نشریه سنجش و ایمنی پرتو
سال هشتم شماره 1 (پیاپی 30، زمستان 1398)

In this research, in order to improve our calculations in treatment planning for proton radiotherapy of ocular melanoma, we improved our human eye phantom planning system in GEANT4 toolkit. Different analytical models have investigated the creating of Spread Out Bragg Peak (SOBP) in the tumor area. Bortfeld’s model is one of the most important analytical methods. Using convolution method, a new analytical model for the creating of SOBP in the eye tumors was introduced. Also, the GEANT4 Monte Carlo toolkit was implemented for the Bragg peak production in the water and realistic eye phantom. Two different phantoms are proposed to study the effect of defining realistic materials on the proton dose distribution. Moreover, for the clinical investigation, the SOBP curves are figured in the water and eye phantom, using CATANA beam line. For proton pencil beams, the SOBP width for the water and eye phantoms was 0.901 and 0.877 cm, respectively. Bragg peak and SOBP calculations show a good agreement between the results of GEANT4, proposed and Bortfeld models. Using the CATANA beam line, the SOBP width difference between the two water and eye phantoms is 0.11 cm.

The method of optimal measurement counting time in Gamma spectrometry for soil samples was studied. Gamma spectrometry was done based on minimum detectable activity using the HPGe- p type with efficiency of 30% and FWHM 1.7keV (for 1.33 MeV 60Co). The samples were IAEA-RG Th-1, IAEA-RGU-1 and IAEA-RGK-1 prepared in bottles. The measurements were done for 12 different counting times from 5 min to 36 hour. The minimum detectable activity (MDA) was determined for daughters’ of 232Th, 238U, 235U and radionuclide 40K. The counting time in which the changes of the MDA is less than 0.05 Bq/kg·h was considered as the optimal counting time.

Three alpha particles are emitted from the P-11B fusion reaction. Alpha particles play an important role in the death of cancer cells. When boron is accumulated in the tumor, protons irradiated out of body can react with the boron in the tumor. Also a fast gamma ray of 719 KeV beam is released from the P-11B fusion reaction which plays a useful role for us. This therapeutic approach includes features and benefits such as the application of Bragg Peak in treatment, precise targeting of the tumor, increased therapeutic effect, and observation and monitoring of the treatment area.

Radiation protection and the selection of suitable materials to reduce radiation effects is one of the important branches of medical science and radiation. In this paper, the mass attenuation coefficients, half value layer thicknesses and thicknesses of 0/5 mm lead of polyvinyl chloride (PVC) composites containing Nano and micro-particles of copper oxide are calculated using experimental method for different voltages of 40, 60and 80 kVp of X-ray. The results show that at 40 kVp and 60 kVp, composites containing nanoparticles have a better protective effect than composites containing microparticles, but at 80 kV, composites have better protective than micro-particles. Also, the mass attenuation coefficient of copper oxide composite was calculated using MCNP simulation for X-ray tube by voltage of 80kVp. The results of MCNP code were found to be in agreement with theoretical X-com. The results showed that in 40 kVp there is a difference between these two values, which is higher than that of 80 kVp.

Estimation of absorbed dose for radiation workers or person involved in different radiological accident is the aim of biodosimetry. Cytogenetic methods are the most current and applicable biodosimetry tools. In chronic or protracted exposure, fluorescence in situ hybridization (FISH), stable chromosomal aberration is used for estimation of absorbed dose. For precise estimation of absorbed dose, every biodosimetry department should prepare standard dose response curve for different dose and dose rates. In this study, after sampling of blood from two healthy males, blood samples irradiated with X- ray of linear accelerator (0.5-2 Gy) and after separation of their lymphocytes, culturing and metaphase spread were prepared. Fluorescence in situ hybridization painting was performed and stable chromosomal aberration was recorded. Dose response curve was prepared for stable chromosomal aberration for different X-ray doses which can use for retrospective biodosimetry in occupational and accidental situations.

This study evaluated the impact of contrast material on the estimation of absorbed dose due to computed tomography pulmonary angiography (CTPA) using the ICRP reference phantoms. To address this issue, we modified the previously developed physiologically based pharmacokinetic (PBPK) model to be conformed to the ICRP reference phantoms. Regarding the standard contrast material injection protocol, we then provided an in-house Maple code to solve the modified PBPK models and obtain the iodine concentration curves for adult male (AM) and adult female (AF) phantoms. The material composition of the phantoms was then adjusted to include the determined iodine mass percentages in different organs and tissues. The dosimetry calculations were performed using Monte Carlo N-Particle extended code (MCNPX) version 2.6.0., and the estimation of absorbed dose was done at 20, 25 and 30 seconds after the injection. The results showed dose increment of 38%-44% in the lungs at 25 s after the injection in comparison with the time before injection, which emphasizes the importance of considering contrast material in the dose estimation. It is known that the image quality of contrast-enhanced CT is related to the amount of iodine concentration in pulmonary vessels. Our calculations showed that the best time for imaging is 25 s after injection (5 s after the end of injection). At this time, the lung absorbed dose is maximized and the absorbed dose to other organs in the scanned region are minimized.

Radiotherapy using linear accelerators is known as an effective modality for cancer treatment. The photons energy of treatment beams significantly affect the dose distribution. Therefore, it is important to accurately evaluate the photon energy spectra. In this study, MCNPX Monte Carlo code (version 2.6.0) was used to simulate an 18 MV photon beam of a Varian 2100C/D linear accelerator. By matching computed and measured percent depth and profile doses, the optimum values of mean energy and full width at half maximum (FWHM) of the radial distribution of beam were found to be 18.5 MeV and 0.14 cm, respectively. The simulation was also used to investigate the impact of parameters, such as depth, source-to-surface distances (SSD), field size, flattening filter material and geometry of treatment head components on the photon spectra. The results showed that the photon spectra were decreased as an exponential function by increasing depth in phantom and SSD. Results also indicated that the photon spectra depend on the Z of the flattening filter materials. Photon spectra for low-Z materials, such as Al, were significantly increased (up to 31.6%) in comparison with using the original material due to the decrease in the photon absorption cross-section. Each component of the linac head has a different effect on the photon spectrum due to its material and special shape. Based on the obtained results, primary collimator and MLC have, respectively, maximum and minimum effect on the mean energy of photons. Moreover, photon spectra were changed considerably with field size. Change in the photon spectra up to 28.3% was obtained when using 40 × 40 cm2 field size compared to the 5 × 5 cm2 because of the increased scatter from the collimator and the phantom.