Presentation of Organ Dose and Effective Dose Conversion Factors in Dual-Energy Computed Tomography: A Monte Carlo Simulation Study.

Background: The same conversion factors (k-factors) of Single CT (SECT) are applied to estimate the Effective Dose (ED) in Dual Energy Computed Tomography (DECT). However, k-factors for different organs need independently validating for DECT, due to the different conditions in DECT. Objective: This study aimed to calculate organ dose and k-factors in different imaging protocols (liver, chest, cardiac, and abdomen) for male and female phantoms. Material and Methods: This Monte Carlo Simulation study used Monte Carlo N-Particle (MCNP) code for modeling a Siemens Somatom Definition Flash dual-source CT scanner. The organ dose, dose length product, and k-factors were calculated for the Medical Internal Radiation Dose (MIRD) of male and female phantoms. Results: For the male phantom, the k-factors for the liver, chest, cardiac, and abdomen-pelvis imaging protocols are equal to 0.020, 0.012, 0.016, and 0.014 mSv.mGy-1cm-1, respectively. For the female phantom, the corresponding values are equal to 0.026, 0.023, 0.036, and 0.018, respectively. These values for DECT are different from those corresponding values for SECT, especially for the female phantom. Conclusion: The calculated k-factors for DECT can be used as reference values for the estimation of ED in DECT.

Organ dose in cardiac dual-energy computed tomography: a Monte Carlo study.

Dual-energy computed tomography (DECT) has appeared as a novel approach with the aim of evaluating artery-related diseases. With the advent of DECT, concerns have been raised about the induction of diseases such as cancer due to high radiation exposure of patients. Therefore, the dose received by patients in DECT should be considered. The parameter most commonly used for patient dosimetry is the effective dose (ED). The purpose of this study is to model and validate a DECT scanner by a developed MCNP Monte Carlo code and to calculate the organ doses, the ED, and the conversion factor (k-factor) used in determining ED in the cardiac imaging protocol. To validate the DECT scanner simulation, a standard dosimetry body phantom was modeled in two radiation modes of single energy CT and DECT. The results of simulated CT dose index (CTDI) were compared with those of ImPACT or measurement data. Then dosimetry phantom was replaced by the male and female ORNL phantoms and the organ doses were calculated. The organ doses were also calculated by ImPACT dose software. In the initial validation stage, the minimum and maximum observed relative differences between results of MNCP simulation and measured were 2.77% and 5.79% for the central CTDI and 1.91% and 5.83% for the averaged peripheral CTDI, respectively. The mean ED of simulation and the ImPACT were 3.23 and 5.55 mSv/100 mAs, and the mean k-factor was 0.016 and 0.032 mSv mGy-1 cm-1 in the male and female phantoms, respectively. The k-factor obtained for males is close to the currently used k-factor, but the k-factor for females is almost twice.

A Monte Carlo Platform for Characterization of X-Ray Radiation Dose in CT Imaging.

BACKGROUND: Computed tomography (CT) is currently known as a versatile imaging tool in the clinic used for almost all types of cancers. The major issue of CT is the health risk, belonging to X-ray radiation exposure. Concerning this, Monte Carlo (MC) simulation is recognized as a key computational technique for estimating and optimizing radiation dose. CT simulation with MCNP/MCNPX MC code has an inherent problem due to the lack of a fan-beam shaped source model. This limitation increases the run time and highly decreases the number of photons passing the body or phantom. Recently, a beta version of MCNP code called MCNP-FBSM (Fan-Beam Source Model) has been developed to pave the simulation way of CT imaging procedure, removing the need of the collimator. This is a new code, which needs to be validated in all aspects. OBJECTIVE: In this work, we aimed to develop and validate an efficient computational platform based on modified MCNP-FBSM for CT dosimetry purposes. MATERIAL AND METHODS: In this experimental study, a setup is carried out to measure CTDI100 in air and standard dosimetry phantoms. The accuracy of the developed MC CT simulator results has been widely benchmarked through comparison with our measured data, UK's National Health Service's reports (known as ImPACT), manufacturer's data, and other published results. RESULTS: The minimum and maximum observed mean differences of our simulation results and other above-mentioned data were the 1.5%, and 9.79%, respectively. CONCLUSION: The developed FBSM MC computational platform is a beneficial tool for CT dosimetry.

Tissue composition effect on dose distribution in radiotherapy with a 6 MV photon beam of a medical linac.

AIM: The aim of this study is to evaluate soft-tissue composition effect on dose distribution for various soft tissues in radiotherapy with a 6 MV photon beam of a medical linac. BACKGROUND: The compositions of various soft tissues are different which could affect dose calculations. MATERIALS AND METHODS: A phantom and Siemens Primus linear accelerator were simulated using MCNPX Monte Carlo code. In a homogeneous cubic phantom, six types of soft tissue and three types of tissue-equivalent materials were defined separately. The soft tissues were muscle (skeletal), adipose tissue, blood (whole), breast tissue, soft tissue (9-component), and soft tissue (4-component). The tissue-equivalent materials included water, A-150 tissue equivalent plastic and perspex. Photon dose relative to dose in 9-component soft tissue at various depths on the beam's central axis was determined for the 6 MV photon beam. The relative dose was also calculated and compared for various MCNPX tallies including *F8, F6, and *F4. RESULTS: The results of the relative photon dose in various materials relative to dose in 9-component soft tissue using different tallies are reported in the form of tabulated data. Minor differences between dose distributions in various soft tissues and tissue-equivalent materials were observed. The results from F6 and F4 were practically the same but differ with the *F8 tally. CONCLUSIONS: Based on the calculations performed, the differences in dose distributions in various soft tissues and tissue-equivalent materials are minor but they could be corrected in radiotherapy calculations to upgrade the accuracy of the dosimetric calculations.

Evaluation of (101)Rh as a brachytherapy source.

PURPOSE: Recently a number of hypothetical sources have been proposed and evaluated for use in brachytherapy. In the present study, a hypothetical (101)Rh source with mean photon energy of 121.5 keV and half-life of 3.3 years, has been evaluated as an alternative to the existing high-dose-rate (HDR) sources. Dosimetric characteristics of this source model have been determined following the recommendation of the Task Group 43 (TG-43) of the American Association of the Physicist in Medicine (AAPM), and the results are compared with the published data for (57)Co source and Flexisource (192)Ir sources with similar geometries. MATERIAL AND METHODS: MCNPX Monte Carlo code was used for simulation of the (101)Rh hypothetical HDR source design. Geometric design of this hypothetical source was considered to be similar to that of Flexisource (192)Ir source. Task group No. 43 dosimetric parameters, including air kerma strength per mCi, dose rate constant, radial dose function, and two dimensional (2D) anisotropy functions were calculated for the (101)Rh source through simulations. RESULTS: Air kerma strength per activity and dose rate constant for the hypothetical (101)Rh source were 1.09 ± 0.01 U/mCi and 1.18 ± 0.08 cGy/(h.U), respectively. At distances beyond 1.0 cm in phantom, radial dose function for the hypothetical (101)Rh source is higher than that of (192)Ir. It has also similar 2D anisotropy functions to the Flexisource (192)Ir source. CONCLUSIONS: (101)Rh is proposed as an alternative to the existing HDR sources for use in brachytherapy. This source provides medium energy photons, relatively long half-life, higher dose rate constant and radial dose function, and similar 2D anisotropy function to the Flexisource (192)Ir HDR source design. The longer half-life of the source reduces the frequency of the source exchange for the clinical environment.