Dosimetric aspects of physical and dynamic wedge of Clinac 2100C linear accelerator

AIM: To investigate variation of wedge factors on field size and depth for physical and dynamic wedges of identical wedge angles for Clinac 2100C linear accelerator and its clinical implementation. MATERIAL AND METHODS: A computer controlled water phantom dosimetric system is used to generate profile data for physical wedges, whereas a 0.6 cm3 ion chamber is used for generation of profiles for dynamic wedge and wedge factors for both types of wedges. The method has been discussed to handle the dynamic wedge dosimetry in absence of linear array of detectors or film densitometer. RESULTS: A systematic dependence on wedge factor is observed for physical wedge, with respect to depth and wedge angle but not depending on field size. Whereas dynamic wedge shows strong dependence on field size and is not systematic because the dynamic wedge is controlled by segmented treatment tables depending on field size and energy and no significant variation is observed on depth for various wedge angles. The handling of beam data in a commercially available treatment planning system is discussed and a comparison has been made for iso-doses of both types of wedges. CONCLUSION: The dynamic wedge isodose curves shows rather straight lines than physical wedge but larger hot spots at thin edge which needs careful consideration during planning.     

Performance evaluation of a diode array for enhanced dynamic wedge dosimetry

The performance of a diode array (Profiler) was evaluated by comparing its enhanced dynamic wedge (EDW) profiles measured at various depths with point measurements using a 0.03 cm3 ionization chamber on a commercial linear accelerator. The Profiler, which covers a 22.5 cm width, was used to measure larger field widths by concatenating three data sets into a larger field. An innovative wide-field calibration technique developed by the manufacturer of the device was used to calibrate the individual diode sensitivity, which can vary by more than 10%. Profiles of EDW measured with this device at several depths were used to construct isodose curves using the percentage depth dose curve measured by the ionization chamber. These isodose curves were used to check those generated by a commercial treatment planning system. The profiles measured with the diode array for both 8 and 18 MV photon beams agreed with those of the ionization chamber within a standard deviation of 0.4% in the field (defined as 80% of the field width) and within a maximum shift of less than 2 mm in the penumbra region. The percentage depth dose generally agreed to within 2% except in the buildup region. The Profiler was extremely useful as a quality assurance tool for EDW and as a dosimetry measurement device with tremendous savings in data acquisition time.     

Calculation of backscatter factors for diagnostic radiology using Monte Carlo methods

Backscatter factors were determined for x-ray beams relevant to diagnostic radiology using Monte Carlo methods. The phantom size considered most suitable for calibration of dosimeters is a cuboid of 30 x 30 cm2 front surface and 15 cm depth. This phantom size also provides a good approximation to adult patients. Three different media were studied: water, PMMA and ICRU tissue; the source geometry was a point source with varying field size and source-to-phantom distance. The variations of the backscatter factor with phantom medium and field geometry were examined. From the obtained data, a set of backscatter factors was selected and proposed for adoption as a standard set for the calibration of dosimeters to be used to measure diagnostic reference doses.     

A dual source photon beam model used in convolution/superposition dose calculations for clinical megavoltage x-ray beams

A realistic photon beam model based on Monte Carlo simulation of clinical linear accelerators was implemented in a convolution/superposition dose calculation algorithm. A primary and an extra-focal sources were used in this beam model to represent the direct photons from the target and the scattered photons from other head structures, respectively. The effect of the finite size of the extra-focal source was modeled by a convolution of the source fluence distribution with the collimator aperture function. Relative photon output in air (Sc) and in phantom (Scp) were computed using the convolution method with this new photon beam model. Our results showed that in a 10 MV photon beam, the Sc, Sp (phantom scatter factor), and Scp factors increased by 11%, 10%, and 22%, respectively, as the field size changed from 3 x 3 cm2 to 40 x 40 cm2. The variation of the Sc factor was contributed mostly by an increase of the extra-focal radiation with field size. The radiation backscattered into the monitor chamber inside the accelerator head affected the Sc by about 2% in the same field range. The output factors in elongated fields, asymmetric fields, and blocked fields were also investigated in this study. Our results showed that if the effect of the backscattered radiation was taken into account, output factors in these treatment fields can be predicted accurately by our convolution algorithm using the dual source photon beam model.     

