فهرست مطالب mohammad javad tahmasebi birgani

Radiomic feature reproducibility assessment is critical in radiomics‑based image biomarker discovery. This study aims to evaluate the impact of preprocessing parameters on the reproducibility of magnetic resonance image (MRI) radiomic features extracted from gross tumor volume (GTV) and high‑risk clinical tumor volume (HR‑CTV) in cervical cancer (CC) patients.

This study included 99 patients with pathologically confirmed cervical cancer who underwent an MRI prior to receiving brachytherapy. The GTV and HR‑CTV were delineated on T2‑weighted MRI and inputted into 3D Slicer for radiomic analysis. Before feature extraction, all images were preprocessed to a combination of several parameters of Laplacian of Gaussian (1 and 2), resampling (0.5 and 1), and bin width (5, 10, 25, and 50). The reproducibility of radiomic features was analyzed using the intra‑class correlation coefficient (ICC).

Almost all shapes and first‑order features had ICC values > 0.95. Most second‑order texture features were not reproducible (ICC < 0.95) in GTV and HR‑CTV. Furthermore, 20% of all neighboring gray‑tone difference matrix texture features had ICC > 0.90 in both GTV and HR‑CTV.

The results presented here showed that MRI radiomic features are vulnerable to changes in preprocessing, and this issue must be understood and applied before any clinical decision‑making. Features with ICC > 0.90 were considered the most reproducible features. Shape and first‑order radiomic features were the most reproducible features in both GTV and HR‑CTV. Our results also showed that GTV and HR‑CTV radiomic features had similar changes against preprocessing sets.

Medical images of cancer patients are usually evaluated qualitatively by clinical specialists which makes the accuracy of the diagnosis subjective and related to the skills of clinicians. Quantitative methods based on the textural feature analysis may be useful to facilitate such evaluations. This study aimed to analyze the gray level co‑occurrence matrix (GLCM)‑based texture features extracted from T1‑axial magnetic resonance (MR) images of glioblastoma multiform (GBM) patients to determine the distinctive features specific to treatment response or disease progression.

20 GLCM‑based texture features, in addition to mean, standard deviation, entropy, RMS, kurtosis, and skewness were extracted from step I MR images (obtained 72 h after surgery) and step II MR images (obtained three months later). Responded and not responded patients to treatment were classified manually based on the radiological evaluation of step II images. Extracted texture features from Step I and Step II images were analyzed to determine the distinctive features for each group of responsive or progressive diseases. MATLAB 2020 was applied to feature extraction. SPSS version 26 was used for the statistical analysis. P value < 0.05 was considered statistically significant.

Despite no statistically significant differences between Step I texture features for two considered groups, almost all step II extracted GLCM‑based texture features in addition to entropy M and skewness were significantly different between responsive and progressive disease groups.

GLCM‑based texture features extracted from MR images of GBM patients can be used with automatic algorithms for the expeditious prediction or interpretation of response to the treatment quantitatively besides qualitative evaluations.

Auditing the treatment planning system (TPS) software for a radiotherapy unit is of paramount importance in any radiation therapy department. A Plexiglas phantom was proposed to measure the ionization of 60Co high dose rate (HDR) source and compare dose points in the planning system for auditing and verifying TPS.

Auditing was performed using a Plexiglas phantom in an end‑to‑end test, and relative dose points were detected by a farmer‑type ionization chamber and compared with the relative dose of similar points in TPS. The audit results were determined as pass optimal level (<3.3%), pass action level (between 3.3% and 5%), and out of tolerance (>5%).

The comparison of the collected data revealed that 80% of the measured values were ≤5% in the pass level, and 20% of the points were out of tolerance (between 5% and 6.99%).

This study documented the appropriateness of the dosimetry audit test and this phantom design for the HDR brachytherapy TPS.

Recently, magnetic resonance imaging (MRI)-based radiotherapy has become a favorite science field for treatment planning purposes. In this study, a simple algorithm was introduced to create synthetic computed tomography (sCT) of the head from MRI.

