Iranian Journal of Medical Physics
Year:2011 | Volume:8 | Issue:1 (30)|Papers Count:8

Introduction: Nowadays new radiochromic films have an essential role in radiotherapy dosimetry. Properties such as high sensitivity, good reproducibility, high spatial resolution, easy readout and portability have made them attractive for dosimetry, especially in high-dose-gradient regions.Material and Methods: In this study, electron-beam dose distributions in homogenous and heterogeneous phantoms were calculated using the MCNPX Monte Carlo code and compared with experimental measurements obtained by GAFCHROMIC® EBT film and p-type silicon diode dosimetry. Irradiation was carried out using an Elekta linear accelerator at two different electron energies (8 and 15 MeV), with a 10x10 cm2 applicator and at 100 cm source-to-surface distance.Results: The results show good agreement (within 2%) between radiochromic film measurements and MCNP results.Conclusions: The results show that the new radiochromic films can be used in electron dosimetry and that they are also reliable in presence of heterogeneous media.

Introduction: Brachytherapy is one of the most common treatment modalities for gynecological cancer. The GZP6 brachytherapy system is one of the devices utilized in Iran. It has been considered particularly due to its low cost compared to other more complete and established systems. This system has some deficiencies including lack of a treatment planning software for non-predefined treatments, inability to change the gradually changeable dosimetric variables and using a point source estimation in dose calculation.This report presents a complementary treatment planning software (CTPS) to the system’s own dedicated program.Material and Methods: First, the dosimetric characteristics of three GZP6 sources were calculated based on the TG-43 protocol using the MCNP4C Monte Carlo code. Then, the calculated dose distribution around the implanted applicators, based on the selected dwell positions and dwell times, was shown in a graphical user interface (GUI) written using the MATLAB software.Results: The computation uncertainty in the resulting TG-43 parameters was about 1% and the calculated parameters were in good agreement with similar studies on cobalt-60 source dosimetry.Furthermore, the GUI is prepared as a user-friendly executable file which can be installed on any operating system.Discussion and Conclusion: Since different patients have distinct anatomy and physical conditions, a program for non-predefined situations of source arrangement is necessary. Using GZP6 CTPS can satisfy this requirement.

Introduction: Hyperthermia created by microwave (MW), infrared, ultrasound and other methods, is often utilized as an adjuvant to sensitize cancer cells to the effects of chemotherapy and radiation therapy. We investigated the efficacy of hyperthermia using MW in synergy with chemotherapy in the presence and absence and gold nanoparticles (GNPs).Material and Methods: After culturing and proliferation of the Saos-2 cell line derived from human osteogenic sarcoma, the cells were incubated at two concentrations of GNPs in two diameters of 20 and 40 nm and in the absence and presence of doxorubicin in different groups.Forty eight hours after irradiating the cells with MW up to a temperature of 42oC, cell survival rate was determined using the MTT method, in order to study the effectiveness of the therapeutic parameters.Results: Cell survival in the presence of GNPs was greater than 95%. After chemotherapy by doxorubicin with and without 40 nm GNPs, cell survival rates were determined as 62.8% and 37.1%, declining down to 17% and 4.1% respectively following the combined treatment with MW and chemotherapy in the presence of 20 and 40 nm GNPs.Discussion and Conclusions: GNPs did not induce any cytotoxicity by themselves; their presence along with MW provided a reduction in survival rate that was comparable in severity with the lethal effects of doxorubicin. MW hyperthermia with GNPs produced a higher treatment efficiency in comparison to similar groups in which GNPs were absent. The synergism observed between hyperthermia and chemotherapy was dependent in GNPs' size and concentration.This finding could be caused by increased uptake of doxorubicin by the cells in the presence of GNPs.

Introduction: Repetitive strain injuries are one of the most prevalent problems in occupational diseases. Repetition, vibration and bad postures of the extremities are physical risk factors related to work that can cause chronic musculoskeletal disorders. Repetitive work on a computer with low level contraction requires the posture to be maintained for a long time, which can cause muscle fatigue. Muscle fatigue in shoulders and neck is one of the most prevalent problems reported with computer users especially during typing. Surface electromyography (SEMG) signals are used for detecting muscle fatigue as a non-invasive method.Material and Methods: Nine healthy females volunteered for signal recoding during typing. EMG signals were recorded from the trapezius muscle, which is subjected to muscle fatigue during typing. After signal analysis and feature extraction, detecting and predicting muscle fatigue was performed by using the MLP artificial neural network.Results: Recorded signals were analyzed in time and frequency domains for feature extraction. Results of classification showed that the MLP neural network can detect and predict muscle fatigue during typing with 80.79%±1.04% accuracy.Conclusion: Intelligent classification and prediction of muscle fatigue can have many applications in human factors engineering (ergonomics), rehabilitation engineering and biofeedback equipment for mitigating the injuries of repetitive works.