In-water calibration of PDR 192Ir brachytherapy sources with an NE2571 ionization chamber

An ionometric calibration procedure for 192Ir PDR brachytherapy sources in terms of dose rate to water is presented. The calibration of the source is performed directly in a water phantom at short distances (1.0, 2.5 and 5.0 cm) using an NE2571 Farmer type ion chamber. To convert the measured air-kerma rate in water to dose rate to water a conversion factor (CF) was calculated by adapting the medium-energy x-ray dosimetry protocol for a point source geometry (diverging beam). The obtained CF was verified using two different methods. Firstly, the CF was calculated by Monte Carlo simulations, where the source-ionization chamber geometry was modelled accurately. In a second method, a combination of Monte Carlo simulations and measurements of the air-kerma rate in water (at 1.0, 2.5 and 5.0 cm distance) and in air (1 m distance) was used to determine the CF. The obtained CFs were also compared with conversion factors calculated with the adapted dosimetry protocol for high-energy photons introduced by T?lli. All calculations were done for a Gammamed PDR 192Ir source-NE2571 chamber geometry. The conversion factors obtained with the four different methods agree to within 1% at the three distances of interest. We obtained the following values (medium-energy x-ray protocol): CF(1 cm) = 1.458; CF(2.5 cm) = 1.162; CF(5.0 cm) = 1.112 (1 sigma = 0.7% for the three distances of interest). The obtained results were checked with TLD measurements. The values of the specific dose rate constant and the radial dose function calculated in this work are in accordance with the literature data.     

Bayesian Model Calibration Using Geotechnical Centrifuge Tests

The predicted performance using a geotechnical prediction model is expected to deviate from reality. A practical approach to assess the model error is through calibration with observed performances in physical model tests. In this paper, a Bayesian framework of model calibration using centrifuge modeling tests is proposed and the procedure of model calibration is illustrated. Two centrifuge tests conducted to investigate the performance of soil slopes under rainfall conditions are used to calibrate a coupled hydromechanical analysis model. It is found that for centrifuge tests with different levels of soil variability, the test with a smaller variability of soil properties is more efficient for model calibration. According to the concept of random field, a centrifuge model with a larger model size and accelerated to a lower acceleration is better for model calibration. When the discrepancy between the performance interpreted from the centrifuge model and the field performance is small, the improvement of the reliability estimation for a new slope is significant. However, when there is little information about the discrepancy, the reliability estimation cannot be significantly improved by the information from centrifuge modeling. The proposed procedure is shown to be able to quantify the calibration effects of centrifuge tests and may be used to achieve a more reliable calibration.     

Calibration of a mosfet detection system for 6-MV in vivo dosimetry

PURPOSE: Metal oxide semiconductor field-effect transistor (MOSFET) detectors were calibrated to perform in vivo dosimetry during 6-MV treatments, both in normal setup and total body irradiation (TBI) conditions. METHODS AND MATERIALS: MOSFET water-equivalent depth, dependence of the calibration factors (CFs) on the field sizes, MOSFET orientation, bias supply, accumulated dose, incidence angle, temperature, and spoiler-skin distance in TBI setup were investigated. MOSFET reproducibility was verified. The correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was studied. MOSFET midplane dosimetry in TBI setup was compared with thermoluminescent dosimetry in an anthropomorphic phantom. By using ionization chamber measurements, the TBI midplane dosimetry was also verified in the presence of cork as a lung substitute. RESULTS: The water-equivalent depth of the MOSFET is about 0.8 mm or 1.8 mm, depending on which sensor side faces the beam. The field size also affects this quantity; Monte Carlo simulations allow driving this behavior by changes in the contaminating electron mean energy. The CFs vary linearly as a function of the square field side, for fields ranging from 5 x 5 to 30 x 30 cm2. In TBI setup, varying the spoiler-skin distance between 5 mm and 10 cm affects the CFs within 5%. The MOSFET reproducibility is about 3% (2 SD) for the doses normally delivered to the patients. The effect of the accumulated dose on the sensor response is negligible. For beam incidence ranging from 0 degrees to 90 degrees, the MOSFET response varies within 7%. No monotonic correlation between the sensor response and the temperature is apparent. Good correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was found (the correlation coefficient is about 1). The MOSFET midplane dosimetry relevant to the anthropomorphic phantom irradiation is in agreement with TLD dosimetry within 5%. Ionization chamber and MOSFET midplane dosimetry in inhomogeneous phantoms are in agreement within 2%. CONCLUSION: MOSFET characteristics are suitable for the in vivo dosimetry relevant to 6-MV treatments, both in normal and TBI setup. The TBI midplane dosimetry using MOSFETs is valid also in the presence of the lung, which is the most critical organ, and allows verifying that calculation of the lung attenuator thicknesses based only on the density is not correct. Our MOSFET dosimetry system can be used also to determine the surface dose by using the water-equivalent depth and extrapolation methods. This procedure depends on the field size used.     

