Monte Carlo study on beam hardening effect of physical wedges

Physical wedges are still widely used as beam modifiers in external beam radiotherapy. However the presence of them in the beam trace may cause beam hardening which may not be considered in many treatment planning systems. The aim of this study is to investigate the beam hardening effect generated by physical wedges via different beam quality indexes as photon spectrum, half value layer, mean energy and tissue-phantom ratio. The effect of physical wedges on the photon beam quality of a 6-18MV Varian 2100C/D accelerator was studied with the BEAMnrc Monte Carlo code. Good agreements were obtained between measured and calculated depth doses and beam profiles for open and wedged photon beams at both energies. It was noticed that for 6 MV photon beams, physical wedges have more significant effects on beam quality than for 18 MV. Also it was obtained that at 18 MV photon beam as the wedge angle increased, the effect of wedge on beam quality becomes reversed and beam softening occurred. According to these results, it is recommended that beam hardening and softening of physical wedges should be considered in treatment planning systems in order to increase the accuracy in dose delivery

Comparison of conventional and 3D conformal treatments using linac energies for prostate cancer

To evaluate the dosimetric difference between conventional and three-dimensional conformal Radiotherapy [3D-CRT] using 6 and 18 MV X-ray photons. Computed tomography scans of 26 pelvic patients were acquired and transferred to the 3D treatment planning system. For each patient, 8 Conventional plans [3, 4, 5 and 6 Fields] and one 3D-CRT plan were prepared using 6 and 18 MV photon energies. The minimum dose [D[min]], maximum dose [D[max]] and mean dose [D[mean]] to target [PTV] and organs at risk [OAR], Integral dose, Homogeneity Index and Conformity Index were compared for each plan. Also, Experimental measurements were performed using farmer ionization chamber on a patient based pelvic phantom. On Average, six-field [6F1] plans, offer minimum dose to critical organs and sufficient dose to prostate. Increasing the beam energy lead to a decrease in D[mean] of the bladder and femoral heads, as well as D[max] of PTV. The CI and ID were decreased by 4% and 11% respectively with increasing the energy and the number of beams. Experimental measurements were also in good agreement with calculations. 3D-CRT reduced D[mean] of bladder, rectum and femoral heads and also CI and ID were significantly improved by 44.6% and 30.8%, respectively. Increasing the photon energy and number of beams, improve the treatment parameters of bladder, femoral heads and PTV, except the rectum. 3D-CRT offered the most conformity in the delivery doses to the prostate while sparing dose to OARs, uninvolved structures with lower integral dose

Monte Carlo estimation of electron contamination in a 18 MV clinical photon beam

The electron contamination may reduce or even diminish the skin sparing property of the megavoltage beam. The detailed characteristics of contaminant electrons are presented for different field sizes and cases. The Monte Carlo code, MCNPX, has been used to simulate 18 MV photon beam from a Varian Linac-2300 accelerator. All dose measurements were carried out using a PTW-MP2 scanner with an ionization chamber [0.6 CC] at the water phantom. The maximum electron contaminant dose at the surface ranged from 6.1% for 5 x 5 cm[2]to 38.8% for 40 x 40 cm[2] and at the depth of maximum dose was 0.9% up to 5.77% for the 5 x 5 cm[2] to the 40 x 40 cm[2] field sizes, respectively. The additional contaminant electron dose at the surface for the field with tray increased 2.3% for 10 x 10 cm[2], 7.3% for 20 x 20 cm[2], and 21.4% for 40 x 40 cm[2] field size comparing to the standard field without any accessories. This increase for field with tray and shaping block was 5.3% and 13.3% for 10 x 10 and 20 x 20 cm[2], respectively, while, the electron contamination decreased for the fields with wedge, i.e. 2.2% for the 10 x 10 cm[2] field. The results have provided more comprehensive knowledge of the high-energy clinical beams and may be useful to develop the accurate treatment planning systems capable of taking the electron contamination in to account

Prioritizing factors effecting on human resources productivity: Viewing of middle class managers in Isfahan University of Medical Sciences-2009

