Shakeel ur Rehman

Absorbed dose estimations of 131I for critical organs using the GEANT4 Monte Carlo simulation code

The aim of this study is to compare the absorbed doses of critical organs of 131I using the MIRD (Medical Internal Radiation Dose) with the corresponding predictions made by GEANT4 simulations. S-values (mean absorbed dose rate per unit activity) and energy deposition per decay for critical organs of 131I for various ages, using standard cylindrical phantom comprising water and ICRP soft-tissue material, have also been estimated. In this study the effect of volume reduction of thyroid, during radiation therapy, on the calculation of absorbed dose is also being estimated using GEANT4. Photon specific energy deposition in the other organs of the neck, due to 131I decay in the thyroid organ, has also been estimated. The maximum relative difference of MIRD with the GEANT4 simulated results is 5.64% for an adults critical organs of 131I. Excellent agreement was found between the results of water and ICRP soft tissue using the cylindrical model. S-values are tabulated for critical organs of 131I, using 1, 5, 10, 15 and 18 years (adults) individuals. S-values for a cylindrical thyroid of different sizes, having 3.07% relative differences of GEANT4 with Siegel & Stabin results. Comparison of the experimentally measured values at 0.5 and 1 m away from neck of the ionization chamber with GEANT4 based Monte Carlo simulations results show good agreement. This study shows that GEANT4 code is an important tool for the internal dosimetry calculations.

Study of CdTe detection efficiency for medical applications using Geant4 based stochastic simulations

The determination of detection efficiency and peak-to-total ratios has been performed for rectangular CdTe detectors for various x-ray and low-energy γ-ray source configurations including parallel beams, point, and cylindrical sources. The dependence of efficiency values on axial and off-axial distances, detector thickness and area, and source dimensions has been studied. The detector model developed in this work has been validated by comparing the Monte Carlo simulated values of detector efficiency for a parallel incident beam with the available published data and good agreement has been found with discrepancies remaining within 2% throughout the energy range. Geant4 simulations show nearly 100% photopeak and total efficiency with peak-to-total ratios approaching a maximum value of 1.0 for photons in the 4-70 keV energy range. Similar high values of detection efficiency have been obtained for brachytherapy I-125 seed sources having cylindrical geometries which indicates the suitability of CdTe detectors for the calibration of sources used in therapy. The logistic power curve was found excellent for empirically fitting the photopeak efficiency variations with axial displacement of the I-125 brachy source in the horizontal configuration. Geant4 simulations clearly show that small thicknesses, of the order of 0.5 mm, of CdTe material are sufficient for attaining almost 100% detection efficiency for low-energy photons having energies up to 100 keV.

Study of thermal neutron albedo for hydrogenous materials using Geant4 simulations

The medical LINACS with energies greater than 8 MeV produce photo-neutrons along with x-rays. These photo-neutrons contaminate the therapeutic beam and are of concern to the personnel working around the treatment room. Consequently, study of neutron albedo is very helpful for radiation protection purposes. In this work, the integral neutron albedo is determined as a function of thickness of target materials using parallel beam of thermal neutrons for a number of materials important in radiation protection and medical purposes, including water, aluminum, lead, iron, beryllium, graphite, skin-ICRP, soft tissue-ICRP, muscle skeletal-ICRP using Geant4 Monte Carlo simulation code. These simulations clearly show that neutron albedo rises quickly with target thickness and reaches saturation after about 15 cm. The value of neutron albedo depends upon scattering material thickness and geometry of reflector and its physical properties. The results of this work have also been compared with those obtained with other experimental results and theoretical calculated values, and found a good agreement with simulated value. These calculations will be helpful for evaluating the outcome of future clinical treatment planning in LINACS, radiation protection and radiation safety. The study of these materials demonstrates that a large proportion of neutrons are reflected at the edge of reflector from 1 to 10 cm, further increasing thickness have relatively small effect on neutron-albedo.

Acquisition of small field electron dosimetry data using ion chamber & Gafchromic® EBT3 film and development of GUI for treatment parameters calculation

Small electron field is a preferential choice of the oncologists in case of the treatment of small superficial lesions (scars). Small field electron dosimetry is necessary to perform prior to the treatment to assure the accuracy of the dose delivery. The main objectives of this work is to acquire small field electron dosimetric data with narrow circular cutouts and to develop the graphical user interface on the basis of this acquired data to extract desired field parameters from the data. Results show that as the cutout size become smaller, percentage depth dose shifts towards surface due to the lack of lateral scatter equilibrium. Thus surface dose is increased i.e. the depth of maximum dose (dmax) is reduced. Similarly the depth of 90% isodose level is decreased as well as the range (i.e. R50 and Rp). Whereas the x-ray contamination is increased with decrease in cutout diameter, field size coverage for 90% isodose curve and the relative output factor is decreased. Results of the radiochromic films were not up to expectations because of the existence of air pockets as the acquisition was performed with solid water phantom slabs. It is, therefore, suggested to use these films in real water. Based on experimental data, GUI enables us to find treatment parameters i.e. energy, cutout dimension, margins, bolus thickness, monitor units etc.

Dose perturbations at heterogeneous interfaces in radiotherapy — An EGSnrc based Monte Carlo investigation

Radiotherapy dose computation in the presence of tissue heterogeneities presents a challenge for the treatment planning system (TPS). This study presents a Monte Carlo (MC) investigation of dose perturbation due to various kinds of heterogeneities (soft tissue interfaces with bone, air, lung and metallic implants) for a range of clinically useful photon energies and field sizes. EGSnrc MC radiation transport code has been used for linear accelerator head modeling and phantom dose calculations. Dose perturbations were quantified in terms of forward- and backscatter factors as well as inside the heterogeneity of low densities for various field sizes including 5 × 5 cm2, 10 × 10 cm2 and 15 × 15 cm2. For lung heterogeneity, both under-dosage and over-dosage has been assessed in relation to the field sizes and energies. These simulations confirm under-dosage in the case of lung heterogeneity for small field sizes, which increases with the energy. However, lung tissues reflect an overdose with smaller energies and larger field sizes.

6Li Atom Percentage Determination by Atomic Absorption–Emission Spectrometry Using a Natural Lithium Hollow Cathode Lamp

A simple method has been developed for the determination of 6Li atom % using combined atomic emission–absorption spectrometry employing a commonly available natural lithium hollow cathode lamp. Unlike in previous practice, there is no need for specially fabricated and high cost 6Li and 7Li monoisotopic lamps in this method. The method requires adjustment of total lithium contents of the sample, i.e., 6Li + 7Li, to 2 μg-mL−1 based upon atomic emission spectroscopy (AES) (Caes) against a 2 μg-mL−1 natural lithium standard. The concentration of the sample was then analyzed by atomic absorption spectroscopy (AAS) measurements (Caas). The difference between the concentration measured by AES and AAS, i.e., Caes — Caas, was calculated. The magnitude of the difference was found to be a function of 6Li fraction in the sample. A calibration curve was constructed by plotting 6Li atom % versus [(Caes — Caas)/Caes] X 100. 6Li atom % of an unknown sample can be evaluated by putting its [(Caes — Caas)/Caes] X 100 value in the calibration curve. The method is fast, convenient, and precise.