A simple atlas-based method was proposed to create sCT images based on the paired T1/T2-weighted MRI and bone/brain window CT. Dataset included 10 patients with glioblastoma multiforme and 10 patients with other brain tumors. To generate a sCT image, first each MR from dataset was registered to the target-MR, the resulting transformation was applied to the corresponding CT to create the set of deformed CTs. Then, deformed-CTs were fused to generate a single sCT image. The sCT images were compared with the real CT images using geometric measures (mean absolute error [MAE] and dice similarity coefficient of bone [DSCbone]) and Hounsfield unit gamma-index (THU) with criteria 100 HU/2 mm.

The evaluations carried out by MAE, DSCbone, and THU showed a good agreement between the synthetic and real CT images. The results represented the range of 78-93 HU and 0.80-0.89 for MAE and DSCbone, respectively. The THU also showed that approximately 91%-93% of pixels fulfilled the criteria 100 HU/2 mm for brain tumors.

This method showed that MR sequence (T1w or T2w) should be selected depending on the type of tumor. In addition, the brain window synthetic CTs are in better agreement with real CT relative to bone window sCT images.

Bifunctional radiosensitizer agents in which nitroaromatic moieties are attached through a linker to antineoplastic moieties have demonstrated higher cytotoxicity and radiosensitizer effects than the corresponding counterparts. This study was conducted to investigate the cytotoxicity and radiosensitizer activities of 2, 4-dinitrobenzene as a radiosensitizer moiety which connected to α, β unsaturated aryl ketones against the radioresistant human HT29 colon cancer cells. A series of bifunctional radiosensitizer derivatives that are composed of electron-affinic 2, 4-dinitrophenyl moiety and thiol reactive unsaturated conjugated ketones were prepared. The designed compounds were synthesized by the reaction of the corresponding 2, 4-dinitrobenzaldehyde, cyclohexanone and different aryl aldehydes. The cytotoxicity and radiosensitizer activity of the tested compounds were examined against HT29 colon cancer cells under aerobic condition. The IC50 value of the tested compounds and percent of survival cells were analyzed by the MTT assay. The clonogenic assay was used to assess the cell viability following treatment with the tested compounds with or without the combination of radiation. This approach demonstrated that the tested compounds at the concentrations utilized have little or no cytotoxicity towards the radioresistant HT29 cell line but have great cytotoxicity and radiosensitizer activity when combined with irradiation. The novel bifunctional unsaturated conjugated aryl ketones which are thiol alkylators found to exhibit radiosensitivity activity. Consequently, these new developed compounds should be evaluated further to assess their potential efficacy with radiotherapy to combat malignancies in a pre-clinical animal model.

Lead has been widely used for internal shielding. It is recognized as a toxic material that pollutes the environment and does not fit well the patient's body because of its inflexibility. In this study, a specific combination (70% W, 18.61% Ni, and 11.39% C) of lead-free and flexible putty metal was introduced and validated for internal shielding by Monte Carlo study. To evaluate the putty metal for internal shielding by Monte Carlo study, Varian 2100 C/D was validated within measurements. Then, using the given energy spectrum of Varian 2100 C/D, the shield thickness and backscatter factors were calculated by Monte Carlo study and compared with those of lead. The results showed that this putty metal shield required a comparable thickness compared to lead for providing protection. In addition, it is nontoxic and flexible and can be easily cut. Internal shielding with high atomic number materials causes dose enhancement that is not taken into account in treatment planning systems. This study showed that this composition as an internal shield causes 5% - 7% reduction in electron backscatter factor compared to lead. It can be concluded that lead can be replaced by putty metal with a specific combination (70% W, 18.61% Ni, and 11.39% C). It is lead-free and flexible and its required thickness for protection is acceptable under clinical condition. It causes a 5% - 7% reduction in electron backscattering.

