Varying Field Sizes Research Articles

SU-FF-T-306: Monte Carlo Modeling and Commissioning of a Bi-Directional Micro Multileaf

Purpose: A new bidirectional micro multileaf collimator (mMLC) from Acculeaf has been installed in our department. The purpose of this study is to commission the micro multileaf collimator system for Monte Carlo dose calculations. Materias and Methods: The mMLC is attached on a Varian 600C linac. The mMLC is able to shape the field in the two orthogonal directions with four banks of leaves. The maximum field size that is allowable with mMLC attached is 10cm × 10cm. The Monte Carlo commissioning of the system was performed in two steps. Commissioning of the 6MV beam alone and then commissioning with the mMLC on. The EGSnrc/BEAMnrc code was used to simulate the linac and the mMLC. Phase space files were scored before the linac jaws, and after the mMLC for several field sizes. When the mMLC is attached the field size defined by the linac jaws is 10cm × 10cm. Results: For the commissioning of the linac, several fields of various sizes were simulated and compared against measurement in water using ion chamber. Several fields, rectangular and irregular were simulated and compared against measurements in water when the mMLC was in place. The data collected were depth dose curves, dose profiles at various depths and output factors for different field sizes. Conclusions: Agreement between measured and calculated data was better than 1% and less than 1.0mm in the penumbra region for the linac part. With mMLC attached the agreement between measurements and calculations is within 2% and 1.5 mm in the penumbra region.

SU-FF-T-127: Comparison of Two Commercial Detector Arrays for IMRT Quality Assurance

Purpose: to evaluate the suitability of two commercial detector arrays for IMRT patient‐specific quality assurance.Method and Materials: The two detector arrays investigated are the MapCHECK diode array and the ImRT MatriXX ionization chamber array. We examined the linearity of the detector response, short‐term and long‐term reproducibility, statistical uncertainty as a function of delivered dose, and the validity of the array calibration. The dependence of the response of detectors on the field size, dose rate, and radiation energy were also studied. Finally, radiochromic EBT films were used to compare the results with those from the two detector arrays for three IMRT fields. Results: Both detector readings showed excellent agreement (<0.5%) with Farmer chamber measurement with varying field size and SSD in both 6 MV and 18 MV photo beams. While the MapCHECK showed a stable short‐term response (<0.3% variation), the MatriXX showed a continuously slow increase in reading during the one‐hour period (an increase of about 0.8%). Both detectors showed a fluctuation of about 1% during the one‐month period. The MapCHECK also showed a slightly better array sensitivity correction with all the detectors having less than 1% discrepancy and more than 90% of the detectors within 0.5% variation. On the other hand, about 60% of the MatriXX detectors showed a less than 0.5% variation and ∼8% exhibited a larger than 1% discrepancy. Using the commonly used 3%/3mm criteria, both detector arrays had more than 95% passing rate for the three IMRT fields when compared with the EBT film. Conclusion: The two detector arrays showed no field size, SSD, and energy dependent response with stability of better than 1% in most cases and are suitable for IMRT patient‐specific quality assurance. This work was supported in part by NCI grants R01‐CA‐100636.

Monte Carlo modelling of a‐Si EPID response: The effect of spectral variations with field size and position

This study focused on predicting the electronic portal imaging device (EPID) image of intensity modulated radiation treatment (IMRT) fields in the absence of attenuation material in the beam with Monte Carlo methods. As IMRT treatments consist of a series of segments of various sizes that are not always delivered on the central axis, large spectral variations may be observed between the segments. The effect of these spectral variations on the EPID response was studied with fields of various sizes and off-axis positions. A detailed description of the EPID was implemented in a Monte Carlo model. The EPID model was validated by comparing the EPID output factors for field sizes between 1 x 1 and 26 x 26 cm2 at the isocenter. The Monte Carlo simulations agreed with the measurements to within 1.5%. The Monte Carlo model succeeded in predicting the EPID response at the center of the fields of various sizes and offsets to within 1% of the measurements. Large variations (up to 29%) of the EPID response were observed between the various offsets. The EPID response increased with field size and with field offset for most cases. The Monte Carlo model was then used to predict the image of a simple test IMRT field delivered on the beam axis and with an offset. A variation of EPID response up to 28% was found between the on- and off-axis delivery. Finally, two clinical IMRT fields were simulated and compared to the measurements. For all IMRT fields, simulations and measurements agreed within 3%-0.2 cm for 98% of the pixels. The spectral variations were quantified by extracting from the spectra at the center of the fields the total photon yield (Ytotal), the photon yield below 1 MeV (Ylow), and the percentage of photons below 1 MeV (Plow). For the studied cases, a correlation was shown between the EPID response variation and Ytotal, Ylow, and Plow.