Introduction: In brachytherapy, radioactive sources are placed close to the tumor, therefore, small changes in their positions can cause large changes in the dose distribution. This emphasizes the need for computerized treatment planning. The usual method for treatment planning of cervix brachytherapy uses conventional radiographs in the Manchester system. Nowadays, because of their advantages in locating the source positions and the surrounding tissues, CT and MRI images are replacing conventional radiographs. In this study, we used CT images in Monte Carlo based dose calculation for brachytherapy treatment planning, using an interface software to create the geometry file required in the MCNP code. The aim of using the interface software is to facilitate and speed up the geometry set-up for simulations based on the patient’s anatomy. This paper examines the feasibility of this method in cervix brachytherapy and assesses its accuracy and speed.Material and Methods: For dosimetric measurements regarding the treatment plan, a pelvic phantom was made from polyethylene in which the treatment applicators could be placed. For simulations using CT images, the phantom was scanned at 120 kVp. Using an interface software written in MATLAB, the CT images were converted into MCNP input file and the simulation was then performed.Results: Using the interface software, preparation time for the simulations of the applicator and surrounding structures was approximately 3 minutes; the corresponding time needed in the conventional MCNP geometry entry being approximately 1 hour. The discrepancy in the simulated and measured doses to point A was 1.7% of the prescribed dose. The corresponding dose differences between the two methods in rectum and bladder were 3.0% and 3.7% of the prescribed dose, respectively.Comparing the results of simulation using the interface software with those of simulation using the standard MCNP geometry entry showed a less than 1% difference of the prescribed dose at 67% of the studied points (minimum<0.01%, maximum 4.8%).Discussion and Conclusions: Using the interface software reduces the overall simulation time. Comparison of the results of the measurements, simulation using the interface software and simulation using standard MCNP geometry entry shows that the use of tomographic images and transforming them into MCNP input file for dose calculation in brachytherapy is feasible and reasonably accurate.

Introduction: The conventional methods of dosimetry are not capable of dosimetry in such a small volume of less than one cubic millimeter. Although the polymer gel dosimetry method based on magnetic resonance imaging (MRI) could achieve three dimensional dosimetry with high resolution, a spatial resolution evaluation based on gel dose modulation transfer function has not been investigated yet. Therefore, in this study, the spatial resolution of two systems of film densitometry and polymer gel dosimetry based on MRI has been evaluated by using the dose modulation transfer function (DMTF).Material and Methods: Kodak therapy verification films and MAGICA polymer gel samples were positioned below a brass absorption grid with different periodic slices (a/2=280, 525, 1125 mm), which was placed in a water bath container to avoid regions of dose build-up just below the absorption grid and then irradiated with Cobalt-60 photons on a Theratron external-beam treatment unit.Dose variation under the brass grid was determined using a calibration curve, while transverse relaxation time (T2) as the selective parameter in a dose image based on multiple echo MRI with 1.5 Tesla GE Signa Echo Speed system (FOV=10 cm, matrix size=512x512, pixel size=0.199x0.199 mm2, TE= 20, 40, 60, 80 ms, TR=4200 ms, NEX=4, slice thickness=2 mm, gap=1 mm) was calculated. DMTF from the modulation depths of T2 and variation in film optical density after calibration would be achieved. The results of polymer gel were compared with film.Results: After deriving the dose distribution profile under the absorption grid, minima and maxima at the smallest period of a=560 mm could scarcely be resolved, but the modulations due to a=2250 mm and a=1050 mm grids could be discerned. The modulation depth for a=2250 mm grid was set to 100% and the other modulations were subsequently referred to this maximum modulation. For film densitometry at a=1050 mm, the modulation depth was reduced to 35% (30% for MR based polymer gel dosimetry; MRPD) and at a=560 mm the modulation in dose was reduced to about 9% of the maximum amplitude (7% for MRPD). The DMTF for the two systems at 200 micron spatial resolution for the 2250, 1050 and 560 mm grids (0.4, 0.9 and 1.7 line pairs per mm) were equal to 1, 0.35 and 0.09 for film densitometry and 1, 0.30 and 0.07 for polymer gel.Discussion and Conclusions: Based on the results of this study, the decrease in DMTF at higher frequencies in a system in its operating resolution limit, is dependent on the type of the dosimetry system. Therefore, the assessment of DMTF for film densitometry system implied a higher spatial resolution in comparison with polymer gel dosimetry at 200 micron spatial resolution.

Introduction: Average glandular dose calculation in mammography with Mo-Rh target-filter and dose calculation for different situations is accurate and fast.Material and Methods: In this research, first of all, x-ray spectra of a Mo target bombarded by a 28 keV electron beam with and without a Rh filter were calculated using the MCNP code. Then, we used the Sobol-Wu parameters to write a FORTRAN code to calculate average glandular dose.Results: Average glandular dose variation was calculated against the voltage of the mammographic x-ray tube for d=5 cm, HVL=0.35 mm Al, and different value of g. Also, the results related to average glandular absorbed dose variation per unit roentgen radiation against the glandular fraction of breast tissue for kV=28 and HVL= 0.400 mmAl and different values of d are presented. Finally, average glandular dose against d for g=60% and three values of kV (23, 27, 35 kV) with corresponding HVLs have been calculated.Discussion and Conclusion: The absorbed dose computational program is accurate, complete, fast and user friendly. This program can be used for optimization of exposure dose in mammography. Also, the results of this research are in good agreement with the computational results of others.

Introduction: Until now, many models have been presented for optical study of the human eye. In recent years, surgery on the anterior section of the eye (such as cataract and photo-refractive surgery) has increased, so a study on the optics of the eye and evaluation of vision quality has become more important.Material and Methods: In this article, some of these models are considered. They include models with spherical and conic-section surfaces (for cornea and lens), simple models and new models with complex surfaces.Results: Evaluation of the optical models of the eye provides the possibility of enhancing the representation of human vision and also increasing the accuracy of surgery on the anterior section of the eye to enable higher quality vision.