paired t test to MU differences of MLC plans and copper compensator plans.Results The γanalysis results showed the planar dose pass rates of 10 plans lie between 90.2％ and 98.2％,which means the IMRf technique based on copper compensator can meet the clinical needs.The MU for copper compensator IMRT plans is less than the MLC IMRT plans (873.9 vs.975.1,P =0.005).Conclusions Compared to conventional IMRT based on MLC,copper compensator has the advantage of lower requirement of hardware and lower scattering dose in patient.Copper compensator can meet the needs of clinical practice.