A probabilistic model for acute bystander exposure and risk assessment for soil fumigants

There is currently significant regulatory interest in estimating exposures and risks to bystanders following applications of soil fumigants. Soil fumigants are generally volatile and some of the fumigant mass will escape the field causing downwind exposures to resident populations. Regulatory agencies in the US are currently considering implementing buffer zones for many of the fumigants. These buffer zones would provide a restricted-entry zone around fumigant applications to mitigate potential exposures. This paper describes a probabilistic dispersion modeling application that has been developed to address this issue. The Probabilistic Exposure and Risk Model for FUMigants (PERFUM) employs the Gaussian dispersion algorithms developed by the US Environmental Protection Agency (US EPA). The model adapts EPA air dispersion algorithms to develop probabilistic estimates of acute exposures to bystanders following fumigant applications. The model considers the potential variability in exposures caused by differences in mass emission rates of the fumigants and the meteorological conditions following the application. The model is generalized to model exposures for fumigants with different exposure averaging times of concern to regulators (1–24 h), varying field sizes (up to 40 acres), and field dimensions. PERFUM also outputs the probabilistic exposures both based on the whole population surrounding the field and for the maximally exposed location only. The model is non-proprietary and publicly available.

Calibration of an amorphous‐silicon flat panel portal imager for exit‐beam dosimetry

Amorphous-silicon flat panel detectors are currently used to acquire digital portal images with excellent image quality for patient alignment before external beam radiation therapy. As a first step towards interpreting portal images acquired during treatment in terms of the actual dose delivered to the patient, a calibration method is developed to convert flat panel portal images to the equivalent water dose deposited in the detector plane and at a depth of 1.5 cm. The method is based on empirical convolution models of dose deposition in the flat panel detector and in water. A series of calibration experiments comparing the response of the flat panel imager and ion chamber measurements of dose in water determines the model parameters. Kernels derived from field size measurements account for the differences in the production and detection of scattered radiation in the two systems. The dissimilar response as a function of beam energy spectrum is characterized from measurements performed at various off-axis positions and for increasing attenuator thickness in the beam. The flat panel pixel inhomogeneity is corrected by comparing a large open field image with profiles measured in water. To verify the accuracy of the calibration method, calibrated flat panel profiles were compared with measured dose profiles for fields delivered through solid water slabs, a solid water phantom containing an air cavity, and an anthropomorphic head phantom. Open rectangular fields of various sizes and locations as well as a multileaf collimator-shaped field were delivered. For all but the smallest field centered about the central axis, the calibrated flat panel profiles matched the measured dose profiles with little or no systematic deviation and approximately 3% (two standard deviations) accuracy for the in-field region. The calibrated flat panel profiles for fields located off the central axis showed a small -1.7% systematic deviation from the measured profiles for the in-field region. Out of the field, the differences between the calibrated flat panel and measured profiles continued to be small, approximately 0%-2% of the mean in-field dose. Further refinement of the calibration model should increase the accuracy of the procedure. This calibration method for flat panel portal imagers may be used as part of a validation scheme to verify the dose delivered to the patient during treatment.

