Dong Hyeok Jeong

EFFECTIVE DOSE MEASUREMENT FOR CONE BEAM COMPUTED TOMOGRAPHY USING GLASS DOSIMETER

During image-guided radiation therapy, the patient is exposed to unwanted radiation from imaging devices built into the medical LINAC. In the present study, the effective dose delivered to a patient from a cone beam computed tomography (CBCT) machine was measured. Absorbed doses in specific organs listed in ICRP Publication 103 were measured with glass dosimeters calibrated with kilovolt (kV) X-rays using a whole body physical phantom for typical radiotherapy sites, including the head and neck, chest, and pelvis. The effective dose per scan for the head and neck, chest, and pelvis were 3.37±0.29, 7.36±0.33, and 4.09±0.29 mSv, respectively. The results highlight the importance of the compensation of treatment dose by managing imaging dose.

Design and Characteristics of Parallel Plate Type Ionization Chamber for the Measurement of High Energy Photon and Electron Beam

The dosimetric system consisting of parallel plate type of air-filled ionization chamber and charge to voltage converter was designed to do constancy check of the high energy photon and electron beam of clinical linear accelerator. The ionization chamber had been fabricated using an acrylic plate for the air cavity and two printed circuit boards for electrical configuration. The air gap between two electrodes ranged from 3 mm, 6 mm and 10 mm. The sensitive volume of the chambers were 0.9 cc, 1.9 cc and 3.1 cc respectively. The major parameters of the chamber characteristics such as zero drift current, leakage current, saturation voltage, reproducibility, linearity, EPM(Effective Point of Measurement), dose rate effect and polarity effect were measured. The experimental results are as followings. Zero drift currents were 0.23 pA for the volume of 0.9 cc, 1.9 cc and 0.45 pA for 3.1 cc. Leakage currents were 0.38 pAfor the volume of 0.9 cc, 1.9 cc and 0.56 pAfor 3.1 cc. Saturation voltages were 300 V, 400 V and 500 V for three kinds of chambers with volume of 0.9 cc, 1.9 cc and 3.1 cc respectively. Standard deviation of reproducibility was less than 0.003, and the linearity was measured within0.5% for all kinds of chambers. EPM(effective point of measurement) shifted towards the geometric center of the chamber as the plate separation increased. These data were comparable to those of commercially available dosimetric system for QA-purpose. This dosimetric system consisting of the ionization chamber, the PCB, and RF coaxial cables is satisfactory for the purpose of the constancy check of the high energy photon and electron beam from the medical linear accelerator.

Study for Stack Ionization Chamber using Carbon Coated Microfilm

We have designed the stack ionization chamber for quality assurance of clinical photon beam. The stack chamber was composed of solid phantom inserted several chambers. The chamber as a mini plane parallel chamber was made of carbon coated microfilms. Estimated transmission rate of stack type ionization chambers for radiation beam by using Monte Carlo. The stack type ionization chambers In here the electrical characteristics of the six chambers in the solid phantom were evaluated using 6 MV photon beam. The leakage currents were 0.5 pA, reproducibility was less than 0.5%, linearity was less than 0.5%, and dose rate effect was less than 0.7%. In addition the effect of dose variation due to other chambers was estimated to maximum 0.8% approximately. The designed dosimeter can be used for quality determination in output dosimetry or measurement of percentage depth dose approximately for clinical photon beam..

Development of Multi-Channel Dosimeter System for Measurement of Dose Distribution in Therapeutic Photon Beams

We investigated the characteristics of the multi-channel dosimeter (MCD) system for dose distribution by the moving wedge in clinical photon beams. Also we developed an electrometer for the multi-channel dosimeter system for radiation therapy. In dynamic field irradiation, the virtual wedge technique and its fraction methods are available through the computer-controlled asymmetric independent collimator. The modified MCD system consists of an electrometer, a solid detector and an array phantom. The MCD is used for the point dose measurement and the field size scanning. Thus, we evaluated the dosimetric characteristics of the virtual wedge and the conventional fixed wedge by the radiation field analyzer (RFA). In our analysis for the 6 and 10 MV photon beams, the maximum dose in the virtual-wedge and the fixed-wedge factors varied from 1.2 % to 1.6 % for square collimator setting ranging from 10 to 20 cm, respectively. We constructed the modified MCD system with the developed electrometer for radiation therapy to improve on the analytical expressions describing the dose distributions for the virtual wedge and the wedge fraction method in 6 and 10 MV clinical photon beams.

A Test code to Calculate the trajectory of Charged Particals in a Medium under External Electric and Magnetic Fields

A computer code to calculate the trajectory of charged particles in a medium under external electric and magnetic fields was designed. Using this code, the trajectories considered the energy loss and the direction change of the primary particles determined by stopping power and Lorentz force respectively could be calculated. The results show the variation of trajectories as a function of kinetic energy and external fields. The code can be useful for finding the information about the enhancement of energy deposition in a medium irradiated by charged particle beam. Since only the energy loss and the direction change for primary particles are considered, there are errors in this calculation. However, the results can be applied to the study of dose enhancement in radiation therapy. More accurate calculations can be achieved by considering multiple scattering of primary particles and the effect of secondary particles.