Since, multidisciplinary and Combination of different factors that affecting on productivity of human resources are different in organizations due to variation in indoor and outdoor features, so the aim of this study was to prioritize the factors effecting on human resources productivity by viewing of middle level managers to make the basis of human resources management decisions to manpower planning and maintenance for reducing costs due to staff quitting. This cross-sectional and descriptive study was carried out in 2009 .Middle class managers in Isfahan University of Medical Sciences examined as a statistical population. Data collected used by researcher constructed questionnaire that validation was confirmed through interviews with faculty members and experts. Cronbach alpha was calculated for reliability [0/935]. Data analyzed by SPSS statistical software; the maximum means score was 5 and the minimum was 1. Management style with a mean score of 4.41 was the most important factors in human resource's productivity. Factors related to individuals with a mean score of 4.3, culture with a mean score of 4.1, organizational structure with a mean score of 4.06, compensation systems with a mean score of 4, courses of training with a mean score of 3.85 and environmental factors that related to physical space with a mean of 3.769 compared to other options had less consequence. Reforming employment systems and organizational structures, job enrichment, needs of educational planning, devolution to the middle levels managers and decision supervised measuring in the volume and balance with each other and further fields. Health incentive programs with job skills and staff development can be affected to increase productivity of human resources and promoting health system

Verifying the accuracy of dose distribution in gamma Knife unit in presence of inhomogeneities using PAGAT polymer gel dosimeter and MC simulation

Polymer gel dosimetry is still the only dosimetry method for direct measuring of threedimensional dose distributions. MRI Polymer gel dosimeters are tissue equivalent and can act as a phantom material. In this study the obtained isodose maps with PAGAT polymer gel dosimeter were compared to those calculated with EGSnrs for singleshot irradiations of 8 and 18 mm collimators of Gamma Knife [GK] unit in homogeneous and inhomogeneous phantoms. A custom-built, 16 cm diameter spherical Plexiglas head phantom was. Inside the phantom, there was one cubic cutout for insertion of gel phantoms, and another cutout for inserting the inhomogeneities. The phantoms were scanned with a Siemens clinical 1.5 T MRI scanner. The multiple spin-echo sequence with 32 echoes was used for the MRI scans. The results of measurement and simulation in homogeneous and inhomogeneous phantoms showed that the presence of inhomogeneities in head phantom could cause spatial uncertainty higher than +/- 2 mm and dose uncertainty higher than 7%. the presence of inhomogeneities could cause dose differences which were not in accordance with accuracy in treatment with GK radiosurgery. Moreover, the findings of Monte Carlo calculation revealed that the applied simulation code [EGSnrc] was a proper tool for evaluation of 3D dose distribution in GK unit

Comparison of measured and Monte Carlo calculated dose distributions from circular collimators for radiosurgical beams

Stereotactic radiosurgery is an important clinical tool for the treatment of small lesions in the brain, including benign conditions, malignant and localized metastatic tumors. A dosimetry study was performed for Elekta 'Synergy S' as a dedicated Stereotactic radiosurgery unit, capable of generating circular radiation fields with diameters of 1-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. The linear accelerator Elekta "Synergy S" equipped with a set of 5 circular collimators from 10 mm to 50 mm in diameter at isocentre distance was used. The cones were inserted in a base plate mounted on the collimator linac head. A PinPoint chamber and Wellhofer water tank chamber were selected for clinical dosimetry of 6 MV photon beams. The results of simulations using the Monte Carlo system BEAM/EGS4 to model the beam geometry were compared with dose measurements. An excellent agreement was found between Monte Carlo calculated and measured percentage depth dose and lateral dose profiles which were performed in water phantom for circular cones with 1, 2, 3, 4 and 5 cm in diameter. The comparison between calculation and measurements showed up to 0.5% or 1mm difference for all field sizes. The penumbra [80-20%] results at 5 cm depth in water phantom and SSD=95 ranged from 1.5 to 2.1 mm for circular collimators with diameter 1 to 5 cm. This study showed that BEAMnrc code has been accurate in modeling Synergy S linear accelerator equipped with circular collimators