Nowadays, magnetic resonance imaging (MRI) in combination with computed-tomography (CT) is increasingly being used in radiation therapy planning. MR and CT images are applied to determine the target volume and calculate dose distribution, respectively. Since the use of these two imaging modalities causes registration uncertainty and increases department workload and costs, in this study, brain synthetic CT (sCT) and synthetic MR (sMR: sT1w/sT2w) images were generated using Atlas-based method; consequently, just one type of image (CT or MR) is taken from the patient. The dataset included MR and CT paired images from 10 brain radiotherapy (RT) patients. To generate sCT/sMR images, first each MR/CT Atlas was registered to the MR/CT target image, the resulting transformation was applied to the corresponding CT/MR Atlas, which created the set of deformed images. Then, the deformed images were fused to generate a single sCT/sMR image, and finally, the sCT/sMR images were compared to the real CT/MR images using the mean absolute error (MAE). The results showed that the MAE of sMR (sT1w/sT2w) was less than that of sCT images. Moreover, sCT images based on T1w were in better agreement with real CT than sCT-based T2w. In addition, sT1w images represented a lower MAE relative to sT2w. The CT target image was more successful in transferring the geometry of the brain tissues to the synthetic image than MR target.

Radiotherapy in one of the main methods of tumor treatment and control. Today, the integrated radiation therapy-MRI systems have been developed. The magnetic fields of imaging systems can have effects on dose distribution in target volume. Therefore, the aim of this study was to investigate the effect of magnetic fields on dose distribution in radiation therapy.
This is a review article which was done through searching the google scholar and PubMed data bases by expressions: radiation therapy and magnetic field photon therapy and magnetic field, electron therapy and magnetic field, proton therapy and magnetic field. Related research papers were sorted and their results were summarized.
Magnetic fields can change the path of charged particles in the medium can enforce the primary charged particles, secondary electrons and positrons to experience a spiral path, if applied perpendicular to beam axes which leads to produce a peak dose. Longitudinal magnetic field decreases the penumbra and lateral deflection of electrons.
Magnetic fields influence the dose distribution in radiotherapy and modification of treatment plan is essential when applying integrated MRI-radiation therapy systems. Also, applying an intensity controlled transverse magnetic field can be an inexpensive approach to adjusting the maximum dose of charged particles in tumor volume while protecting normal tissues.

The percent depth dose (PDD) is used as a dose calculation method in radiation therapy. PDD varies with source surface distance (SSD) according to inverse square law. In most clinical situations it is necessary to convert standard SSD to that must be used in practice. Therefore; this research proposes a new factor to calculate PDD corresponding to SSD.
Electa compact accelerator for 6MV photon beam, Scanditronix blue phantom the size of 50 ×50 ×50cm3 and two ionization chamber with sensitive volume of 0.13 cc were used. Dosimetry was done for field sizes: 8×8, 10×10¡ 6.4×6.4 cm2 at SSD=80 and 100cm. Ks0 and Ks taking into account the collimator scatter, for fields in the standard SSD and the practical SSD, respectively. The ratio of Ks0 and Ks was applied to correct the Maynord F factor.
This corrected Maynord F factor revealed better results for 8×8 and 10×10 cm2 fields further more the corrected Maynord F factor, reached the more precision in buildup region for the 6.4×6.4 cm2 field for PDD calculations.
For small fields with less collimator scatter, Maynord F factor method is more accurate to calculate PDD changes with varying SSD. Although, for large depths or SSDs, using the corrected Maynord F factor will show better results for calculating PDD variations with SSD changes.

In the recent decades the Monte Carlo simulation in the field of radiation therapy has been used a lot. In this study photon contamination of Lead free and flexible internal shield was investigated in the electron field.
Subjects and To evaluate photon contaminationthat caused by this new material in the electron field by Mont Carlo study. Varian 2100 C/D was validated within measurement and then by given energy spectrum shield thickness and photon contamination were obtained and compared to those of lead.
The results show that this material required 1.2 times thickness of those leadfor providing protection. In addition, it causes photon contamination approximately equal those of lead for energy 6 and 9 MeV.
It can be concluded that thickness of specific combination (70% W, 18.61% Ni, 11/39% c) which has equal transmission same as lead caused photon contamination equal those of Lead that it can be acceptable under clinical condition.