Pollen dispersal of oilseed rape: estimation of the dispersal function and effects of field dimension

Summary Debate continues regarding the ecological impacts of genetically modified (GM) crops and their coexistence with non‐GM crops in Europe. In this debate, quantitative predictions of gene dispersal by pollen are necessary, and as a result numerous plot‐to‐plot gene flow experiments have been performed with various crops. However, plot‐to‐plot cross‐pollination rates (CPR) depend on spatial configuration of plots, implying that (i) they are difficult to compare among experiments and (ii) functions directly fitted on CPR data are inappropriate for predictions in other spatial contexts. Modelling pollen dispersal via an individual dispersal function (IDF) circumvents these problems by accounting for spatial designs. We detail for oilseed rape how this approach can be used to both estimate an IDF from field data and predict CPR between two neighbouring fields of various sizes and shapes. Predictions were used to investigate the sensitivity of CPR to the family of IDF, the uncertainty in parameter estimates and the effects of field dimensions and isolation distances. We fitted a range of families of IDF, including several types of tails, on previously published data. The best IDF was a fat‐tailed power‐law function, meaning frequent long‐distance dispersal. The choice of IDF appeared crucial when predicting CPR between fields, occasionally being even more important than the distance between fields. Width of the source field and depth of the recipient field were next in importance. When approximated CPR were calculated without considering field dimensions, using distance between field centres gave better performance than field margins. Synthesis and applications. This study demonstrates the value of IDF for quantitative predictions of pollen flow in variable spatial configurations. A spatially explicit model of agro‐ecosystems used to define management rules for the commercial release of GM crops in Europe already employs IDF but underestimates long‐distance dispersal for oilseed rape. These new parameter estimates will refine the performance of these models. Moreover, the detailed guidelines for estimating an IDF should encourage such statistical analysis of other dispersal data, enabling comparisons of dispersal data obtained for different environments and species and providing new IDF for management models.

323 A-Si EPID image prediction for fields of various sizes and off-axis positions using Monte Carlo methods

323 A-Si EPID image prediction for fields of various sizes and off-axis positions using Monte Carlo methods

Effects of internal and external scatter on the build-up characteristics of Monte Carlo calculated absorbed dose for electron irradiation

The effects of internal and external scatter on surface, build-up and depth dose characteristics simulated by Monte Carlo code EGSnrc for varying field size and SSD for a 10 MeV monoenergetic electron beam with and without an accelerator model are extensively studied in this paper. In particular, sub-millimetre surface PDD was investigated. The percentage depth doses affected significantly by the external scatter show a larger build-up dose. A forward shifted Dmax depth and a sharper fall-off region compared to PDDs with only internal scatter considered. The surface dose with both internal and external scatter shows a marked decrease at 110 cm SSD, and then slight further changes with the increasing SSD since few external scattered particles from accelerator model can reach the phantom for large SSDs. The sharp PDD increase for the 5 cm x 5 cm field compared to other fields seen when only internal scatter is considered is significantly less when external scatter is also present. The effect of external scatter on surface PDD is more pronounced for large fields than small fields (5 cm x 5 cm field).

Effects of treatment distance and field size on build-up characteristics of Monte Carlo calculated absorbed dose for electron irradiation

Surface, build-up and depth dose characteristics of a monoenergetic electron point source simulated by Monte Carlo code MCNP4c for varying field size and SSD are extensively studied in this paper. MCNP4c (Monte Carlo N-Particle Transport Code System) has been extensively used in clinical dose simulation for its versatility and powerful geometrical coding tool. A sharp increase in PDD is seen with the Monte Carlo Modelling immediately at the surface within the first 0.2 mm. This effect cannot be easily measured by experimental instruments for electron contamination, and may lead to a clinical underdosing of the basal cell layer, which is one of the most radiation sensitive layers and the main target for skin carcinogenesis. A high percentage build-up dose for electron irradiation was shown. No significant effects in surface PDDs were modelled with different SSD values from 95 cm to 125 cm. Three depths were studied in detail, these being 0.05 mm, the lower depth of the basal cell layer; 0.95 mm, the lower depth of the dermal layer; and 0.95 cm, a position within the subcutaneous tissue. Results showed only small surface PDD differences were modelled for SSD variations from 95 cm to 125 cm and field sizes variation from the values between 5 cm and 10 cm squares to 25 cm. When the field side length is smaller than this, the surface dose shows an increasing trend by about 7% at 5 x 5 cm2.

