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PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.

We present a method for condensing the photon energy and angular distributions obtained from Monte Carlo simulations of medical accelerators. This method represents the output as a series of correlated histograms and as such is well-suited for inclusion as the photon-source package for Monte Carlo codes used to determine the dose distributions in photon teletherapy. The method accounts for the isocenter-plane variations of the photon energy spectral distributions with increasing distance from the beam central axis for radiation produced in the bremsstrahlung target as well as for radiation scattered by the various treatment machine components within the accelerator head. Comparison of the isocenter energy fluence computed by this algorithm with that of the underlying full-physics Monte Carlo photon phase space indicates that energy fluence errors are less than 1% of the maximum energy fluence for a range of open-field sizes. Comparison of jaw-edge penumbrae shows that the angular distributions of the photons are accurately reproduced. The Monte Carlo sampling efficiency (the fraction of generated photons which clear the collimator jaws) of the algorithm is approximately 83% for an open 10x10 field, rising to approximately 96% for an open 40x40 field. Data file sizes for a typical medical accelerator, at a given energy, are approximately 150 kB, compared to the 1 GB size of the underlying full-physics phase space file.

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Depth-dose curve measurements and Monte Carlo simulations for a catheter-based 32P intravascular brachytherapy source wire are described. The measured dose rates were obtained using both radiochromic-dye film and an extrapolation chamber (EC). Calibrated radiochromic-dye films were irradiated at distances between 0.5 and 5 mm from the source axis in polystyrene phantoms, and scanned with high-resolution densitometers. Measurements with an automated EC with a 1 mm diameter collecting electrode were also performed at a distance of 2 mm from the source in polystyrene. The measured dose rates obtained from the film and EC were divided by the measured source activity to obtain measured values of dose rate per unit contained activity. Dosimetric calculations of the catheter-based 32P wire geometry were also obtained using several Monte Carlo codes (CYLTRAN, MCNP, PENELOPE, and EGS). The measured and calculated values of dose rate per unit contained activity are in good agreement (<10%) within the relevant treatment distances (1 to 4 mm). With carefully selected input parameters, the calculated depth-dose curves using these codes were within 5% at 4 mm depth. At greater depths the discrepancies between the codes increase. We discuss likely mechanisms for these differences.

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Monte Carlo photon-electron transport calculations have been done to derive new wall corrections for the six NBS-NIST standard graphite-wall, air-ionization cavity chambers that serve as the U.S. national primary standard for air kerma (and exposure) for gamma rays from 60Co, 137Cs, and 192Ir sources. The data developed for and from these calculations have also been used to refine a number of other factors affecting the standards. The largest changes are due to the new wall corrections, and the total changes are +0.87 % to +1.11 % (depending on the chamber) for 60Co beams, +0.64 % to +1.07 % (depending on the chamber) for 137Cs beams, and −0.06 % for the single chamber used in the measurement of the standardized 192Ir source. The primary standards for air kerma will be adjusted in the near future to reflect the changes in factors described in this work.

We report precise measurements of differential x-ray scattering cross sections in Ne and He from 11 -22 keV and develop a method for obtaining predictions of comparable accuracy (1%). The measurement of ratios (total scattering in Ne to He and Compton to Rayleigh scattering in Ne) facilitates comparison to theories. We find evidence for the need to include nonlocal exchange, electron correlation, and dynamic effects for an accurate description at low Z and conclude that no single current theory is sufficient. [S0031-9007(98)06921-X] PACS numbers: 32.80.Cy, 07.85. -mThe use of x rays has been of fundamental importance in a number of fields, from demonstrating the validity of the quantum theory of radiation [1] to determining macromolecular structures such as DNA [2]. Recent advances in experimental techniques, such as the use of modern synchrotron sources that permit accurate experiments with well-defined initial conditions, coupled with similar advances in theory, such as the development of computer codes that calculate the relativistic S-matrix, now make it possible to perform precision investigations necessary to probe details in the description of atomic x-ray scattering. Among the phenomena that have recently been investigated are the need for multipoles in describing photon-atom interactions [3] and the effects of electron correlations on atomic processes ([4], and references therein). In this Letter we describe the need for nonlocal effects in describing x-ray scattering, even at relatively high energies, and confirm the need for inclusion of electron correlation and dynamic effects. In obtaining these results, we performed the first experimental decomposition of Compton scattering from Rayleigh scattering in free atoms. We also describe experimental and theoretical methods to obtain absolute scattering cross sections at an accuracy of ഠ1%. Our results have broad implications for calculations of elastic and inelastic photon scattering from light elements, which are widely used to determine crystallographic structures and electron momentum distributions.Relativistic S-matrix calculations of elastic photon scattering have been available for some time [5]. These have mainly been tested by high energy x-or g-ray scattering on solid targets composed of heavy elements [6] where they are superior to other, simpler techniques, leading to their wide acceptance as a benchmark [7]. Only recently [8] has a similar theory been successfully applied to Compton scattering. This theory has not been as extensively tested against measurements. In both cases, the emphasis has been on an accurate description of the photon-atom interaction. The main approximations are made in the description of the atom, which is assumed to be spherically symmetric and with the electron-electron interactions included only within the independent particle (IPA) and local exchange approximations.Simpler predictions for these cross sections can be obtained by calculating form factors (FF) for Rayleigh scattering, and Compton profiles, using the impu...