Monte Carlo characterization of biocompatible beta-emitting 90Y glass seed incorporated with the radionuclide 153Sm as a SPECT marker for brachytherapy applications.

A glass seed consisting of the ß--emitting radionuclide 90Y incorporated with radionuclide 153Sm as SPECT marker is proposed for potential application in brachytherapy in order to reduce the undesirable dose to healthy adjacent organs. The aim of this work is to determine the dosimetric characteristics, as suggested in the AAPM TG-60/TG-149 reports, for this seed using Monte Carlo simulation. Monte Carlo codes MCNP5, EGSnrc, and FLUKA were used to calculate the absorbed dose distribution around the seed. Dosimetric parameters, such as reference absorbed dose rate, radial dose function, and one-dimensional (1D) and two-dimensional (2D) anisotropy functions, were obtained. The computational results from these three codes are in agreement within 5.4% difference on average. The absorbed dose rate at the reference point was estimated to be 5.01 cGy h-1 µCi-1 and self absorption of YAS glass seed amounted to 30.51%. The results showed that, with thermal neutron bombardment of 5 hours in a typical flux, sufficient activity for applications in brachytherapy may be achieved. With a 5 mCi initial activity, the total dose of a YAS glass seed was estimated to be 1.38 Gy at 1.0 cm from the seed center. Comparing with gamma emitting seeds, the 90Y seed could reduce undesirable doses to adjacent organs, because of the rapid dose falloff of beta ray. Because of the high R90 value of 5.5 mm, fewer number of 90Y seeds will be required for an interstitial brachytherapy treatment using permanent implant, in comparison with other beta-emitting seeds. The results would be helpful in the development of the radioactive implants using 90Y glass seeds for the brachytherapy treatment.

An analytical model to determine interseed attenuation effect in low-dose-rate brachytherapy.

Brachytherapy treatment planning systems (BTPS) are employing the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43)-recommended dosimetric parameters of sources, which are measured in water. The majority of brachytherapy implant volumes are not homogeneous media. Particularly, an implant with multiple seeds significantly changes homogeneity of the implant volume. Heterogeneities, such as attenuation by adjacent seeds or interseed attenuation (ISA), are neglected to this day in all BTPS. The goal of this project is to determine a novel analytical method to evaluate the impact of the dose perturbations (P-value) and/or interseed attenuation effect (ISA-value). This method will be validated for low- and high-energy brachytherapy seeds such as 125I and 192Ir using Monte Carlo (MC) simulation techniques. In this analytical model, determination of dose perturbation and interseed attenuation in a multisource brachytherapy implant is based on MC-simulated 3D kernels of P-values and ISA data for single active and single dummy configurations, arranged at different distances and orientations relative to each other. The accuracy of the final model in multisource implant configurations has been examined by a comparison of the calculated P-values and ISA-values with full Monte Carlo water simulations (FMCWS). This model enabled us to determine the total perturbation and ISA values for any multisource implant, and the results are in excellent agreement with the FMCWS data. The advantage of this model to FMCWS for daily clinical application is the speed of the calculations and ease of the implementation. The new perturbation and ISA formulism have shown a better accuracy for 192Ir than 125I due to Compton scattering and its independence of the atomic number of the chemical composition of the phantom materials. The maximum difference between the ISA model and FMCWS for all cases was less than 5%. This new model can provide inputs for brachytherapy planning software to consider the ISA effect in dose calculations based on TG-43U1 algorithm. This approach is applicable for energy range of 125I to192Ir sources.

SYNTHESIS OF LIF:MG,TI NANOPARTICLES BY CO-PRECIPITATION METHOD AND EVALUATION OF INCREASING THE 5A/5 RATIO IN ALPHA AND GAMMA RADIATION.

The peak 5 in LiF: Mg, Ti includes sub-peaks 5a and 5b, which occur at the temperatures lower and higher than that of peak 5, respectively. Peak 5a in LiF:Mg,Ti occurs due to the localized recombination of trapping/luminescence center (TC/LC), in which the electron is released from the electron trap by obtaining energy from heat and recombines through the tunneling phenomenon with a hole located in the adjacent luminescence center at a distance of 3 nm. Concerning the standard TLD tablets, which are composed of micron-sized particles, the peak 5a either does not occur or appears with very low intensity, which is insignificant in terms of dosimetry. Thus, the present study focuses on synthesizing thermoluminescent nanoparticles by co-precipitation method in several stages by citing models based on the maintenance of linear behavior of thermoluminescence nanopowders up to high doses and its relationship with localized electron-hole recombination. In addition, by changing the concentration of ingredients, altering the temperature of the reaction medium and presence or absence of surfactant, nanoparticles with suitable geometric shapes were achieved. The synthesized nanopowders were irradiated with different doses of alpha and gamma, and after analyzing the glow curves, the increase in peak 5a/5 was reported as the main factor in nanodosimetry. Based on the results, the LiF: Mg, Ti thermoluminescence nanopowders can increase the 5a/5 ratio and can be used as a convenient, inexpensive and practical tool to estimate the amount of energy deposited by the beams in nanoscale.