Compensators for intensity-modulated beams

This study describes the importance of attenuator scatter in the construction of compensators. The attenuator used in this study was Lipowitz metal, commonly referred to as cerrobend. Linear attenuation coefficients of cerrobend were measured in air for different thickness of cerrobend sheets and different field sizes for a 6-MV photon beam. The magnitude of the dose contribution from photons scattered by the attenuator was measured. The variations of beam hardening and the scatter to primary ratio as a function of the thickness of cerrobend and varying field size were investigated. The compensators in this study were produced using a simple exponential attenuation model and the measured linear attenuation coefficients. It was found that the beam hardening effect was significant, and can lead to an error of 6.2% in the transmission, for 6 cm of cerrobend in the beam. The maximum scatter contribution to the measured fluence was 19.8% of the transmitted primary dose for a 20 × 20-cm 2 field size, and 6 cm of cerrobend in the beam. For a simple wedge-step compensator; there was a maximum deviation of 6% between the measured and our predicted fluence profile. For simple compensators, this deviation can be attributed to scatter.

Grating acuity with varying field size in patients and normal infants

PURPOSE To investigate the effect of field size on grating acuity assessment in normal children and pediatric ophthalmological patients. METHOD A prototype set of acuity cards was produced with three acuity steps per octave and a card size of 26 × 70cm, bearing a grating field of 26 × 35cm. Field size reduction was achieved by masking. Large fields of the prototype versus TAC-sized fields were tested in normal infants and ophthalmological patients. RESULTS Teller acuity cards (TAC) and masked prototype cards of equal field size gave identical results in normal controls and pediatric ophthalmological patients.A better response to large fields was found in all normal infants up to 7 months of age and in none older than 15 months. In patients, an effect of field size was found in decreasing numbers up to 5 years of age. The interocular acuity difference was larger when small fields were presented. CONCLUSION The effect of field size in patients is considered to be a developmental delay due to the individual history of disease. It may possibly have prognostic value for treatment.

Size distribution and dynamics of oil and gas field discoveries in petroleum basins

The size distribution of oil and gas fields within an arbitrary petroleum basin is approximated by the truncated Pareto distribution. We propose definite expressions for assessing the number and total resources of fields having any preset limits of field sizes and describe the application of simulational mathematical modeling for assessing the resource structure of a petroleum basin. The exploration filter model, which is a formal expression of oil and gas exploration strategy, is proposed. A definite expression for the filter operating in a given basin can be obtained from the history of oil and gas discoveries in that basin. The proposed concept can be used to predict the sequence of oil and gas fields of various sizes that are likely to be discovered in the basin during further exploration.

On the need for monitor unit calculations as part of a beam commissioning methodology for a radiation treatment planning system

This paper illustrates the need for validating the calculation of monitor units as part of the process of commissioning a photon beam model in a radiation treatment planning system. Examples are provided in which this validation identified subtle errors, either in the dose model or in the implementation of the dose algorithm. These errors would not have been detected if the commissioning process only compared relative dose distributions. A set of beam configurations, with varying field sizes, source-to-skin distances, wedges, and blocking, were established to validate monitor unit calculations for two different beam models in two different radiation treatment planning systems. Monitor units calculated using the treatment planning systems were compared with monitor units calculated from point dose calculations from tissue-maximum ratio (TMR) tables. When discrepancies occurred, the dose models and the code were analyzed to identify the causes of the discrepancies. Discrepancies in monitor unit calculations were both significant (up to 5%) and systematic. Analysis of the dose computation software found: (1) a coordinate system transformation error, (2) mishandling of dose-spread arrays, (3) differences between dose calculations in the commissioning software and the planning software, and (4) shortcomings in modeling of head scatter. Corrections were made in the beam calculation software or in the data sets to overcome these discrepancies. Consequently, we recommend incorporating validation of monitor unit calculations as part of a photon beam commissioning process. PACS number(s): 87.53.–j, 87.66.–a

Ionization chamber, electrometer, linear accelerator, field size, and energy dependence of the polarity effect in electron dosimetry

Plane-parallel ionization chambers are the instrument of choice for use in electron calibration and dosimetry, but these chambers may exhibit large polarity effects. This study concentrates on measuring the dependence of the polarity error at various mean energies using different linear accelerator, field size, ion chamber, and electrometer combinations. The polarity error was shown to increase for all four ionization chambers as the mean energy at depth decreased, but was always less than one percent at d(max). Polarity error dependence was also observed for similar plane-parallel chambers, varying field sizes, and different linear accelerators, but no polarity error dependence was observed for similar cylindrical chambers and different electrometers. Measurements of the polarity error can be used to develop correction factors for future measurements that will help minimize the time spent performing electron dosimetry and calibrations. These correction factors can be used to calculate the correct reading without the need to reverse the chamber bias, thus reducing the number of measurements required.

