Dosimetric Verification For Nasopharyngeal Carcinoma Patient Treated With Intensity Modulated Radiation Therapy

Objective: The purpose of study is to carry out dosimetric verification for nasopharyngeal carcinoma patient treated with intensity modulated radiation therapy(IMRT).Use ionization chamber for absolute dose verification,and analyze the effect of volume of ionization chamber on absolute dose measurement. Meanwhile, use 2 dimensional ionization chamber array for relative dose verification of IMRT. The study aims at ensuring the accuracy of carrying out therapy plan for the patients who would accept IMRT, and testing the accuracy of therapy plan system(TPS).And it also offers basis for adjusting therapy plan.Methods: Thirty patients confirmed pathologically with nasopharyngeal carcinoma are treated with IMRT. The therapy plans are made in TPS,and the dose distribution should meet the requirement of clinical treatment. Transfer their IMRT plans to the 2 dimensional ionization chamber array and phantom scanned by CT,and then generate quality assurance plan(QA plan).Choose 15 out of 30 IMRT plans,using different volume of ionization chamber(0.6cc and 0.13cc) for absolute dose verification.Both ionization chambers are placed phantom respectively,and then carry out QA plan. Ionization chamber area dose calculated in TPS is plan dose,while the dose measured by ionization chamber according to QA plan is actual dose, Compare plan dose and actual dose then get the error.Relative error=(plan dose-actual dose)/actual dose.Error more than±5% indicates that there is a big dose error when IMRT plan is performed. Meantime,use 2 dimensional ionization chamber array for relative dose verification in 30 IMRT plans. According to QA plan, treat for 2 dimensional ionization chamber array with an actual gantry angle,and then output the measured plane dose distribution and identical plan plane dose distribution to ominiPro-IMRT software. Afterwards carry out plan and measured sectional image analysis,plan and measured isodose curve analysis,and gamma analysis by turns.For gamma analysis, set up the tolerance limit of 4% and 4mm distance deviation.For the gamma value [γ( rm )] of the measurement point, [γ( rm )]≤1 demonstrate point calculation passes,the passes point more than 90% indicate that QA plan passes for relative dose verification .Results: In absolute dose verification, among 15 patients treated with 0.6cc ionization chamber,11 patients'plans dose error is within±5%. Among 15 patients treated with 0.13cc ionization chamber,14 patients'plans dose error is within±5%.Dose error cases rise in clinical permission limit,so the results have statistical significance(P=0.002).In relative dose verification, 27 out of 30 patients'passed point in IMRT plans are more than 90% which means the calculation passes. The other three patients'passed point are 85.89%,86.56%,80.53%, which means the calculation fails. The pass rate is 90℅.Conclusion: In order to ensure the quality of IMRT plan, dosimetric verification is necessary. In absolute dose verification, ionization chamber with small volume is far more valuable. So we recommend the small volume ionization chamber. In relative dose verification, 2 dimensional ionization chamber array can be used as a tool for plane dose distribution verification of IMRT. What's more, it is easy and convenient to use, and in favor of verifying routinization.