Evaluation of dose calculation accuracy of various algorithms in lung equivalent inhomogeneity: Comparison of calculated data with Gafchromic film measured results.

AIM: To evaluate dose calculation accuracy of various algorithms in lung equivalent inhomogeneity comprising tumor within it and comparison with Gafchromic film data. MATERIALS AND METHODS: Gafchromic film measured central axis absorbed dose in lung insert (-700 Hounsfield unit [HU]), in racemosa wood cylindrical inhomogeneity (-725 HU) and at three surfaces of tumor (-20 HU) created in cylindrical inhomogeneity, put in the cavity of computerized imaging reference systems (CIRS) thorax phantom were compared with convolution (CON), superposition (SP), fast SP (FSP), and X-ray voxel Monte Carlo (XVMC) algorithms calculated dose using 6 MV beams of field size 2 cm × 2 cm, 3 cm × 3 cm, 4 cm × 4 cm, 5 cm × 5 cm, and 8 cm × 8 cm. RESULTS: XVMC was in good agreement with film measured results for all selected field sizes except 3 cm × 3 cm. SP under estimated by 5.7% at the center of the lung insert while deviation up to 6% was found at the cent of wood inhomogeneity in 2 cm × 2 cm. Except CON, increase in dose from proximal to the central surface of the tumor and then dose falloff from central to the distal surface for field size 2 cm × 2 cm to 4 cm × 4 cm was recorded. The change in film measured percentage depth dose from 2 cm × 2 cm to 3 cm × 3 cm field sizes was found -8% however for consecutive field size(s) larger than 3 cm × 3 cm this difference was less. CON and FSP produced overestimated results. CONCLUSION: Out of four algorithms, XVMC found consistent with measured data. The electronic disequilibrium within and at the interface of inhomogeneity make the accurate dose predictions difficult. These limitations results in deviations from the expected results of the treatments.

Monte Carlo study of dosimetric parameters and dose distribution effect of inhomogeneities and source position of GammaMed Plus source.

BACKGROUND: The conventional treatment planning system (TPS) gives analytical calculations with approximately ± 15-20% dose uncertainty, which may lead to over exposure of critical organs or under dose of target as well as the presence of inhomogeneities, and the position of source affects the exact dose calculation like in breast and intraluminal brachytherapy. AIM: To obtain dose distribution parameters of GammaMed Plus high dose rate (HDR) 192 Ir source using Monte Carlo (MC) EGSnrc and GEANT4 codes as well as to find the effect on dose distribution due to source position, and due to presence of air and cortical bone by using MC GEANT4 code, and to find the similarity of both studies with any past study of any HDR brachytherapy source for either as input to TPS or verification of TPS calculations. SETTINGS AND DESIGN: It is done using different software of the computer, e.g., excel, MS word, etc. MATERIALS AND METHODS: The source, source position for different studies, water phantom, water characteristics, points of measurements, air and cortical bone inhomogeneities, and position of inhomogeneities were simulated. STATISTICAL ANALYSIS USED: For uncertainties calculation, mean and probability are used. RESULTS: The calculated dose rate constant, radial dose function, and 2D anisotropy function of the source show similarity with published data. Calculated dose distribution differences due to presence of air and cortical bone, and position of source in water phantom also show similarity with published data. CONCLUSION: These results can either be implemented in TPS or can be used for verification of TPS calculations.