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 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 table of phantom scatter factors of photon beams as a function of the quality index and field size

Phantom scatter factors, for square fields of various sizes, have been determined at a fixed reference depth of 10 cm, separately in different institutes, for a large number of linear accelerators under the auspices of the Netherlands Commission on Radiation Dosimetry. The method used for these measurements has been described in a previous paper. The present article describes the conversion of the measured values into a comprehensive and consistent data set, that gives the phantom scatter factor as a function of field size (from 4 cm up to 40 cm) and quality index (from 0.600 up to 0.800).

A transmission electron microscopic study of the Bethany iron meteorite

The Bethany iron meteorite which is a part of the Gibeon shower is a fine octahedrite with zoned plessite fields of various sizes. The optically irresolvable microstructural details inside the plessitic fields have been studied by transmission electron microscopy, and the crystallographic relationships between the primary kamacite (α) and the parent taenite (γ), and between the α and γ particles in the coarse plessite, have been examined using electron diffraction. In the case of primary kamacite the orientation-relationship with γ was close to the Nishiyama-Wasserman relationship, whereas, for the plessitic α, the orientation-relationship with γ was close to Kurdjumov-Sachs. It was also found that the (111)γ and (110)α planes were not strictly parallel. Additionally, measurements of the composition profile through the zoned plessite have been made using STEM microanalysis technique, and related to microstructure.

Electron-beam arc therapy using a high energy betatron.

Dose distributions in electron-beam arc therapy were investigated using the electron beam from a Brown Boveri 45 MeV betatron. This variable isocenter machine is equipped with an angular speed control device that synchronizes the radiation dose rate with the rotation speed about the isocenter. An Alderson Rando phantom was used with Kodak RP/V film to measure isodensity distributions that were then converted to isodose curves. The effects of varying field size, isocenter depth, and air gap between collimator and phantom were examined, along with changes in the dose distributions produced by wax bolus in part of the arc.

Motion and vision. II. Stabilized spatio-temporal threshold surface.

The stabilized contrast-sensitivity function measured at a constant retinal velocity is tuned to a particular spatial frequency, which is inversely related to the velocity chosen. The Fourier transforms of these constant-velocity passbands have the same form as retinal receptive fields of various sizes. At low velocities, in the range of the natural drift motions of the eye, the stabilized contrast-sensitivity function matches the normal, unstablized result. At higher velocities (corresponding to motions of objects in the environment), this curve maintains the same shape but shifts toward lower spatial frequencies. The constant-velocity passband is displaced across the spatio-temporal frequency domain in a manner that is almost symmetric about the constant-velocity plane at v = 2 deg/s. Interpolating these diagonal profiles by a suitable analytic expression, we construct the spatio-temporal threshold surface for stabilized vision, and display its properties in terms of the usual frequency parameters; e.g., at low spatial frequencies, the temporal response becomes nearly independent of spatial frequency, while at low temporal frequencies, the spatial response becomes independent of temporal frequency.

Stem corrections for ionization chambers.

Ionization chambers often exhibit a stem effect, caused by interactions of radiation with air near the chamber end, or with dielectric in the chamber stem or cable. These interactions contribute to the apparent measured exposure. To determine the stem efffect for several common ionization chamber systems, exposures were measured with TLD capsules placed at the center of 60Co fields of various sizes. These exposure measurements then were repeated with various ionization chamber systems, including two Victoreen R meters (25- and 100-R chambers), a Capintec 192 dosimeter with a Farmer 0.6-cm3 probe, a PTW transit dose probe, and an EG and G IC-18 probe with a Keithley 610-B electrometer. From a comparison of TLD and ionization chamber measurements of the variation in exposure rate with field size, stem corrections for the different systems were determined within 1%.