The output factor is an important factor in calculating the dose in the treatment of cancer patients with Megavoltage photon beams. Since in the most cases, the treatment field is asymmetric, therefore the calculation of this factor is very important.
Subjects and Measurements were done for Siemens Primus Plus accelerator, Golestan Hospital using Farmer ionization chamber 6cc. The output factors was measured for the symmetric fields and calibration curves were plotted for collimator scatter factor. For each symmetric square field, many asymmetrical fields were defined. Using analysis and comparison with measured values for Siemens Linac, a tuning parameter was calculated for each symmetric square field. Then, the tuning parameter was applied to calculate collimator scatter factor for the corresponding asymmetric field. Finally the calculation results were compared with experimental values.
The tuning parameter for the two symmetric fields 30 × 30 and 25 × 25 were 0.452±0.001 and 0.481 ± 0.001 respectively. Taking into account this tuning parameter for asymmetric fields, the maximum percentage difference between calculated and measured results, was 0.7%.
Results indicate a reduction in the asymmetric collimator scatter factor relative to the symmetrical field. In addition, with increase in distance from center of symmetry, it becomes more significant decline. Also according to the calibration curve, the equivalent square field side for every asymmetric field can be obtained.

Access to appropriate images of fetal brain can greatly assist to diagnose of probable abnormalities. The aim of this study was to compare the suitability of T2-True Fast Imaging with Steady State Free Precession (T2-TRUFI), and T2-Half-Fourier Acquisition Single-Shot Turbo Spin-Echo (T2- HASTE( magnetic resonance imaging (MRI) to extract the fetal brain using the level set algorithm.
T2-TRUFI and T2-HASTE MRI of the uterus were performed. The fetal brain was cropped from the image manually, with an adequate margin of maternal tissues; and then the fetal brain was extracted using level set. The outcome was statistically analyzed to compare its success, error (Sensitivity and Specificity), and similarities (Dice and Jaccard), with those of images obtained by radiologist.
The mean values of statistical tests to evaluate the similarity (Dice and Jaccard) and the success and error (Sensitivity and specificity) between two T2-TRUFI and T2-HASTE were calculated as 97.35%, 94.98%, 95.88%, 95.88%, 99.45%, 91.10%,83.82%, 86.44% and 99.11%, respectively. However, the results from two images showed high scores to extract the fetal brain, but images from the T2-HASTE technique resulted in better visually output.
Based on our results, the T2-HASTE MR images are preferred to fetal brain extraction by level set algorithm compared to the T2-TRUFI MR images.

Rapid dose assessment using biological dosimetry methods is essential to increase the chance of survival of exposed individuals in radiation accidents.
We compared the expression levels of the FDXR and RAD51 genes at 6 and 18 MV beam energies in human peripheral blood lymphocytes. The results of our study can be used to analyze radiation energy in biological dosimetry.
For this in vitro experimental study, from 36 students in the medical physics and virology departments, seven voluntary, healthy, non-smoking male blood donors of Khuzestan ethnicity with no history of exposure to ionization radiation were selected using simple randomized sampling. Sixty-three peripheral blood samples were collected from the seven healthy donors. Human peripheral blood was then exposed to doses of 0, 0.2, 0.5, 2, and 4 Gy with 6 and 18 MV beam energies in a Linac Varian 2100C/D (Varian, USA) at Golestan hospital in Ahvaz, Iran. After RNA extraction and cDNA synthesis, the expression levels of FDXR and RAD51 were determined 24 hours post-irradiation using the gel-purified reverse transcription polymerase chain reaction (RT-PCR) technique and TaqMan strategy (by real-time PCR).
The expression level of FDXR gene was significantly increased at doses of 2 Gy and 4 Gy in the 6 - 18 MV energy range (P The data suggest that the expression analysis of the FDXR gene, contrary to that of the RAD51 gene, may be suitable for assessment of high-energy X-ray. In addition, RAD51 is not a suitable gene for dose assessment in biological dosimetry.

