PENEASY and PENEASYLINAC can simulate the considered Varian Clinacs both in an accurate and efficient manner. Fan splitting is crucial to achieve simulation results for the off-axis field in an affordable amount of CPU time. Work to include Elekta linacs and to develop a graphical interface that will facilitate user input is underway.

Plaque brachytherapy is an effective eye and vision-sparing method to treat patients with intraocular tumors. Practitioners are encouraged to use ABS-OOTF guidelines to enhance their practice.

We investigate possible usage of single-walled carbon nanotubes (SWNTs) as an efficient storage and separation device of hydrogen−methane mixtures at room temperature. The study has been done using Grand Canonical Monte Carlo simulations for modeling storage and separation of hydrogen−methane mixtures in idealized SWNTs bundles. These mixtures have been studied at several pressures, up to 12 MPa. We have found that the values of the stored volumetric energy and equilibrium selectivity greatly depend on the chiral vector (i.e., pore diameter) of the nanotubes. The bundle formed by [5,4] SWNTs (nanotube diameter of 6.2 Å) can be regarded as a threshold value. Below that value the densification of hydrogen or methane is negligible. Bundles with wider nanotube diameter (i.e., 12.2, 13.6, 24.4 Å) seem to be promising nanomaterials for hydrogen−methane storage and separation at 293 K. SWNTs with pore diameters greater than 24.4 Å (i.e., [18,18]) are less efficient for both on-board vehicle energy storage and separation of hydrogen−methane mixture at 293 K with pressures up to 12 MPa. SWNTs comprised of cylindrical pores of 8.2 and 6.8 Å in diameter (equivalent chiral vector [6,6] and [5,5], respectively) are the most promising for separation of the hydrogen−methane mixture at room temperature, with the former selectively adsorbing methane and the latter selectively adsorbing hydrogen. We observed that inside the pores of [6,6] nanotubes absorbed methane forms a quasi-one-dimensional crystal when the system has thermalized. The average intermolecular distance of such a crystal is smaller than the one of liquid methane in bulk at 111.5 K, therefore exhibiting the quasi-one-dimensional system clear compression characteristics. On the other hand, for a smaller nanotube diameter of 6.8 Å the hydrogen can enter into the tubes and methane remaining in bulk. We found that in the interior of [5,5] nanotubes adsorbed/compressed hydrogen forms a quasi-one-dimensional crystal.

No abstract

PRIMO is a self-contained user-friendly system that facilitates the Monte Carlo simulation of dose distributions produced by most currently available linacs. This opens the door for routine use of Monte Carlo in clinical research and quality assurance purposes. It is free software that can be downloaded from http://www.primoproject.net.

Improvements beyond the primitive approximation in the path integral Monte Carlo method are explored both in a model problem and in real systems. Two different strategies are studied: The Richardson extrapolation on top of the path integral Monte Carlo data and the Takahashi-Imada action. The Richardson extrapolation, mainly combined with the primitive action, always reduces the number-of-beads dependence, helps in determining the approach to the dominant power law behavior, and all without additional computational cost. The Takahashi-Imada action has been tested in two hard-core interacting quantum liquids at low temperature. The results obtained show that the fourth-order behavior near the asymptote is conserved, and that the use of this improved action reduces the computing time with respect to the primitive approximation.

A technique for accelerating the simulation of multileaf collimators with Monte Carlo methods is presented. This technique, which will be referred to as the movable-skin method, is based on geometrical modifications that do not alter the physical shape of the leaves, but that affect the logical way in which the Monte Carlo code processes the geometry. Zones of the geometry from which secondary radiation can emerge are defined as skins and the radiation transport throughout these zones is simulated accurately, while transport in non-skin zones is modelled approximately. The skins method is general and can be applied to most of the radiation transport Monte Carlo codes used in radiotherapy. The code AUTOLINAC for the automatic generation of the geometry file and the physical parameters required in a simulation of a linac with the Monte Carlo code PENELOPE is also introduced. This code has a modularized library of all Varian Clinac machines with their multileaf collimators and electron applicators. AUTOLINAC automatically determines the position of skins and the parameter values employed for other variance-reduction techniques that are adequate for the simulation of a linac. Using the adaptive variance-reduction techniques presented here it is possible to simulate with PENELOPE an entire linac with a fully closed multileaf collimator in two hours. For this benchmark a single core of a 2.8 GHz processor was used and 2% statistical uncertainty (1sigma) of the absorbed dose in water was reached with a voxel size of 2 x 2 x 2 mm(3). Several configurations of the multileaf collimator were simulated and the results were found to be in excellent agreement with experimental measurements.

The agreement between Monte Carlo simulations and experimental data proved that the evaluated Varian phase-space files for FFF beams from TrueBeam can be used as radiation sources for accurate Monte Carlo dose estimation, especially for field sizes up to 10 × 10 cm(2), that is the range of field sizes mostly used in combination to the FFF, high dose rate beams.