Transmission ionization chambers for measurements of air collision kerma integrated over beam area. Factors limiting the accuracy of calibration

Kerma-area product meters (KAP meters) are frequently used in diagnostic radiology to measure the integral of air-collision kerma over an area A (integral of A Kc,air dA) perpendicular to the x-ray beam. In this work, a precise method for calibrating a KAP meter to measure integral of A Kc,air dA is described and calibration factors determined for a broad range of tube potentials (40-200 kV). The integral is determined using a large number of TL dosimeters spread over and outside the nominal field area defined as the area within 50% of maximum Kc,air. The method is compared to a simplified calibration method which approximates the integral by multiplying the kerma in the centre of the field by the nominal field area Anom. While the calibration factor using the precise method is independent of field area and distance from the source, that using the simplified method depends on both. This can be accounted for by field inhomogeneities caused by the heel effect, extrafocal radiation and scattered radiation from the KAP meter. The deviations between the calibration factors were as large as +/- 15% for collimator apertures of 5-100 cm2 and distances from the source of 50-160 cm. The uncertainty in the calibration factor using the precise method was carefully evaluated and the expanded relative uncertainty estimated to be +/- 3% with a confidence level of 95%.     

Microdosimetric specification of the radiation quality of a d(48.5)+Be fast neutron therapy beam produced by a superconducting cyclotron

The proportional counter microdosimetric technique has been employed to quantify variations in the quality of a d(48.5)+Be fast neutron beam passing through a homogeneous water phantom. Single event spectra have been measured as a function of spatial location in the water phantom and field size. The measured spectra have been separated into component spectra corresponding to the gamma, recoil proton and alpha plus heavy recoil ion contribution to the total absorbed dose. The total absorbed dose normalized to the "monitor units" used in daily clinical use has been calculated from the measured spectra and compared to the data measured with calibrated ion chambers. The present measurements agree with the ion chamber data to within 5%. The RBE of the neutron beam is assumed to be proportional to the microdosimetric parameter y* for the dose ranges pertinent to fractionated neutron therapy. The relative variations in y*, assumed to be representative of variations in the RBE are mapped as a function of field size and spatial location in the phantom. A variation in the RBE of about 4% for points within and 8% for points outside a 10 cm x 10 cm field is observed. The variations in the RBE within the beam are caused by an increase in the gamma component with depth. An increase in the RBE of about 4% is observed with increasing field size which is attributed to a change in the neutron spectrum. Compared to the uncertainties in the prescribed dose, associated with uncertainties in the clinically used RBE, variation in the RBE between various tissues, and other dosimetric uncertainties caused by factors such as patient inhomogeneities, patient setup errors, patient motion, etc., the measured spatial RBE variations are not considered significant enough to be incorporated into the treatment planning scheme.     

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.     

Correction factors for water-proofing sleeves in kilovoltage x-ray beams

This paper investigates the effect of the waterproofing sleeve on the calibration of kilovoltage photon beams (50-300 kV). The sleeve effect correction factor, ps has been calculated using the Monte Carlo method as the ratios of the air kerma in an air cavity of a cylindrical chamber without the waterproofing sleeve to that with a sleeve. Three sleeve materials have been studied, PMMA, nylon and polystyrene. The calculations were carried out using the EGS4 (Electron Gamma Shower version 4) code system with the application of a correlated-sampling variance-reduction technique. The results show that the sleeve correction factor for 1-mm thick nylon and polystyrene sleeves, ps varies from 0.992 to 1.000 and from 0.981 to 1.000, respectively, for the same beam quality range. The ps factor varies with sleeve thickness, beam quality and phantom depth. No significant dependence of the ps factor on field size and source-surface distance has been found. Measurements for PMMA, nylon and polystyrene sleeves of various thicknesses have also been carried out and show excellent agreement with Monte Carlo calculations.     

