Evaluation of Dosimetric Properties in External Beam Radiation Therapy

Abstract

A great level of accuracy and efficiency is required to execute the various paradigms of the radiotherapy processes. The work described in the dissertation was aimed to verify the quality of treatment plans for different techniques in order to enhance the accuracy of planning processes involved. Also dose profile uniformity was verified to improve the Quality Assurance (QA) of radiotherapy process. This project was organized into three major components comprising of ‘comparison of treatment plan quality of RapidArc and Intensity Modulated Radiotherapy (IMRT) using various dosimetric indices’, ‘investigation of tolerance dose to Organs at Risk (OARs) and assessing degree of homogeneity and conformity for Head and Neck (H&N) cancer patients by using Simultaneous Integrated Boost Intensity Modulated Radiotherapy (SIB IMRT) technique’, and ‘determination of quality of 6 MV and 15 MV wedge-shaped radiation beams by analyzing the effect of symmetry parameter on depth, field size, beam energy, and wedge angle.’ First, analysis and comparison of the plan quality of IMRT and RapidArc techniques using various dosimetric indices was carried out to find out the better treatment modality for treating patients with cervix cancer. Thirteen cervical cancer patients treated with IMRT were selected for analysis and original plans were subsequently re-optimized based on the RapidArc technique. Inverse treatment planning was done by Eclipse (Varian Medical Systems, Palo Alto, CA) Treatment Planning System (TPS) for 15 MV photon beams from computed tomographic data. Double arcs were used for RapidArc plans. Quality of treatment plans were evaluated by calculating Conformity Index (CI), Homogeneity Index (HI), Gradient Index (GI), coverage, and Unified Dosimetry Index (UDI) for each plan. RapidArc plans resulted in better Planning Target Volume (PTV) coverage as evident by its superior Conformation Number (CN), coverage, CI, HI, GI, and UDI. Regarding Organs at Risk, RapidArc plans exhibit superior organ sparing as found from integral dose comparison. Difference between both techniques was determined by statistical analysis. For all cases under study, modest differences between IMRT and RapidArc treatment were observed. In conclusion, RapidArc-based treatment planning is safer with similar planning goals compared to the standard fixed IMRT technique. Furthermore, degree of conformity, homogeneity and tolerance dose to organs at risk for head and neck cancer patients were investigated. Fifteen patients receiving treatment based on inverse planned SIB IMRT technique were analyzed in this study. Using a beam energy of 6 MV, two dose levels of 70 Gy and 55·4 Gy were used to treat the tumor. Doses of 2 Gy in 35 fractions and 1·68 Gy in 33 fractions were simultaneously delivered for Effective Planning Target Volume (PTV1) and Boost Planning Target Volume (PTV2) respectively. Dose distribution in PTV and critical organs lie within tolerance dose guidelines protecting spinal cord, brain stem, optic chiasm, and optic nerve. Minor deviation from tolerance limit was observed for parotid glands. This technique provided highly conformal and homogenous dose distribution as well as offered best solution for preserving organ function by keeping dose below tolerance level. Finally, symmetry parameter of wedge-shaped photon beam profile was investigated to determine quality of radiation beams produced by linear accelerator. Analysis of beam profile parameter symmetry was performed for 6 MV and 15 MV photon beam energies by varying different factors such as depth, field size and wedge angle. Photon beam profile was measured from Varian’s Clinac DHX using PTW array detector. Data management, analysis, and processing of isodose curves was carried out by PTW MP3-M water tank and PTW MEPHYSTO version 7.33 software. Variation in symmetry was analyzed to match acceptance criteria as recommended by the American Association of Physicists in Medicine (AAPM) protocols. Results implied that symmetry parameter can be treated as being independent of depth and linearly increase with field size and wedge angle. For wedged beam profiles, measured values of symmetry were generally good and exceeded only slightly from the AAPM’s standard criteria for symmetry for open fields. The difference between open field beam profiles and wedge-shaped profiles were considered so that possible errors involved in radiotherapy process can be rectified.