An investigation into the dose rate and photon energy dependence of the GENA gel dosimeter in the MeV range.

PURPOSE: In the current study, the energy and dose rate dependence of a new genipin-based gel dosimeter, named GENA gel dosimeter, were investigated. METHODS: Prepared gel dosimeters exposed using a Varian clinical linac. Beam qualities of 6 and 18 MV were applied to investigate the GENA gel dosimeter's energy dependence. Furthermore, the gel dosimeters were exposed to 50, 100, 200, and 350 cGy/min dose rates, ranging from 0 to 8 Gy. The irradiated gel dosimeters were read out using a double beam UV-Visible spectrophotometer. The absorbance peak (AP) and area under spectrum (AUS) were evaluated. RESULTS: Absorbance-dose sensitivities of (8.0 ± 0.18) × 10-3 cm-1Gy-1 and (7.8 ± 0.15) × 10-3 cm-1Gy-1 were obtained for GENA gel dosimeter for 6 and 18 MV beam qualities, respectively. Results specified no significant difference (p > 0.05) between the GENA gel dosimeter's sensitivities irradiated using the two energies mentioned above. For the mentioned dose rates, AP-dose sensitivities of (8.2 ± 0.22) × 10-3, (8.1 ± 0.21) × 10-3, (8.1 ± 0.2) × 10-3 and (8.0 ± 0.18) × 10-3 cm-1Gy-1 were obtained, respectively. Results showed no significant difference (p > 0.05) between the GENA gel dosimeter's sensitivities for the investigated dose rates and energies. In addition, results revealed that when the incident photon energy and dose rate changed, there were no significant differences (p > 0.05) between the GENA gel dosimeter's dose resolution values. CONCLUSIONS: It is concluded that the response of the GENA gel dosimeter is not dependent on the energy and dose rate (p > 0.05) within the studied energy and dose rate ranges.

Modeling the time dependent distribution of a new (153)Sm complex for targeted radiotherapy purpose.

BACKGROUND: For radioimmunotherapy purposes, a chemical complex with high absorption in cancer tumor is required. New chemicals are to be examined for their concentration in tumor and healthy organs. These are labeled with ß-emitting radioisotopes to irradiate the tumor while deposited inside it. AIM: To study the capability of recently developed chemical complex in targeting cancer tumor and investigate the distribution of (153)Sm-TPTTC in rat organs as function of time. MATERIALS AND METHODS: The chemical complex - [Tris(1,10-phenanthroline)Samarium(III)] trithiocyanate was prepared and labeled with (153)Sm radioisotope. The labeled complex was injected to a population of tumor bearing mice. In 2, 4, 24, 48, 96 h after injection the animals were sacrificed and the concentration of Samarium complex was measured in various organs such as blood, heart, intestine, colon, liver, spleen, kidney, sternum and bone. RESULTS: The concentration of the radiopharmaceutical in various organs was measured at different times. The temporal behavior of biodistribution of (153)Sm-TPTTC was modeled and drawn as function of time. CONCLUSION: It is shown that (153)Sm-TPTTC is concentrated in tumor tissue and liver much more than in other organs. The variation of pharmaceutical concentration in all organs is described with summation of eight exponential terms and it approximates our experimental data with precision better than 2%.

Monte Carlo calculation of buildup factors for 50 keV-15 MeV photons in tungsten up to 15 mean free paths.

Exposure buildup factor values of tungsten for a point isotropic source in energy range 0.05-15 MeV up to 15 mfp are computed using Monte Carlo N-Particle code. Buildup factors of this study and ANSI/ANS-6.4.3 at some selected energies and penetration depths are compared. Highest discrepancies are observed for high-energy and/or high penetration depths. Highest error occurs at 15 MeV, amounting to 32%. In the energy range 2-6 MeV, our results show smaller values for all penetration depths and vice versa at higher energies.