The source point of the irradiated electron beam must be considered to estimate the output factor and dose distribution during electron therapy.
The aim of this study was to determine the effective source-surface distance (SSDeff) of an electron linear accelerator (Linac), and its dependence on energy and depth.
A Varian Linac 2100CD with electron energies of 4, 6, 9, 12, and 15 MeV, electron applicator size of 20 × 20 cm2, nominal SSDs of 97 to 113 cm, and air gaps of 2 to 18 cm were studied. Using a Farmer (0.13 cc) ionizing chamber, the percentage depth doses were measured in the water phantom (50 cm3) and then the SSDeff was calculated by applying the inverse square law.
For the 100% PDD, the SSDeff values were calculated as 79, 91, 92, 93, and 92 cm for 4, 6, 9, 12, and 15 MeV, respectively. At a depth with a certain PDD, increasing energy also increases SSDeff, and a similar increase is observed at a distinctive energy by increasing the PDD.
Using the maximum dose depth from PDD curves and the inverse square law, the required SSDeff to calculate the dose distribution of the electron beam can be calculated.

Recently, electron beams are widely used for superficial and skin lesions treatment. Since the accelerator devices, create electron beams with finite energies, the treatment area is limited to a specified depth. The objective of this study was to assess the effect of combination of electron beams with different energies and different contribution in order to produce PDD curve with a more uniform central dose distribution for the shallow superficial lesions treatment (up to cm 3). The results were compared with using a single electron beam energy. Subjects and at First, percentage depth dose (PDD) was measured by using a Varian accelerator devices for energies 4, 6, 9, 12 Mev of electron beams and field size 20 × 20 cm2. Then, PDD curve was converted to dose curve. 4 Gaussian function was fit to dose curve using MATLAB software and the best combination was selected by combining different proportions of each of this functions. percentage of the build up dose increased to about %90-%95 maximum dose and also percentage of the surface dose increases to about %90 maximum dose by combination of electron beams with different energies and different contribution. low energy electron beams Combination cause optimization of the dose distribution PDD curve. Furthermore, we can produce electron beam with arbitrary energy by this method.

Mean inactivation dose is a useful radiobiological parameter for the comparison of human cell survival curves. Given the importance and accuracy of these parameters, in the present study, the radio sensitivity enhancement of colon cancer (HT-29) cells in the presence of gold nanoparticles (GNPs) were studied using the mean inactivation dose (MID). Naked-GNPs with 50 nm diameters were incubated with HT-29 cells. The cytotoxicity and uptake of these particles on HT-29 cells were assessed. After determining the optimum GNPs concentration, the cells were incubated with gold nanoparticle for 24 hours. The change in the MID value as well as the radio sensitization enhancement under irradiation with 9 MV X-ray beams in the presence of GNPs were evaluated by multiple (3-(4, 5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium)MTS assay. Cell survival in the presence of GNPs was more than 90% and the maximum uptake of GNPs was observed at 60 µM of gold nanoparticles. In contrast, in the presence of GNPs combined with radiation, cell survival and MID value significantly decreased, so that the radio sensitization enhancement was 1.4. Due to the significant reduction in the mean inactivation dose of colon cancer cells in the presence of gold nanoparticles, it seems that GNPs are suitable options to achieve a new approach in order to improve radiotherapy efficiency without increasing the prescribed radiation dose.

Quality control techniques used to test the components of the radiological system and verify that the equipment is operating satisfactorily. In this study, quality control (QC) assessment of conventional radiology devices was performed in frequently visited radiology centers of Khuzestan province, Iran. Fifteen conventional radiology devices were examined, based on the protocol proposed in Report No. 77 by the Institute of Physics and Engineering in Medicine (IPEM). Ten standard QC tests, including voltage accuracy and reproducibility, exposure time accuracy and reproducibility, tube output linearity (time and milliampere), filtration (half-value layer), tube output (70 kV at FSD =100 cm), tube output reproducibility and beam alignment were performed and assessed. All measurements were performed, using Barracuda multi-purpose detector. Thereproducibility of voltage, exposure time and dose output, as well as output linearity, met the standard criteria in all devices. However, in 60% of the units, the results of the beam alignment test were poor. We also found that 66.7% of the studied units offer services to more than 18,000 patients annually or 50 patients per day. Despite the fact that radiological devices in Khuzestan province are relatively old with high workload, the obtained results showed that these devices met the standard criteria. This may be mainly related to proper after-sale services, provided by the companies. Although these services may be expensive for radiology centers, the costs may be significantly reduced if QC is defined as a routine procedure performed by qualified medical physicists or radiation safety officers.