Theoretical and experimental determination of phantom scatter factors for photon fields with different radial energy variation

The output factor used for monitor unit determination in radiotherapy can be divided into two factors: the head scatter factor and the phantom scatter factor. Theoretical and experimental phantom scatter factors have been compared for different beam qualities between 4 MV and 50 MV and field sizes from 5 cm x 5 cm to 30 cm x 30 cm. The theoretical data were obtained through a convolution method based on Monte Carlo calculated energy spectra and dose kernels. The calculations have been performed both for accelerators with a rather large energy variation within the field and for accelerators with a constant energy distribution in the field. Deviations between theoretical and experimental data were found to be less than 1%.     

Can the average particle section size in a metallographic plane be larger than the true average particle size in a three-dimensional microstructure?

It is shown that the average particle section size observed in a metallographic plane can be larger or smaller than the true average particle size in the three-dimensional (3-D) microstructure. The ratio of the average particle section size in a representative metallographic plane to the true average particle size in the 3-D microstructure is linearly related to the square of the coefficient of variation (CV) of the size distribution function in the 3-D microstructure. For most of the size distributions encountered in material microstructures, the average particle section size in a metallographic plane is expected to be larger than the true average particle size in the 3-D microstructure. Experimental data on hollow spherical carbon particles in a polymer matrix composite are presented to illustrate the theoretical results.     

Characterization of a sudden expansion flow chamber to study the response of endothelium to flow recirculation

In order to simulate regions of flow separation observed in vivo, a conventional parallel plate flow chamber was modified to produce an asymmetric sudden expansion. The flow field was visualized using light reflecting particles and the size of the recirculation zone was measured by image analysis of the particles. Finite element numerical solutions of the two and three-dimensional forms of the Navier-Stokes equation were used to determine the wall shear stress distribution and predict the location of reattachment. For two different size expansions, numerical estimates of the reattachment point along the centerline of the flow chamber agreed well with experimental values for Reynolds numbers below 473. Even at a Reynolds number of 473, the flow could be approximated as two-dimensional for 80 percent of the chamber width. Peak shear stresses in the recirculation zone as high as 80 dyne/cm2 and shear stress gradients of 2500 (dyne/cm2)/cm were produced. As an application of this flow chamber, subconfluent bovine aortic endothelial cell shape and orientation were examined in the zone of recirculation during a 24 h exposure to flow at a Reynolds number of 267. After 24 h, gradients in cell orientation and shape were observed within the recirculation zone. At the location of reattachment, where the wall shear stress was zero but the shear stress gradients were large, cells plated at low density were still aligned with the direction of flow. No preferred orientation was observed at the gasket edge where the wall shear stress and shear stress gradients were zero. At higher cell densities, no alignment was observed at the separation point.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Penetration Rate on Cone Penetration Resistance in Saturated Clayey Soils

In this paper, the effects of penetration rate on cone resistance in saturated clayey soils are investigated. Shear strength rate effects in clayey soils are related to two physical processes: the increase of shear strength with increasing rate of loading and the increase of shear strength as the process transitions from undrained to drained. Special focus is placed on this second effect. Cone penetration tests were performed at various penetration rates both in the field and in a calibration chamber, and the resulting data were analyzed. The field cone penetration tests were performed at two test sites with fairly homogeneous clayey silt and silty clay layers located below the groundwater table. Additionally, tests with both cone and flat-tip penetrometers in sand-clay mixtures were performed in a calibration chamber to investigate the change in drainage conditions from undrained to partially drained and from partially drained to fully drained. A series of flexible-wall permeameter tests were conducted in the laboratory for various clayey sand mixtures prepared at various mixing ratios in order to obtain values of the coefficient of consolidation, which is required to estimate the penetration rates below which penetration is drained and above which penetration is undrained. A correlation between cone resistance and drainage conditions was established based on the results of the calibration chamber and field penetration tests.     

Appropriateness of doping silumins with titanium and zirconium additives

Using a Thermo-Calc software package, isothermal and polythermal joins of the Al-Si-Zr and Al-Si-Ti phase diagrams are constructed and analyzed. Using the calculation and experimental procedures, liquidus lines for these systems are constructed at the Si content of 5 and 10%. It is shown that the addition of titanium and zirconium strongly increases the liquidus, which requires an increase in the melting and casting temperature compared with the modes usual for silumins. The nonoptimality of the compositions of some branded silumins (namely, AK8M3ch, AK8l, AK8M, AL4M, and V124), which admit up to 0.3% Ti and 0.2% Zr, is substantiated.     

