Objective To investigate the differences between Monte Carlo (MC) calculated doseto-water (Dw) and dose-to-medium (Dm) for lung cancers treated with intensity-modulated radiotherapy (IMRT).Methods A total of 10 lung carcinoma patients with 5-field IMRT treatment plans were stratified sampling randomly selected for this study,which were performed on Monaco treatment planning system (TPS) with MC algorithm.Using the patients' own CT images as quality assurance (QA) phantoms,two kinds of QA plan were calculated,one was the Dm,and another was the Dw plan.Dose volume histogram (DVH) parameters and the subtraction of two plans were used to evaluate the spatial distribution of the difference between the Dm and Dw.Results Differences between dose-volume indices computed with Dm and Dw for the PTV65 and PTV50 doses (D50％,D98％ and D2％) were-0.3％,-0.2％,0.3％ and 0.1％,-0.6％,0.4％,respectively,of which the D50％ of PTV65 and D98％ of PTV50 had statistical difference (t =-2.536,-3.776,P ＜ 0.05).For normal tissues,spinal cord,heart,lung and esophagus,the D50％ differences between Dm and Dw were 0.3％,1.1％,-0.2％ and -0.1％,of which the Dm of spinal cord and heart were slightly lower than the Dw (t =2.535,3.254,P ＜ 0.05).For the D2％ of the normal tissues,the differences were 0.3％,-0.6％,-0.7％ and 0.6％,the differences were statistically significant (t =2.311,-4.105,-3.878,6.214,P＜0.05).All the differences were within 2％.Meanwhile planned subtraction analysis showed the differences between the Dm and Dw varied very much with the other body parts of the patient,especially for bone tissues,and the two doses were significant difference (＞ 5％).Conclusion In the course of clinical application,the relative differences between Dm and Dw for lung cancers MC calculations should be noted when considering the dose limitations of bone tissue.

Objective To verify IMRT plans in point,planar and 3D dose,and to concretely analyze the dose differences of 3D anatomic structure based on Gamma passing rate.Methods Thimble ion-chamber,Matrixx and ArcCheck were separately used to measure six nasopharyngeal carcinoma treatment plans and six lung cancer treatment plans.The dose measurement deviation of the center point was compared as well as the Gamma passing rate of dose verification under the criteria of both 3％/3 mm and 2％/2 mm,the group t-test and one-way ANOVA were also proceeded.3DVH system was used to analyze the dose measurement deviation of target volume (TV) and organ at risk (OAR) through DVH.Results For IMRT and VMAT treatment plans,the mean deviation of point dose was (0.59 ± 1.31) ％ and (-1.00 ± 1.03)％ respectively,and the maximum deviation was less than 3％.Under the criterion of 3％/3 mm,the Gamma passing rate measured by Matrixx,ArcCheck and 3DVH for IMRT plans was 96.28％,97.55％ and 99.02％ respectively,and for VMAT plans,the corresponding results of three different detectors were 97.24％,99.67％ and 98.48％.The results analyzed and compared by 3DVH showed that even under the condition of high Gamma pass rate (more than 95％ for a Gamma criterion of 3％/3 mm),the DVH metrics of both TV and OAR in two cases (account for 16.7％ of the total plan) were significantly different on the clinical parameters,including GTV,spinal cord and brain stem etc.Conclusions The analysis of dose difference of the measurement results based on Gamma pass rate and on anatomic structure of 3D images can more effectively evaluate the influence of dose error to the implementing of clinical plan and the impact to the clinical treatment.

Objective To investigate the γ pass rate and contributing factors by summarizing the plan verification results of 260 volumetric modulated arc therapy (VMAT) plans.Methods From 2010 to 2012,two different detector arrays (MapCheck2,Sun,Nuclear,USA ; Delta4 ScandiDos,Swenden) were used for plan verification in 260 patients.The measured dose distributions were compared with the calculation results of treatment planning system using γ pass rate (Pγ ≤ 1 for 2％/2 mm,3％/3 mm and 5％/3 mm,threshold 10％).And the results were put under independent-samples t test.The impact of multi-leaf collimator (MLC) on the γ pass rate (3％/3 mm) was analyzed.Results The average γ pass rates of 2％/2 mm,3％/3 mm and 5％/3 mm of 260 VMAT plans were 91.7％,98.5％ and 99.7％,respectively.Among 260 VMAT plans,3 VMAT plans (1.2％ of the total) failed to meet the clinical acceptable pass rate and needed to be redesigned or re-optimized.The γ pass rate of 2％/2 mm was slightly different between two measurement devices (90.0％ vs 93.5％,P =0.000),while the γ pass rate of 3％/3 mm showed no significant difference between two measurement devices and two accelerators (98.5％ vs 98.5％,P =0.926 and 98.5％ vs 98.6％,P =0.670).The γ pass rate (3％/3 mm) of the treatment plan before MLC calibration was 61.1％,compared to 94.1％ after calibration.Conclusions Most dose verification results of treatment plans can meet the clinical requirement.Gantry rotation may influence the γ pass rate of VMAT dose verification under stricter pass rate standard (2％/2 mm).MLC calibration is essential for VMAT.

OBJECTIVETo study the correction of algorithm for Varian enhanced dynamic wedge(EDW) factors and compare the dose/monitor unit (MU) deviation measured at the central axis of EDW field with that obtained by manual calculation or using the treatment planning system.

METHODSEDW factors and dose were measured with Thimble ion chamber at 10 cm depth under the water for 6 MV and 10 MV photon on Varian linear accelerator. The corresponding calculations were done with the radiation treatment planning system. An analytic formula, namely the MU Fraction model, was used to calculate the EDW factor, which was corrected with a constant factor. The MU of conventional 2-D planning derived from manual calculating, treatment planning system, and actual measurements were compared.

RESULTSWith the measured results as the standard, the corrected manual calculation deviation of EDW factors was significantly reduced. For photon 6 MV, the maximum deviation reduced from 4.2% to 1.3% for 60° symmetry fields was, and from -4.7% to -1.8% for asymmetric fields. For photon 10 MV, the maximum deviation for all EDW fields was reduced from -3.0% to 1.1%. Comparison of the manual calculations with the measured results showed a MU deviation for symmetric fields within 2%, and more than 5% for some asymmetric fields. The deviation between the calculations of the treatment planning and the measured results was less than 1.5%.

CONCLUSIONConstant factor correction can effectively reduce the deviation of manual calculation. For MU calculation of EDW field in conventional 2-D dimensional treatment planning, the corrected results of symmetric fields meet clinical requirements. While the minimum distance between the field edge and the central axis was less than 4 cm in asymmetric fields, the corresponding special method, measurement or the treatment planning system should be used to calculate the dose/MU.

Algorithms ; Models, Theoretical ; Particle Accelerators ; Radiotherapy Dosage ; Radiotherapy Planning, Computer-Assisted