Abstract
Modeling of linac head (VARIAN Trilogy) for 6 MeV photon beam was performed using BEAMnrc code package (BEAMnrc 2017). The DOSXYZnrc code was used to determine the percentage depth dose (PDD profiles) and beam profiles for different symmetric square field sizes, i.e., 5cm X 5cm; and 40cm X 40cm. The DICOM images of Alderson Radiation Therapy (ART) RANDO Phantom was used. Four field 3D-CRT treatment plans were generated using AAA, PBC, and Monte-Carlo (MC). It was found that nominal energy of 5.7 MeV with FWHM of 1.2 mm provides best matching of modelled and working linac. All three 3D-CRT plans calculated with AAA, PBC and MC on a Pelvic Rando phantom, were compared using CERR (Computational Environment for Radiotherapy Research) and MATLAB 2013b. It was found that AAA and PBC have comparable results, although in case of tissue interfaces and inhomogeneous media AAA provides better accuracy in comparison to PBC. It can also be observed that AAA and PBC underestimate doses in comparison to MC in the soft muscle tissue which includes OARs such as bladder, bowel bag, and PTV TOTAL. It was found that both AAA and PBC fails to account for tissue air interface accurately and shows variation of 30-40% whereas for surface dose variation of +/-10% was observed. In homogeneous media (muscle tissue) AAA and PBC underestimate doses in comparison to MC. These commercially available algorithms overestimates and underestimates dose values as compared to MC based dose calculation for low and high dose regions specially.
Highlights
In medicine, the radiation is primarily used either for the diagnosis or the treatment of benign or malignant tumors
(a) dose delivered over a period of a few weeks so that the normal tissues which are exposed to ionizing radiation repairs from sub-lethal damages during the time between two successive fractions, (b) irradiating the tumor with radiation beams from several directions so that the doses to surrounding tissue are kept much smaller than dose to the tumor and (c) use of appropriate drugs before treatment which either increases the radio sensitivity of tumor cells or makes normal cell radio-resistant [1]
To compute the percentage depth dose (PDD) and beam profiles for different field sizes of the modeled linac head the simulations were performed in mainly three steps: (i) phase space files for that particular field size were scored just below the secondary collimators (Jaws) using BEAMnrc, (ii) these phase space files were used as an input for the water phantom modeled (50 X 50 X 50 cm3, voxel size of 0.2 X 0.2 X 0.2 cm3) in DOSXYZnrc and 3ddose files were obtained, (iii) STATDOSE standalone code provided with EGSnrc was used as a tool for analyzing and indexing the voxels to take into account the region of interest to determine the beam profiles and PDD profiles
Summary
The radiation is primarily used either for the diagnosis or the treatment of benign or malignant tumors. The commercially available treatment planning systems (TPS) are using dose calculation algorithms which are applying several approximations for computing the dose delivered by the radiation beam. These algorithms apply analytical approximations for the calculation of the delivered dose [3]. Monte-Carlo (MC) methods have established reputation to accurately calculate the dose delivered by the radiation beam as these methods do not inherently use any approximations for computation of dose distribution. These methods have a capability of determining the dosimetric and spectral characteristics of radiation beam accurately. The dose distribution in different organs at risk (OARs) and target volumes was studied
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