Evaluation of dosimetric characteristics of a ternary nanocomposite based on High Density Polyethylene/Bismuth Oxide/Graphene Oxide for gamma-rays.

This research aims to investigate a ternary nanocomposite based on High Density Polyethylene/ Bismuth Oxide/Graphene Oxide (HDPE/Bi2O3/GO) at various concentrations. Solution method was used to fabricate the samples. FESEM-EDX mapping, AFM, TEM, XRD, XPS, FTIR, and TGA/DTG analyses were carried out on the samples. XRD analysis demonstrated a semi-crystalline behavior for the samples. TEM analysis exhibited a cauliflower-like structure of the material. The sample was irradiated by gamma-rays of 60Co source over the dose rate of 30-254 mGy/min and the electric current was measured as the response of the real-time dosimeter. Thus, various dosimetric characteristics were performed, namely linearity, angular dependence, energy dependence, bias-polarity, field size, and repeatability of the data. Results showed that response of the dosimeter was linear in the range of the investigated dose rate. The sensitivity of the 60 wt% Bi2O3 sample was measured as 3.4 nC·mGy-1. The angular response variation was 20% for normal beam incidence. The response of the dosimeter to assess the energy dependency was obtained as 2.2% at the radiation field of the 137Cs and 60Co beams. The dosimeter response was dependent on the bias-polarity, with maximum discrepancy of 11.1%. The dosimetry response was highly dependent upon the radiation field size. The repeatability of the dosimeter response was measured with standard deviation less than 1%. As well, the dosimeter response during the one-hour irradiation was stable with a standard deviation of 0.66%. Results showed that considering some correction factors, this material can be used for dosimetry of gamma-rays at the therapy level.

Calculation of externally applied electric field intensity for disruption of cancer cell proliferation.

It is already known that electrostatic, magnetostatic, extremely low-frequency electric fields, and pulsed electric field could be utilized in cancer treatment. The healing effect depends on frequency and amplitude of electric field. In the present work, a simple theoretical model is developed to estimate the intensity of electrostatic field that damages a living cell during division. By this model, it is shown that magnification of electric field in the bottleneck of dividing cell is enough to break chemical bounds between molecules by an avalanche process. Our model shows that the externally applied electric field of 4 V/cm intensity is able to hurt a cancer cell at the dividing stage.

The Measurement of Thyroid Absorbed dose by Gafchromic™ EBT2 Film and Changes in Thyroid Hormone Levels Following Radiotherapy in Patients with Breast Cancer.

BACKGROUND: Radiotherapy is a main method for the treatment of breast cancer. This study aimed to measure the absorbed dose of thyroid gland using Gafchromic EBT2 film during breast cancer radiotherapy. In addition, the relationship between the absorbed dose and thyroid hormone levels was evaluated. METHODS: Forty-six breast cancer patients, with the age ranged between 25 and 35 years, undergoing external radiotherapy were studied. The patients were treated with 6 and 18 MV X-ray beams, and the absorbed thyroid dose was measured by EBT2 film. Thyroid hormone levels, thyroid-stimulating hormone (TSH), triiodothyronine (T3), and thyroxin (T4), were measured before and after the radiotherapy. Pearson's, Spearman's, and Chi-square tests were performed to evaluate the correlation between the thyroid dose and hormone levels. RESULTS: The mean thyroid dose was 26 ± 9.45 cGy with the range of 7.85-48.35 cGy. There were not any significant differences at thyroid hormone levels between preradiotherapy and postradiotherapy (P > 0.05). There was a significant relationship between increased thyroid absorbed dose and changes in TSH and T4 levels (P < 0.05), but it was not significant in T3 level (P = 0.1). CONCLUSION: Regarding the results, the thyroid absorbed dose can have an effect on its function. Therefore, the thyroid gland should be considered as an organ at risk in breast cancer radiotherapy.

Evaluation of Bremsstrahlung radiation dose in stereotactically radiocolloid therapy of cystic craniopharyngioma tumors with 32P radio-colloid.