Race differences in brain size.

Replies to M. Peters's (see record 1996-14159-001) comments on Rushton's (see record 1995-17150-001) reply to a critique of his work (e.g., see record 1993-40406-001) on brain size differences among human racial groups (i.e., Mongoloids, Caucasoids, and Negroids). Rushton reports on 4 independent ways of estimating brain size, and confirms that brains of East Asians average about 17 cm–3 larger than those of Europeans, whose brains average about 80 cm–3 larger than those of Africans. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Improving calibration accuracy in gel dosimetry

A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is presented which has intrinsically higher accuracy than current methods, and requires less gel. Two test-tubes of gel (inner diameter 2.5 cm, length 20 cm) are irradiated separately with a 10 x 10 cm2 field end-on in a water bath, such that the characteristic depth-dose curve is recorded in the gel. The calibration is then determined by fitting the depth-dose measured in water, against the measured change in relaxivity with depth in the gel. Increased accuracy is achieved in this simple depth-dose geometry by averaging the relaxivity at each depth. A large number of calibration data points, each with relatively high accuracy, are obtained. Calibration data over the full range of dose (1.6-10 Gy) is obtained by irradiating one test-tube to 10 Gy at dose maximum (Dmax), and the other to 4.5 Gy at Dmax. The new calibration method is compared with a 'standard method' where five identical test-tubes of gel were irradiated to different known doses between 2 and 10 Gy. The percentage uncertainties in the slope and intercept of the calibration fit are found to be lower with the new method by a factor of about 4 and 10 respectively, when compared with the standard method and with published values. The gel was found to respond linearly within the error bars up to doses of 7 Gy, with a slope of 0.233 +/- 0.001 s(-1) Gy(-1) and an intercept of 1.106 +/- 0.005 Gy. For higher doses, nonlinear behaviour was observed.     

Measurement of photon beam backscatter from collimators to the beam monitor chamber using target-current-pulse-counting and telescope techniques

Backscattered radiation (BSR) arising from field-defining collimators and entering the beam monitor chamber (BMC) may contribute to observed variations in medical linear accelerator photon beam output with collimator setting. Measuring the magnitude of such contributions for particular accelerators under specified operating conditions is therefore important when attempting to understand and model accelerator head scatter. The present work was conducted to confirm some backscatter measurements for collimating jaws reported previously and to extend these to include other accelerators and a multileaf collimator (MLC). BSR reaching the BMC from the jaws of Clinac 600C, 2100C and 2300CD accelerators and from an MLC on the 2300CD was investigated using both target-current-pulse-counting and telescope methods. Our measurements show that for the Clinac 600C BSR-dependent output variations are negligible. However, for the 2100C and 2300CD BSR-dependent relative output increased in an almost linear fashion, by up to 2.4% for 15 and 18 MV beams, and by up to 1.7% for 6 MV beams, as the field size varied from 5 x 5 cm2 to 40 x 40 cm2. The magnitude of BSR dependent upon collimator location in the head, as expected, thereby contributing to the collimator exchange effect. An earlier study at our centre using the telescope method had reported higher BSR levels. This discrepancy was resolved when corrections for telescope block and room scatter, previously assumed negligible, were made.     

Recurrence of major depressive disorder in hospitalized children and adolescents

Global studies of within-group genetic variation have revealed a tendency for some traits, but not all, to show higher heterozygosity in sub-Saharan African populations. Although excess African diversity has been interpreted as reflecting a greater "age" of sub-Saharan African populations, more recent research has shown that this excess is more likely a consequence of a larger African long-term effective population size. The observation that certain traits, particularly classic genetic markers and RFLPs, do not show this pattern has been interpreted as ascertainment bias. Here, I examine another possible factor: that excess African heterozygosity is in part a function of mutation rate. Simple equilibrium and nonequilibrium models of absolute excess heterozygosity are examined. The results indicate that there is little excess African heterozygosity for traits with low mutation rates and greater excess heterozygosity for traits with moderate to high aggregate mutation rates. Observed data are consistent with these models. Also, depending on population size and time depth, traits with high levels of mutation might show less excess heterozygosity than those with moderate to high mutation rates. Another measure of diversity, mean sequence divergence, shows an increase in excess diversity for traits with high mutation rates.