Achievable accuracy in brain tumors by in vivo dosimetry with diode detectors

In vivo measurements of applied dose during radiotherapy treatment, is important to ensure accurate dose delivery to patients. Uncertainty in dose delivery should fall within +/- 5% of the prescribed dose as recommended by ICRU. Assessment of dose for radiotherapy applications performed with various types of detectors. In this study, semiconductor diodes were used which have some advantages for clinical dosimetry. The brain tumors have generally treated with two fields using SSD technique. Entrance and exit dose were measured for each patient with diodes during treatment. Entrance and exit dose measurements have converted to midline dose. Measured entrance and exit doses have compared with calculated ones and large deviations [more than 5%] have observed. A farmer ionization chamber [0.6 cm] was used as the reference dose detector and a Perspex water phantom [30 30cm2 area and thickness ranging from 5 cm to 30 cm] were used to determine calibration and correction factors. Correction factors were determined and variations more than 1% have used to obtain correct doses. Large deviations between measured and calculated for entrance [5.3%], exit [42%] and midline [47%] were detected. The difference was not found to be significant when comparing the measured entrance dose with the calculated one [p=0.696] and the measured exit dose with calculated one [p=0.643] and measured midline dose with calculated one [p=0.104]. In vivo dosimetry is very useful to check the dose delivered to the patient. A high precision obtained when the calibration and correction factors for each parameter of influence on the diode response are carefully determined and applied to convert the diode signal in the adsorbed dose. In this study, the target-absorbed doses were estimated from the measured entrance dose and the measured transmission

Comparison of MCNP4C, 4B and 4A Monte Carlo codes when calculating electron therapy depth doses

Monte Carlo simulation of radiation transport is considered to be one of the most accurate methods of radiation therapy dose calculation. There are different Monte Carlo codes for simulation of photons, electrons and the coupled transport of electrons and photons. MCNP is a general purpose Monte Carlo code that can be used for electron, photon and coupled photon-electron transport. In this study the MCNP4A, 4B and 4C have been compared when calculating electron beam doses in water. For simulating, the geometry and other parameters were the same for three codes. By choosing two energy indexing algorithm [ITS and MCNP], absorbed doses were scored in water. 10[6] Particles were followed in these three cases. MCNP4C and 4B gave different results compared to 4A when the ITS algorithm was used in 4B and 4C versions. There was a good agreement between versions 4B and 4C. For the energy spectrum, there were significant differences between these three versions in two planes. Because of new improvements in electron transport in 4C, this version is reliable for electron transport and also requires a shorter time than the two previous versions. These results, in addition to the practical measurements acquired with MCNP4B by other investigators, suggest that in electron transport the user should use the ITS indexing energy algorithm

effect of X-ray vertical angulation on radiographic assessment of alveolar bone loss

Radiographs provide unique information about the status of periodontium and permanent records of condition of bone throughout the course of the disease. Interproximal images record the distance between the Cementoenamel Junction and the crest of the interradicular alveolar bone, more accurately. The aim of this study was to investigate the effect of different vertical angles of x-ray tube [0, -10, + 10] degrees in horizontal and vertical bite wing techniques in assessment of bone loss. : One hundred and twenty bitewing radiographs from 150 interproximal surfaces from 10 dried skull were by three degrees [0, -10, +10] prepared. The level of alveolar bone loss was measured using ruler on the skulls and radiographs and data were registered. Type of bone loss was horizontal. The maximum differences between invitro and radiographic findings were +3 and -2.745 mm. In all areas, radiographic data were less than clinical diameters. In all areas, by using zero degree vertical angulation, there was less difference between mean of radiographics and clinical measurements. With changing vertical angulations, using 0, + 10, and -10 degrees, a wide range of measures [2.75-3mm] in amount of alveolar bone loss were ohtained for evaluating periodontal disease or following them up, accurate and reproducible images are necessary. In this study we concluded that zero vertical angulation degree decrease the angulation errors and reduce understimation of radiographic assessment of alveolar bone loss

