Evaluation of Setup Uncertainty on the CTV Dose and Setup Margin Using Monte Carlo Simulation / 의학물리

The effect of setup uncertainties on CTV dose and the correlation between setup uncertainties and setup margin were evaluated by Monte Carlo based numerical simulation. Patient specific information of IMRT treatment plan for rectal cancer designed on the VARIAN Eclipse planning system was utilized for the Monte Carlo simulation program including the planned dose distribution and tumor volume information of a rectal cancer patient. The simulation program was developed for the purpose of the study on Linux environment using open source packages, GNU C++ and ROOT data analysis framework. All misalignments of patient setup were assumed to follow the central limit theorem. Thus systematic and random errors were generated according to the gaussian statistics with a given standard deviation as simulation input parameter. After the setup error simulations, the change of dose in CTV volume was analyzed with the simulation result. In order to verify the conventional margin recipe, the correlation between setup error and setup margin was compared with the margin formula developed on three dimensional conformal radiation therapy. The simulation was performed total 2,000 times for each simulation input of systematic and random errors independently. The size of standard deviation for generating patient setup errors was changed from 1 mm to 10 mm with 1 mm step. In case for the systematic error the minimum dose on CTV Dstat/Dmin was decreased from 100.4 to 72.50% and the mean dose Dsyst was decreased from 100.45% to 97.88%. However the standard deviation of dose distribution in CTV volume was increased from 0.02% to 3.33%. The effect of random error gave the same result of a reduction of mean and minimum dose to CTV volume. It was found that the minimum dose on CTV volume Drand/Dmin was reduced from 100.45% to 94.80% and the mean dose to CTV Drand was decreased from 100.46% to 97.87%. Like systematic error, the standard deviation of CTV dose DeltaDrand was increased from 0.01% to 0.63%. After calculating a size of margin for each systematic and random error the "population ratio" was introduced and applied to verify margin recipe. It was found that the conventional margin formula satisfy margin object on IMRT treatment for rectal cancer. It is considered that the developed Monte-carlo based simulation program might be useful to study for patient setup error and dose coverage in CTV volume due to variations of margin size and setup error.

Set-up uncertainties with radiation therapy for nasopharyngeal carcinoma / 中华放射肿瘤学杂志

Objective This study is to investigate the set-up accuracy of thermoplastic mask used for immobilization of nasopharyngeal carcinoma (NPC) patients being treated by Intensity Modulated Radia- tion Therapy (IMRT).Methods Nineteen patients with early stage nasopharyngeal carcinoma (T1- T2N0M0),treated by fractionated intensity-modulated radiation therapy,underwent repeated CTs during their 6-week treatment course.We evaluated their anatomic landmark coordinates in a total of 85 repeated CT data sets and respective x,y and z shifts relative to their position in the 19 treatment-planning reference CTs.Results The translation in x,y,and z-axes with their mean value derived from both positive and negative set-up errors was-0.84 mm(x-axis),+0.65 mm(y-axis) and +0.01 mm(z-axis).Mean target isocenter translation was (0.89?0.69) mm,(0.82?0.79) mm,(0.95?1.24) mm in x,y and z-direc- tions,respectively.The mean total magnitude vector and 95% CI of isocenter motion were 1.87 mm and 4.65 mm.The data measured over the 6-week fractionated course of treatment revealed a slight deterioration of isocenter accuracy.The 95% CI,considered by us to be a valuable parameter for characterizing the sys- tem,of 4.17 mm for measurement within the first 3 weeks increased to 5.12 mm in the last 3 weeks of treat- ment.Conclusions The sequential CT scanning is a pronounced valuable method of evaluating the quality of departmental specific patient positioning procedures.The thermoplastic mask is eyed as well suited tool for immobilization and repositioning of NPC patients receiving intensity-modulated radiation therapy.