IRANIAN JOURNAL OF MEDICAL PHYSICS
Year:2005 | Volume:2 | Issue:7|Papers Count:9

Introduction: Monte Carlo simulation is the most accurate method of simulating radiation transport and predicting doses at different points of interest in radiotherapy. A great advantage of the Monte Carlo method compared to the deterministic methods is the ability to deal accurately with any complex geometry. Its disadvantage is the extremely long computing time required to obtain a dose distribution with good statistical accuracy.Materials and Methods: The MCNP-4C Monte Carlo code was used to simulate a 9 MV photon beam from a Neptun 10PC linear accelerator. The accelerator was modeled as a complete unit consisting of a target, exit window, initial collimator, primary collimator, flattening filter, monitor chamber and secondary collimator. The geometrical details and the composition of each component was either obtained from the manufacturer or was directly measured. The simulation of the source was performed in a two step process. Initially, the electron source was defined. Secondly, the bremsstrahlung energy spectra and the fluence distribution at the scoring planes were used to define the photon source. The simulated electron beam energy followed a Gaussian distribution, with FWHM equal to 12% in nominal energy. The used intensity distribution of the electron beam also followed a Gaussian distribution with a FWHM equal to 0.34 cm. To compute the photon beam data a 50 x 50 x 40 cm3 water phantom located at SSD = 100 cm was simulated. The depth dose and the dose profile curves were calculated for four different field sizes (5x5, 10x10, 20x20 and 30x30 cm2) and compared against the measured values. The low-energy cut-off for the photons and electrons was 10 and 500 KeV, respectively. The measurements were carried out by using a Scanditronix dose scanning system and a 0.12 cm3 RK ionization chamber.Results: To verify the simulated model, the calculated Monte Carlo dose data were compared against the corresponding measured values. The energy spectra and the angular distribution of the x-ray beam generated by the Neptun 10PC linac was examined. The result showed an efficiency of about 73% for the production of bermsstrahlung photon by the target. The agreement between the calculated and the measured depth dose and the dose profile was generally better than 2% for all the fields.Discussion and Conclusion: The simulation of the Neptun 10PC linac performed in this work is capable of computing the depth dose data and the beam profiles in water phantom for all the predefined fields including 5x5, 10x10, 20x20 and 30x30 cm2. Therefore, it can be concluded that MCNP-4C is a suitable tool for the dose calculation in radiotherapy. The simulated linac machine and the resulting data can be used to predict the dose distribution in all complex fields.

Introduction: During radiotherapy, high accuracy in the dose delivery is required because there is a strong relationship between the absorbed dose, local tumor control and particularly the normal tissue damage. In many institutions, in vivo dosimetry using diodes is performed to check the actual dose delivered. In general, the uncertainty in the dose delivered should fall within ±5% of the prescribed dose as recommended by the International Commission on Radiation Units and Measurements (ICRU).Materials and Methods: The combined entrance and exit dose measurements have been performed for brain tumors by diode detectors. In vivo detectors used in this study were P-type semiconductor diodes used for determination of absorbed dose and exit transmission (Tex). A Perspex water phantom (30x30 cm3 area and thickness ranging from 5 to 30 cm) and a farmer type ionization chamber (0.6 cm3) were used for the measurements. The calibration and correction factor are calculated and the relevant curves have been obtained. The SSD correction factor (SSD = 80 cm for all set-up), directional dependence and temperature dependence (<1%) were ignored in the determination of the absorbed dose.Results: Errors more than 5% between the measured and the calculated entrance, exit and midline doses were detected. The measured entrance, exit and midline doses were compared with the calculated ones, and no significant difference (P>0.1) was observed.Discussion and Conclusion: In vivo measurements have been shown to be very useful as a check of the dose delivered to a given patient.

Introduction: Stereotactic radiosurgery is a technique for the treatment of intracranial lesions with highly collimated x-ray beam. Linac-based radiosurgery is currently performed by two techniques: couch or pedestal mounted. In the first technique, a device is required to affix patient's head to a couch and moreover to translate it accurately. Structure of such a device constructed by the authors in addition to the acceptance test which was performed to evaluate the device has been described in this article.Materials and Methods: A head docking device has been designed and constructed according to the geometry of linac's couch and desired functions. The device is completely made from aluminum and consists of four major components: attachment bar, lower structure with four movements, upper structure with two movements equipped with a lock, two handles and a mounting ring for stereotactic frame. Translating accuracy, mechanical stability and isocentric accuracy were assessed in the frame of acceptance test.Results: Translating accuracy, mechanical stability and isocentric accuracy within 95% confidence intervals were found to be 1, 1.64 and 3.2 mm, respectively.Discussion and Conclusion: According to AAPM report no.54 recommendation head docking device should translate head with accuracy of 1 mm, the property which was met by the constructed device. Moreover considering the measured isocentric accuracy, encompassing mechanical stability, constructed device can confidently be used in stereotactic treatment.