There are several techniques for the treatment of whole skins in the standing position of patient, in spite of some patients can not able to stand. In addition, most of these techniques require a large treatment rooms, but there is no problem in translational technique. In this study we try to design transfer technique for treating skin on the linear accelerator in Ahvaz Golestan hospital. Subjects and Measurements were made at radiation oncology department of Ahvaz Golestan hospital by Varian linear accelerator with 10×10 cm2 applicator, using a parallel plate dosimeter and water phantom with dimensions 20 × 30 × 30 cm3 at 4 and 6 Mev energies. Water phantom moves in front of electron sources to get a steady dose by all points at Zref depth. We could obtain a couch rate of displacement under the applicator. Dosimetry results showed that if the couch moves symmetricallyas mach as equal to twice the length of the applicator plus the length of the phantom, all the points at any depth will get nearly a uniform dose. The results revealed that the technique a good one to deliver a uniform dose to all points of the phantom at Zref.

There are some complications for TMR(Tissue Maximum Ratio) calculation in radiotherapy treatment planning system. For example in TMR calculation from PDD(Percentage depth dose), BSF(Back Scatter Factor) or measurement is difficult and inaccurate. The goal of this study was to calculate TMR from PDD in absence of scatter factors and its conversion to a computer software for promotion of treatment planning system(TPS) in radiotherapy departments. Subjects and Measurements of PDD was performed using Varian and Siemens accelerators at Ahvaz Golestan Hospital radiotherapy department, by ionization chamber CC13 and scanditronix water phantom, for both 6 and 18 MV therapeutic energies. Then an analytical equation for TMR calculation from PDD was defined. By this equation TMR values for square fields in different depths for 2-50MV therapeutic energies were determined. Finally we employed MATLAB software to produce a computer program to be used in TPS software. There was a good agreement between TMR values calculated by presented analytical equation and TMR values in the BJR(British Journal of Radiology) report. In most energies for therapeutic fields, the difference between analytical and BJR values was less than 2% for depths up to 10 cm, and less than 4% for depths up to 20cm. The presented analytical equation can quick calculate and quantify TMR for all therapeutic fields and energies.

Background and Electron beams are extensively used in superficial cancer therapy. Unlike photon therapy, percentage depth dose curves (PDD) do not specify a maximum point in high energy electron therapy and include a depth range. The aim of this study was to obtain PDD by an analytical formulation which can be helpful to acquire the size of treatment area and other treatment factors such as the depth of maximum dose, the most probable energy (Ep), practical range (Rp), and the depth which the dose reaches the peak of 50% (R50). Measurements of PDD was done in radiotherapy department of Ahvaz Golestan Hospital for Siemens Primus Plus accelerator, using ionization chamber CC13 dosimeter and a water phantom for energies of 6, 9, 12, 15, 18, and 21 MeV and different field sizes at various depths. Data was analyzed using MATLAB 7.8 software. The function which had the best fit to the data was selected. This function was used to calculate the, Rp and R50 points for each field size and energy. The dependence of these points to field size and energy were also obtained. We found that 4 Gaussian function had the best fit to the data (). Rapid dose reduction was observed after a relatively flat area. The curve had a sequence that was due to photon contamination. R50and Rp points obtained for the various fields and energies by analytical function revealed that R50 depends on the field size and energy. But, Rp depends only on energy and does not depend on field size variation.

In radiation therapy,depending on the depth and type of the tumor, photon or electron irradiation is often applied to treat malignant tumors. Normal tissues are inevitably damaged, due to dose absorption. Since the electron beam is deviated by the magnetic field, it could be applied to detour the dose deposition from normal tissues to tumor mass. Subjects and 15MeV electron beam, produced with Varian clinac 2100C/D, was used in this research. Relative dosimetry process was done by use of 3D water phantom and CC13 cylindrical chamber. This process was accomplished with and without magnetic field. After use of magnetic field the surface dose was increased. In addition the dose in buildup region had incremented.the depth of maximum dose in presence of magnetic field had a shift rather than without magnetic field. The penumbra had increment in some directions and decrease in some other directions. By applying the magnetic field one can produce a region with increased and adjacent to a lower dose. It seems that arrangement of this region over normal and tumoral region can lead to increase in the efficiency of radiation therapy.

