ABSTRACT
Context: Electronic portal imaging devices (EPIDs) could potentially be useful for patient setup verification and are also increasingly used for dosimetric verification. The accuracy of EPID for dose verification is dependent on the dose-response characteristics, and without a comprehensive evaluation of dose-response characteristics, EPIDs should not be used clinically. Aims: A scatter correction method is presented which is based on experimental data of a two-dimensional (2D) ion chamber array. An accurate algorithm for 2D dose reconstruction at midplane using portal images for in vivo dose verification has been developed. Subjects and Methods: The procedure of scatter correction and dose reconstruction was based on the application of several corrections for beam attenuation, and off-axis factors, measured using a 2D ion chamber array. 2D dose was reconstructed in slab phantom, OCTAVIUS 4D system, and patient, by back projection of transit dose map at EPID-sensitive layer using percentage depth dose data and inverse square. Verification of the developed algorithm was performed by comparing dose values reconstructed in OCTAVIUS 4D system and with that provided by a treatment planning system. Results: The gamma analysis for dose planes within the OCTAVIUS 4D system showed 98% ±1% passing rate, using a 3%/3 mm pass criteria. Applying the algorithm for dose reconstruction in patient pelvic plans showed gamma passing rate of 96% ±2% using the same pass criteria. Conclusions: An accurate empirical algorithm for 2D patient dose reconstruction has been developed. The algorithm was applied to phantom and patient data sets and is able to calculate dose in the midplane. Results indicate that the EPID dose reconstruction algorithm presented in this work is suitable for clinical implementation
ABSTRACT
Objective To investigate the dosimetric effect of carbon fiber couch through virtual simulation in the XiO treatment planning system (TPS).Methods A treatment couch model of iBEAM evo Extension 650 was scanned with a big bore spiral CT and its contour was stored in the XiO TPS.The attenuation coefficient of couch was obtained by measuring the attenuated dose with and without a solid water phantom on the couch at different gantry angles (100°-180°).The optimal relative electron density (RED) values of the carbon fiber (CF) cover and foam core (FC) were adjusted according to the comparison between measured and simulated attenuation dose.The effects of the couch in the TPS on pass rate were evaluated by Octavius 4D phantom with 10 cases with lung cancer.Results The optimal RED values of CF and FC were 0.75 and 0.10 g/cm3,respectively.The measured attenuation error was the maximal at gantry angle of 120° (4.84％) without the treatment couch in the TPS.The average measured attenuation errors without the couch in the TPS dropped significantly from (2.54 ± 1.48) ％ to (-0.04 ± 0.36) ％ after inclusion of the treatment couch during dose calculation (Z =-3.621,P ＜ 0.05).The three-dimensional dose verification γ pass rate (3 mm/3％) without the couch increased significantly from (91.79± 1.25)％ to (94.74± 1.69)％ after inclusion of the couch in the dose calculation (t =6.027,P ＜ 0.05).Conclusions The effect of couch on the attenuation dose is significant.Inclusion of a virtual model of couch in XiO TPS can simulate the attenuation effect properly and improve the accuracy of dose calculation.
ABSTRACT
Objective To investigate the gamma (γ) passing rates for volumetric-modulated arc therapy (VMAT) dosimetric verification with different techniques.Methods A total of 12 VMAT plans for the treatment of different anatomical sites in cancer patients were chosen.The Octavius 4D system was used to measure the dose distributions in two different settings:the gantry was rotating (three-dimensional (3D) and 2D γ-analysis) and the gantry was fixed at 0°(2D γ-analysis).The γ passing rates were analyzed with 3%/3 mm and 2%/2 mm criteria, using the paired t test or Wilcoxon signed-rank test.The 2D γ passing rates for different irradiation methods were calculated.Results For the 3D and 2D dose distributions obtained at a rotating gantry angle as well as the 2D dose distribution obtained at zero gantry angle, the average γ passing rates were 96.03%, 96.98%, and 98.90% for 3%/3 mm (P=0.227, P=0.000, P=0.003);82.08%, 84.04%, and 90.90% for 2%/2 mm (P=0.379, P=0.000, P=0.000).For the 2D dose distributions obtained with different irradiation methods, the average γ passing rate was 98.99% for 3%/3 mm and 93.68% for 2%/2 mm.Conclusions The VMAT dosimetric verification based on a 3D volumetric dosimeter at a rotating gantry position can be clinically useful for delivery quality assurance (QA), and can achieve the most reliable dose calculation for VMAT, which has more referential values.
ABSTRACT
Objective To study the feasibility of dose verification with Octavius 4D system in volumetric modulated arc therapy (VMAT).Methods Twenty patients were treated with the VMAT.Their treatment plans were performes and then acquired the measured data with the Octavius 4D system.In addition,the treatment plans are used in the CT phantom for Octavius 4D system,and acquire the simulated data.We use the γ pass rate to analysis the measured and simulated results for the conditions:the dose deviations are 2％3％5％,the displacement deviations are 2 mm/3 mm/5 mm and the thresholds are 5％ 10％,and got the Gamma pass rate and fitness of profile curve.Results The distributions of measured and simulated dose are matched well,and the fitness of profile curve are also matched well.The averaged Gamma pass rate is 97.78％ in the case 3 mm 3％ 10％.Conclusions Octavius 4D system can meet the dose verification for VMAT treatment.The measured plane is always following the rotating frame and perpendicular to the beam.