Introduction: Arterial stiffness is an important predictor of cardiovascular risk. Several indices have been introduced to estimate the arterial stiffness based on the changes in the brachial blood pressure. Since the substitution of the blood pressure changes in the central arteries such as carotid with the blood pressure changes in the brachial results in error in the blood flow, it is of importance to present an elastic parameter based on the mechanical models without any reliance on the brachial blood pressure. Materials and Methods: Initially, a suitable dynamic model is introduced for pulsatile blood flow in the arteries based on Navier-Stokes the equations in fluid mechanics. Then, according to the theory of elasticity, the equations governing arterial wall are described and coupled with the equations of fluid flow. The attained system of equations is completed by the clinical information obtained from the carotid artery Doppler ultrasound images of healthy male subject. Therefore, the Doppler ultrasound images are recorded and saved in computer after which the center-line blood velocity, the arterial wall thickness, the period of a cardiac cycle and the arterial radius are measured by off-line processing.Results: The results from the analytic solution of the completed equations show that the elastic modulus for this healthy subject is 51 kpa which is in close agreement with the result obtained from other researches.Discussion and Conclusion: By applying this method, a non-invasive method of clinically evaluating the arterial stiffness will be possible by the Doppler ultrasound measurement of common carotid artery without any measurement of the local blood pressure.

Introduction: The purpose of this study is to define the optimal parameters for the tomographic reconstruction procedure in a routine single photon emission tomography. The Hoffman brain phantom is modified to evaluate the reconstruction method. The phantom was imaged in a 3 and 2-dimensional conformation and the results were compared.Materials and Methods: The 2D phantom (slices) was imaged independently. The tomographic image was performed using the phantom in 3D conformation. Planar image was used as the reference image for each corresponding reconstructed slice image. A set of projection data with different count densities were obtained from 99m Tc with the phantom. The data were transformed into intertile format and transferred to personal computer. A software was designed in Matlab (Version 7) to support the phantom and the automatic calculation. All the data were reconstructed using back-projection technique with the ramp, Shepp-Logan, Cosine, Hamming, Hanning (each filter having so different parameters), Butterworth, Metz and Wiener (each filter having 2000 different parameters). The quality of the images was estimated by comparing the reconstructed images with the reference image by the use of universal image quality index (UIQI).Results: All the filters tested score substantially better than the control (P<0.05). The data from this study suggests that the Hanning and Hamming are the weakest of the seven filter types tested (mean of UIQI= 0.2218, 0.2219). This is due to response of Hanning and Hamming filters at the low frequencies. The quantitative results of this study indicate that Wiener filter provided best image quality of the phantom than the other filters regardless of which collimator is used. The mean of UIQI was the most for wiener filter in comparison with the other filters (P<0.05).Discussion and Conclusion: These results clearly show that Wiener filter improves quality image and the extent of the improvement has been quantified. The optimum parameters obtained for the filters show direct relation with total counts in the study.

Introduction: Laser interstitial thermotherapy (UTT) is an internal ablation therapy method consisting of a percutaneous or intraoperative insertion of laser fibers directly into the liver tumor with maximum diameter of 5 cm. In this treatment method, there isn't any general information about the relationship between increasing the exposure power, coagulation and carbonization areas with the changes in temperature. In this' study, according to the power range of UTI the changes in the temperature of liver tissue and the diameter of the necrotic area were measured.Materials and Methods: In vitro UTT was performed on fleshly sheep liver tissue using a bare-tip optical fiber from a Nd: Yag laser. A power setting of 2, 2.4, 3, 3.4, and 4 watt were used for an exposure time of 300 sec. The temperature monitoring was performed during the heating and cooling down by fixing micro thermocouples at 2.5 mm from the fiber tip. The thermal lesions which include necrosis and carbonization areas were compared for each power.Results: The result of the temperature monitoring was expressed as the mean value for each power. The temperature charts show that at 2.5 mm from the fiber tip the max tissue temperature is increased from 276.20°C (for a power setting of 2 watt and a 308 sec of exposure time) to 728.2°C (for a power setting of 3.4 watt and a 365 sec exposure time). At 6 mm from the fiber tip the max. temperature was measured to be 86.4°C for a power setting of 4 watt and 325 sec exposure time. For each power a non linear regression analysis was performed during the heating and cooling down for the dependent (temperature) and independent (time) parameters. The max. value for the cubic equation is shown to be R = 0.99 during the heating and for the exponential equation to be R=0.89 during the cooling down. A p value of 0.01 is considered significant. The diameter of the necrotic liver tissue increases from 12.95 mm at 600 joules to 16.15 mm at 1200 joules of energy. When the total applied energy is increased from 600 to 1200 joules, the thermal ablation increased by 25% while there was a 56% increase in the carbonization area. Increasing the carbonization area caused a decreases in the penetrability of the laser beam. Discussion and Conclusion: A useful treatment planning based on a non-linear regression analysis could be prepared for the treatment of hepatocellular carcinoma. In this analysis, the temperature changes in the necrotic area are monitored as a function of power setting in the range of 2-4 watts in UTT.

