Quality Assurance of Three Dimensional Dosimetry for Radiation Applications in Medicine

Abstract

The radiation therapy field is advancing continuously to achieve higher degrees of accuracy and efficiency. An optimized radiotherapy treatment planning guarantees the achievement of the treatment goals. The intent of this dissertation was to investigate quality of Intensity Modulated Radiotherapy (IMRT) treatment plans through analysis of dose volume histograms, Simultaneous Integrated Boost (SIB) IMRT and RapidArc plan verification using gamma index, and the analysis of wedge-shaped beam profile parameters with the viewpoint of optimizing the accuracy of the processes involved. This work has been split into three components with the same focus of attention of verifying and improving the quality assurance procedure. Firstly, dose conformity and homogeneity of IMRT plans for pelvic cancer patients had been explored. Then evaluation of pre-treatment dose verification was performed for H&N and cervix cancer patients by using gamma index. Lastly, analysis of wedge-shaped isodose curves has been determined to verify quality of radiation beams used. Firstly, this study aimed to investigate degree of conformity and homogeneity for pelvic patients using IMRT technique. Three different formulas of Conformity Index (CI) and four formulas of Homogeneity Index (HI), described in literature were analyzed using 15MV photon beam. In total, 18 patients were selected at random and dose of 5000 cGy was delivered in 25 equal fractions. CI suggested by International Commission on Radiation Units and Measurements (ICRU), Radiation Conformity Index (RCI) and CI PITV had mean ± standard deviation values of 1.02 ± 0.018, 0.98 ± 0.017, and 1.63 ± 0.333, respectively. Values of CI PITV exceeded acceptable value for 27% patients with minor deviation. Mean ±SD of HI A, HI B, HI C and HI D were 1·12±0·02, 0·13± 0·04, 0·10±0·02 and 0·99±0·03, respectively. Coefficient of determination between each set of HI values was known by calculating R2 value. No statistically significant differences were observed for three CIs reported. All the studied HIs can be effectively used for assessment of uniformity inside the target volume. However, values of HI C were closest to ideal value as compared with other three formulas; hence, it is considered a better measure to compute homogeneity of dose within target volume. 8 Secondly, this study aimed to estimate point-by-point difference between measured and calculated dose distribution in terms of both Distance to Agreement (DTA) and Dose Difference (DD) by using gamma analysis. Gamma evaluation was performed for dose verification of SIB IMRT and RapidArc plans. The portal dosimetry software (Varian Medical Systems, Palo, Alto, CA) performed gamma analysis. The dose difference and distance to agreement criteria can be defined individually. All gamma calculations were based on 3 mm DTA and 3% DD criteria for RapidArc plans and 3 mm DTA and 5% DD criteria for SIB IMRT plans. Plans of this study were deemed acceptable when passing rate was 95%. Thirteen SIB IMRT and all RapidArc cases pass tolerance criteria of 95% set by our institution. Findings of this work underlined the importance of gamma analysis method to predict the quality of dose calculation. Passing rate of 95% is achieved in 93% of SIB IMRT and for all RapidArc plans which is adequate level of accuracy for analyzed plans thus assuring the robustness of gamma analysis technique. Lastly, this exploration was intended to analyze photon beam profile parameters; symmetry, Central Axis (CAX) deviation, Field 50%, left and right penumbra, Dmin, Dmax, and homogeneity for 6 MV and 15 MV photon beam energies of wedge-shaped isodose curves to improve the efficiency of treatment plans. Treatment parameters like depth, wedge angle, and field size were varied to observe their effect on dosimetric characteristics of beam profiles in a water phantom, generated by Varian Clinac linear accelerator. It was concluded that these beam profile characteristics precisely agreed with the acceptance criteria as recommended by the radiotherapy protocols with very slight deviation. Wedge angle, field size, and depth dependency on wedge-shaped isodose curves should be taken into account to prevent any significant tumordose discrepancies for the cancer patients.