Brachytherapy polymer gel dosimetry with xCT

Polymer gels are an emerging new class of dosimeters which are being applied to the challenges of modern radiotherapy modalities. Research on gel dosimetry involves several scientific domains, one of which is the imaging techniques with which dose data is extracted from the dosimeters. In the current work, we present our preliminary results of investigating capability of X-ray CT for extracting brachytherapy dose distributions from a normoxic gel dosimeter. A normoxic radiosensitive polymer gel was fabricated under normal atmospheric conditions and poured into three phantoms. Using Cs137 brachytherapy sources, the phantoms were irradiated with different dose distributions with a LDR Selectron remote after-loader. To improve SNR, 25 images were obtained of each slice for image averaging and an averaged background image of an un-irradiated gel phantom was then subtracted for artifact removal. To further improve the accuracy, a self-consistent normalized method was used for calibration of the dosimeters based on an assumption of a linear dose response between zero and maximum dose regions in the gel. Although results reveal very similar CT-number gradients to that of brachytherapy dose distributions, but the method does not fulfill brachytherapy dosimetry requirements. This might be due to the high prescribed doses in this study which in turn results in a large change in the CT numbers. This change in the CT numbers of the images can not be considered to have a linear relationship with dose which was the basic assumption of our calibration method, so the results are just qualitatively comparable. In this study, the results of using X-ray CT for brachytherapy polymer gel dosimetry is promising but not still satisfying. Improving a proper calibration method for correlating CT numbers to dose will be significantly helpful for performing measurements with CT. The main limitation for CT is still a low signal to noise ratio especially in lower dose areas

[Comparison of different computer speeds in calculating of Co[60] depth doses by MCNP4A and MCNP4B Monte Carlo codes]

Background and Objective: MCNP is a general-purpose Monte Carlo code that is used for simulating of neutrons, photons and electrons transport in different media. Recently this code has been used for radiotherapy dosimetry and treatment planning. In recent investigations, the reasonable run-time was not acquired for clinical use of Monte Carlo method. In this research, the speeds of the computers available in Iran were compared in running a percent depth dose calculation [PPD] for CO[60] teletherapy machine

Methods: Geometry of a typical CO[60] teletherapy machine and a water phantom were simulated. Both version of MCNP code were installed on Pentium 233, 866, 1500 MHz, 700 MHz Duran and Athelon 1333MHz personal computers. Percent depth dose of CO[60] gamma rays in water phantom for 10×10 cm was calculated by each computer

Findings: The time required to computer the PDD by F6 tally was 60 times greater than the F8 tally. In all the cases, the 4A version was approximately 5% faster than 4B version. This suggests that in radiotherapy application like our test problem there is not considerable computing time difference between 4A and 4B version

Conclusion: The results recommend the use of F6 tally in radiotherapy application by CO[60] gamma rays where the point of interest are not situated in electronic disequilibrium regions and when the time of calculation is important

Application of MCNP4C Monte Carlo code in radiation dosimetry in heterogeneous phantom

In treating patients with radiation, the degree of accuracy for the delivery of tumor dose is recommended to be within +/- 5% by ICRU in report 24. The experimental studies have shown that the presence of low-density inhomogeneity in areas such as the lung can lead to a greater than 30% change in the water dose data. Therefore, inhomogeneity corrections should be used in treatment planning especially for lung cancer. The usual methods for inhomogeneity correction are the Tissue-Air Ratio [TAR] method, the power low tissue-air ratio [Batho] method, and the Equivalent Tissue-Air Ratio [ETAR] method. But they are not able to calculate the dose with required accuracy in all cases. New and more accurate methods are based on Monte Carlo methods. They are able to account for all aspects of photon and electron transport within a heterogeneous medium. The focus of this paper is the application of MCNP [Monte Carlo N-Particle] code in radiotherapy treatment planning. Materials and methods: Some special test phantoms were made of cork and Perspex instead of lung and normal tissue respectively [with electron densities relative to water equal to 0.2 and 1.137 respectively]. Measurements were obtained using cobalt-60 radiation for four different fields. Then the results of RTAR, Batho and MCNP methods were compared to the measurements. RTAR method has an error equal to 10% approximately. Also Batho method has an error especially in the low-density material. At least, MCNP method calculates correction factors very accurately. Its average error is less than 1% but it takes a long time to calculate the dose. Monte Carlo method is more accurate than other methods and it is currently used in the process of being implemented by various treatment planning vendors and will be available for clinical use in very near future