Introduction: A major problem facing the patients with chronic liver diseases is the diagnostic procedure. The conventional diagnostic method depends mainly on needle biopsy which is an invasive method. There are some approaches to develop a reliable noninvasive method of evaluating histological changes in sonograms. The main characteristic used to distinguish between the normal, hepatitis and cirrhosis liver is the texture of liver surface. The problem of defining a set of meaningful features that explores the characteristics of the texture, leads to several methods of determining tissue texture. Some of these methods, which have been developed so far, are based on wavelet transform. The selection of wavelet transform type affects the accuracy of determining the texture. In this study, an optimal wavelet transform called Gabor wavelet was introduced and three different methods of determining tissue texture were evaluated. These include statistical, dyadic wavelet transform and Gabor wavelet transform methods.Materials and Methods: The proposed algorithm was applied to differentiate ultrasonic liver images into two disease states (hepatitis and cirrhosis) and normal liver. In this experiment, 50 liver sample images for each three states which already been proven by needle biopsy were used. These images are taken from a Toshiba Sonolayer SSA250A device using a 3.75 MHz transducer. For each image, a region of interest (ROI) with 75×35 pixels is selected. The ROI is chosen to include only liver tissue without major blood vessel or hepatic duct.The classification method used for this work is "Minimum Distance", where the distance is calculated between feature vectors of test image and reference images. In order to evaluate the diagnostic results, two quantities named "Sensitivity" and "Specificity" were calculated for each method.Results: The obtained results show that Gabor wavelet has 85% and dyadic wavelet has 71% sensitivity in the hepatitis liver images. On the other hand, Gabor wavelet shows 86% sensitivity in the cirrhosis liver images, while dyadic wavelet has 78%. The specificity of Gabor wavelet in the hepatitis and cirrhosis liver images is 77% and 79% respectively, while the specificity of dyadic wavelet is 65% and 72%, respectively. Discussion and Conclusion: Based on this experiment, the Gabor wavelet is more appropriate than the dyadic wavelet and statistical based method for the texture classification as it leads to higher classification accuracy, because the dyadic wavelet loses some middle-band information, while the Gabor wavelet preserves it. Based on what was observed, the most significant information regarding the texture is mainly located in the middle-frequency bands of wavelet decomposition. Therefore, using Gabor wavelet, a more flexible decomposition of the entire frequency band can be achieved leading to a superior differentiation of the texture information.

Introduction: Esophageal cancer is one of the most frequently occurring cancers in Iran and having a high incidence rate among other countries. Radiotherapy is one of the three methods (surgery, radiotherapy and chemotherapy) for radical or palliative treatment of esophageal cancer. In this method of treatment, the organs such as heart and spinal cord are regarded as organs at risk (OAR) which their dose should be kept below the tolerance level.Different techniques have been used in conventional radiotherapy, among which two parallel opposed fields (POP) is used more often either for the whole course of the treatment or 2/3 of the fractions. In this technique, a great portion of the spinal cord may be involved in the treatment volume. The rate of treatment success may depend on the tolerance dose of heart and cord which act as the limiting factor.Materials and Methods: In this study, 10 patients with S.C.C. of esophagus having indication for external radiotherapy were selected. The CT scan simulations were performed for all the cases and their data were digitized to be used in computerized treatment planning system. Treatment planning for three photon beam energies (Co-60, 6 and 10 MY) are obtained for each individual patient under the same geometry and dose. The integral dose, absorbed dose and dose volume histogram (DVH) of the heart and target were calculated for all the cases.Results: The maximum point dose in the heart is 140, 125 and 115% of the reference point for Co-60, 6 and 10 MV, respectively. The integral dose for the largest A-P patient was found to be 55.33, 54.17 and 51.66 Gy.Kg for Co-60, 6 and 10 MY, respectively. In using 10 MY beam, 7% reduction in the integral dose of the heart was obtained. Alternatively, by using 6 MV beam, a reduction of only 2% was observed. The integral dose of the heart for the 10MY beam had a 5% reduction in comparison to 6 MY beam.Discussion and Conclusion: Since lower energy beams cause a higher integral dose to the heart and cord, the use of higher energy photon beams in the case of deep seated tumors and large size patients are unavoidable. In spite of the advantages of cobalt machines, its energy is not suitable for deeply suited tumors.