Over 90% of craniopharyngeal brain tumors are cystic, which enables the injection of beta emitters such as phosphorus-32 (32P) radio-colloid into cysts for their treatment. The aim of this study was to evaluate the clinical and theoretical modelling of Bremsstrahlung radiation dose resulting from stereotactic radio-colloid therapy of cystic craniopharyngioma tumors with 32P. 32P radio-colloid with appropriate activity concentration was injected to a head phantom, and then the Bremsstrahlung radiation spectrum and planar images were obtained using a gamma camera. Both phantom and gamma camera were simulated using MCNPX code, and the results were compared with practical results. Bremsstrahlung radiation spectrum was measured using a handheld gamma spectrometer for two patients treated with stereotactic radio-colloid therapy with 32P in different positions and compared to Monte Carlo simulation. Results of counting and determining sensitivity coefficients in the air and the attenuating environment were obtained. Also, comparing the counting sensitivity from practical and simulation methods indicated the agreement of the data between the two methods. Comparison of the spectra from different positions around patient's head indicated the ability to use this detector to quantify the activity in the operating room. Selection of the spectrum is important in Bremsstrahlung radiation imaging. We can take advantage of spectrometry measurement using gamma camera, handheld gamma spectrometer for patient, and theoretical modeling with Monte Carlo code to evaluate radiopharmaceutical distribution, leakage, as well as estimate activity and predict therapeutic effects in other adjacent structures and ultimately optimize radio-colloid therapy in cystic craniopharyngeal patients.

Cell damaging by irradiating non-thermal plasma to the water: Mathematical modeling of chemical processes.

Recently non-thermal plasma (NTP) is applied for many therapeutic applications. By NTP irradiating to the tissues or cell-lines, the water molecules (H2O) would be also activated leading to generate hydrogen peroxide (H2O2). By irradiating plasma to bio-solution, its main output including vacuum UV to UV causes the photolysis of H2O leading to generate hydroxyl (OH) molecules in couple forms with ability to convert to H2O2. Additionally, other plasma's output the oxygen atoms could also penetrate under the liquid's surface and react with H2O to generate H2O2. In NTP applications for killing unwanted-cells of microorganisms (e.g. sterilization) or cancerous tissues, the H2O2 molecule is the main reactive species for cell death via inducing DNA damage in mammalian cells. In this paper we proposed a mathematical model for NTP application describing the formation of hydroxyls in the bio solution and other subsequent reactions leading to DNA damage in vitro. The instant concentrations of the OH and H2O2, the main species for DNA oxidation were obtained and investigated in this simulation. In order to validate the model, the cellular response to NTP stimulation was compared with some experimental findings from viewpoint of DNA damage to show the significant consistency.

Extremely High Frequency Electromagnetic Fields Facilitate Electrical Signal Propagation by Increasing Transmembrane Potassium Efflux in an Artificial Axon Model.

Among the many biological effects caused by low intensity extremely high frequency electromagnetic fields (EHF-EMF) reported in the literature, those on the nervous system are a promising area for further research. The mechanisms by which these fields alter neural activity are still unclear and thus far there appears to be no frequency dependence regarding neuronal responses. Therefore, proper in vitro models for preliminary screening studies of the interaction between neural cells with EMF are needed. We designed an artificial axon model consisting of a series of parallel RC networks. Each RC network contained an aqueous solution of lipid vesicles with a gradient of potassium (K+) concentration as the functional element. We investigated the effects of EHF-EMF (53.37 GHz-39 mW) on the propagation of the electric impulse. We report that exposure to the EHF-EMF increases the amplitude of electrical signal by inducing a potassium efflux from lipid vesicles. Further, exposure to the EHF-EMF potentiates the action of valinomycin - a K+ carrier - increasing the extent of K+ transport across the lipid membrane. We conclude that exposure to the EHF-EMF facilitates the electrical signal propagation by increasing transmembrane potassium efflux, and that the model presented is promising for future screening studies of different EMF frequency spectrum bands.

Evaluating the expression of cyclooxygenase-2 enzyme by immunohistochemistry in normal and tumoral tissue before and after neoadjuvant chemoradiotherapy in patients with esophageal cancer in Khorasan Province.