Magnetic fields are capable of altering the trajectory of electron beams andcan be used in radiation therapy.Theaim of this study was to produce regions with dose enhancement and reduction in the medium. The NdFeB permanent magnets were arranged on the electron applicator in several configurations. Then, after the passage of the electron beams (9 and 15 MeV Varian 2100C/D) through the non-uniform magnetic field, the Percentage Depth Dose(PDDs) on central axis and dose profiles in three depths for each energy were measured in a 3D water phantom. For all magnet arrangements and for two different energies, the surface dose increment and shift in depth of maximum dose (dmax) were observed. In addition, the pattern of dose distribution in buildup region was changed. Measurement of dose profile showed dose localization and spreading in some other regions. The results of this study confirms that using magnetic field can alter the dose deposition patterns and as a result can produce dose enhancement as well as dose reduction in the medium using high-energy electron beams. These effects provide dose distribution with arbitrary shapes for use in radiation therapy.

For dose measurement in Megavoltage (MV) photon beams with ion chambers, the effect of volume occupied by the air cavity is not negligible. Therefore, the result of measurement should be corrected with a displacement perturbation correction factor (Pdis) or using an effective point of measurement (EPOM). The aim of this study is to calculate the EPOM for cylindrical ion chamber and to evaluate the fixed EPOM that was recommended by standard dosimetry protocols. Percent depth doses (PDDs) for 6 MV and 18 MV were measured with two types of chambers for different depths and field sizes. The EPOM was calculated using results obtained from measurement data for two types of chambers, comparison of the readings, and using dosimetry, mathematical, and statistical consideration. For displacement correction factor 12∆r''> =0, 12∆r''> = 0.6r and different 12∆r''>, the minimum standard deviations ratio (SDRs) were calculated at several depths and field sizes. Maximum level of SDRs was about 0.38% and 0.49% (when assuming variable 12∆r''>) for 6 MV and 18 MV, respectively (which was less than 0.5% and acceptable). This quantity was greater than one (for assuming 12∆r''> = 0.6r) and greater than 2 when there was no shift (12∆r''> =0) The results show that the recommended shift for cylindrical ion chamber in dosimetry protocols (upstream of 0.6r) is not correct and using a fixed value for the EPOM at all photon beam energies, depths, and field sizes is not suitable for accurate dosimetry.

The dose distribution plays an importance role in the commercial treatment planning algorithm; therefore، a patient-independent model is needed to calculate the modulated profile. A general formula has been proposed to calculate the profiles for any field shape in the wedged and non-wedged، blocked and non-blocked fields under symmetry and asymmetry photon beam conditions. Percentage depth doses (PDDs) are measured for a large number of square fields for both energies، and 45º wedge. Accessing the accuracy of the model for profile prediction، several measurements were carried out for some special treatment fields. Comparison between measurements and calculation showed that using the γ-index in the range of 3% error in the low dose gradient region and a 3mm isodose shift in the penumbra region. This analytical approach provides a general fast and accurate algorithm to calculate dose distribution for any treatment field in conventional radiotherapy.

The aim of radiotherapy is deliver enough doses to the tumor and protecting organs at risk that are around the tumor. In order to get appropriate dose distribution one can use radiation modifiers and compensators. So knowing attenuation coefficient of absorbers is necessary for treatment calculations. In this study mercury was introduced as suitable material for radiotherapy shielding. A new method is presented by physics of interaction of photon with matter and complex integration calculation to calculate attenuation coefficient for any material with different thickness and energies. Radiation quality changes with thickness of absorber and this variation were significant. However، changes in field sizes did not produce any significant variations. Variation of attenuation coefficient with thickness of absorber can not be ignored and it is necessary for accurate radiotherapy treatment. On the other hand using one number as an attenuation coefficient for radiotherapy compensators or modifiers is not accurate enough and variation of attenuation coefficient with thickness of absorber must be considered in radiotherapy treatment planning systems.