BACKGROUND: Esophageal cancer is the third most common cancer in Iran. Neoadjuvant chemoradiotherapy (NCRT) is the appropriate treatment for esophageal cancer. AIM: This study investigated the expression of cyclooxygenase (COX)-2 enzyme in normal and tumoral tissues before any treatment in patients with esophageal cancer, this study also assessed the effect of NCRT on the expression of COX-2 enzyme in normal and tumoral tissue in samples derived by surgery furthermore, and this study investigated the relationship between expression of COX-2 enzyme and the pathologic tumor regression grade (PTRG) patients. MATERIALS AND METHODS: In this study, a total of 120 patients admitted to Omid Hospital, Imam Reza Hospitals, and Reza-Mashhad Medical Center, who were treated with NCRT, were recruited and the expression of the COX-2 enzyme in normal and tumoral tissues was assessed by immunohistochemistry before and after treatment by an expert pathologist between zero and 300. PTRG was determined by a pathologist after treatment. RESULTS: The mean levels of COX-2 expression, obtained from tumoral and normal tissue baseline biopsy in patients, were 177.69 and 64.29, respectively, while in surgical specimen were 177.25 and 49.84, respectively. A significant association was found between PTRG of surgical specimen and COX-2 expression in normal tissue (baseline biopsy) at diagnosis (P = 0.034). CONCLUSIONS: The results indicated that expression of COX-2 in tumoral tissues exceeds the expression of COX-2 in normal tissue of the baseline biopsy. Patients with a high expression of COX-2 in baseline tumor biopsies had less response to treatment of pathology compared to patients with lower expression of COX-2 in baseline tumor biopsies.

Response of single cell with acute angle exposed to an external electric field.

It is known that the electric field incurs effects on the living cells. Predicting the response of single cell or multilayer cells to induced alternative or static eclectic field has permanently been a challenge. In the present study a first order single cell with acute angle under the influence of external electric field is considered. The cell division stage or the special condition of reshaping is modelled with a cone being connected. In the case of cell divisions, anaphase, it can be considered with two cones that connected nose-to-nose. Each cone consists of two regions. The first is the membrane modelled with a superficial layer, and the second is cytoplasm at the core. A Laplace equation is written for this model and the distribution of its electric field is a sharp point in the single cell for which an acute angle model is calculated.

Effects of Wi-Fi (2.45 GHz) Exposure on Apoptosis, Sperm Parameters and Testicular Histomorphometry in Rats: A Time Course Study.

OBJECTIVE: In today's world, 2.45-GHz radio-frequency radiation (RFR) from industrial, scientific, medical, military and domestic applications is the main part of indoor-outdoor electromagnetic field exposure. Long-term effects of 2.45-GHz Wi-Fi radiation on male reproductive system was not known completely. Therefore, this study aimed to investigate the major cause of male infertility during short- and long-term exposure of Wi-Fi radiation. MATERIALS AND METHODS: This is an animal experimental study, which was conducted in the Department of Anatomical Sciences, Faculty of Medicine, Zanjan University of Medical Sciences, Zanjan, IRAN, from June to August 2014. Three-month-old male Wistar rats (n=27) were exposed to the 2.45 GHz radiation in a chamber with two Wi-Fi antennas on opposite walls. Animals were divided into the three following groups: I. control group (n=9) including healthy animals without any exposure to the antenna, II. 1-hour group (n=9) exposed to the 2.45 GHz Wi-Fi radiation for 1 hour per day during two months and III.7-hour group (n=9) exposed to the 2.45 GHz Wi-Fi radiation for 7 hours per day during 2 months. Sperm parameters, caspase-3 concentrations, histomorphometric changes of testis in addition to the apoptotic indexes were evaluated in the exposed and control animals. RESULTS: Both 1-hour and 7-hour groups showed a decrease in sperm parameters in a time dependent pattern. In parallel, the number of apoptosis-positive cells and caspase-3 activity increased in the seminiferous tubules of exposed rats. The seminal vesicle weight reduced significantly in both1-hour or 7-hour groups in comparison to the control group. CONCLUSION: Regarding to the progressive privilege of 2.45 GHz wireless networks in our environment, we concluded that there should be a major concern regarding the timedependent exposure of whole-body to the higher frequencies of Wi-Fi networks existing in the vicinity of our living places.

Simultaneous production of mixed electron--photon beam in a medical LINAC: A feasibility study.

PURPOSE: The electron or photon beams might be used for treatment of tumors. Each beam has its own advantage and disadvantages. Combo beam can increase the advantages. No investigation has been performed for producing simultaneous mixed electron and photon beam. In current study a device has been added to the Medical Linac to produce a mixed photon-electron beam. METHODS: Firstly a Varian 2300CD head was simulated by MCNP Monte Carlo Code. Two sets of perforated lead sheets with 1 and 2 mm thickness and 0.2, 0.3, and 0.5 cm punches then placed at the top of the applicator holder tray. This layer produces bremsstrahlung x-ray upon impinging fraction electrons on it. The remaining fraction of electrons passes through the holes. The simulation was performed for 10 × 10, 6 × 6, and 4 × 4 cm(2) field size. RESULTS: For 10 × 10 cm(2) field size, among the punched targets, the largest penumbra difference between the depth of 1 and 7 cm was 72%. This difference for photon and electron beams were 31% and 325% respectively. A maximum of 39% photon percentage was produced by 2 mm target with 0.2 cm holes diameter layer. The minimum surface dose value was 4% lesser than pure electron beam. For small fields, unlike the pure electron beam, the PDD, penumbra, and flatness variations were negligible. CONCLUSIONS: The advantages of mixing the electron and photon beam is reduction of pure electron's penumbra dependency with the depth, especially for small fields, also decreasing of dramatic changes of PDD curve with irradiation field size.

Enhancing the properties of beam forming bolus in hyperthermia: numerical simulation and empirical verification.

In this paper we present a simulation study of the induced specific absorption rate (SAR) within the phantom produced by radiofrequency radiation from a 8 MHz capacitive applicator. The main focus of the current study is on demonstrating the beam shaping properties of the bolus system as well as its effect on controlling the therapeutic area. Different electrical conductivities and geometries of the bolus were considered in the simulation of induced SAR distributions in a muscle-equivalent model with uniform dielectric properties. To validate the presented model, we carried out a comparison between the SAR simulation results and the temperature measurements in an agar split-phantom and an excellent agreement was observed.

Computer implementation of a new therapeutic model for GBM tumor.

Modeling the tumor behavior in the host organ as function of time and radiation dose has been a major study in the previous decades. Here the effort in estimation of cancerous and normal cell proliferation and growth in glioblastoma multiform (GBM) tumor is presented. This paper introduces a new mathematical model in the form of differential equation of tumor growth. The model contains dose delivery amount in the treatment scheme as an input term. It also can be utilized to optimize the treatment process in order to increase the patient survival period. Gene expression programming (GEP) as a new concept is used for estimating this model. The LQ model has also been applied to GEP as an initial value, causing acceleration and improvement of the algorithm estimation. The model shows the number of the tumor and normal brain cells during the treatment process using the status of normal and cancerous cells in the initiation of treatment, the timing and amount of dose delivery to the patient, and a coefficient that describes the brain condition. A critical level is defined for normal cell when the patient's death occurs. In the end the model has been verified by clinical data obtained from previous accepted formulae and some of our experimental resources. The proposed model helps to predict tumor growth during treatment process in which further treatment processes can be controlled.

The effects of cutouts on output, mean energy and percentage depth dose of 12 and 14 MeV electrons.

Electron field-shaping cerrobend cutouts on the linear accelerator applicator have some effects on the output and percentage depth dose. These effects which arise from the lateral scatter nonequilibrium are particularly evident in higher energies and in cutouts with smaller radius. Dose measurements for circular, square, and triangular cutouts as well as open field was performed in a 10 × 10 cm applicator, using plane parallel type ion chamber with a 100 cm source surface distance. The Percentage Depth Doses curves were drawn and the outputs were measured for each of these cutouts. The output factors, normalized to open 10 × 10 cm field, varied between 0.891 and 0.996 depending on the energy, cutout shape, and cavity area. With the use of cutouts, R(100) shifted toward the surface. The shifts ranged from 9 to 0 mm and from 13 to 0 mm for 12 and 14 MeV, respectively, depending on the shape and cavity area. For R(90), R(80), and R(50) the ranges for observed shifts narrowed down and practically no shifts were observed for R(20). We present these changes in the form of predictive formulas, which would be useful in clinical applications.

Estimation of gamma- and X-ray photons buildup factor in soft tissue with Monte Carlo method.

Buildup factor of gamma- and X-ray photons in the energy range 0.2-2MeV in water and soft tissue is computed using Monte Carlo method. The results are compared with the existing buildup factor data of pure water. The difference between soft tissue and water buildup factor is studied. Soft tissue is assumed to have a composition as H(63)C(6)O(28)N. The importance of such work arises from the fact that in medical applications of X- and gamma-ray, soft tissue is usually approximated by water. It is shown that the difference between water and soft tissue buildup factor is usually more than 10%. On the other hand, buildup factor in water resulted from Monte Carlo method is compared with the experimental data appearing in references. It seems there is around 10% error in the reference data as well.