Collimator design for experimental minibeam radiation therapy.

PURPOSE: To design and optimize a minibeam collimator for minibeam radiation therapy studies using a 250 kVp x-ray machine as a simulated synchrotron source. METHODS: A Philips RT250 orthovoltage x-ray machine was modeled using the EGSnrc/BEAMnrc Monte Carlo software. The resulting machine model was coupled to a model of a minibeam collimator with a beam aperture of 1 mm. Interaperture spacing and collimator thickness were varied to produce a minibeam with the desired peak-to-valley ratio. RESULTS: Proper design of a minibeam collimator with Monte Carlo methods requires detailed knowledge of the x-ray source setup. For a cathode-ray tube source, the beam spot size, target angle, and source shielding all determine the final valley-to-peak dose ratio. CONCLUSIONS: A minibeam collimator setup was created, which can deliver a 30 Gy peak dose minibeam radiation therapy treatment at depths less than 1 cm with a valley-to-peak dose ratio on the order of 23%.

The effects of dose calculation resolution on dose accuracy for radiation therapy treatments of the lung. Part I. A Monte Carlo model of the lung.

PURPOSE: The purpose of this work was to create an anatomically detailed EGSnrc Monte Carlo based model of the right lung. The resulting model, called BRANCH, includes an accurate representation of the right bronchial, arterial, and venous branching networks down to a scale of 0.1 mm. The model may be varied to represent lung shape and density at any phase of the respiration cycle. METHODS: Polynomial surfaces were used to approximate the anatomic boundaries that define the right lung surface at several phases of the respiration cycle. A branching network algorithm was used to generate the bronchial, arterial, and venous trees within the anatomic boundaries. The branching networks were modeled as a series of bifurcating cylinders connected by spherical junctions. The validity of the BRANCH dose calculation was verified using an all-water version of the model. RESULTS: The geometric dimensions of the BRANCH model corresponded well with published data. The bronchial tree model contained 27 798 branches ranging from 0.02 to 0.54 cm in diameter. The arterial tree model had 27,957 branches ranging from 0.02 to 1.2 cm in diameter. The venous model tree had 26 347 branches ranging from 0.02 to 0.34 cm in diameter. A gamma analysis indicated that the all-water BRANCH Monte Carlo code produced dose distributions that agreed within 0.1 cm and 0.5% to conventional DOSXYZnrc results. CONCLUSIONS: The BRANCH model is a useful tool for performing detailed dosimetric studies within a realistic representation of the lung.

The effects of dose calculation resolution on dose accuracy for radiation therapy treatments of the lung. Part II. A comparison of dose distributions from an explicit lung model to dose distributions derived from a CT representation.

PURPOSE: Due to limitations in computer memory and computation time, typical radiation therapy treatments are calculated with a voxel dimension on the order of several millimeters. The anatomy below this practical resolution is approximated as a homogeneous region uniform in atomic composition and density. The purpose of this article is to examine whether the exclusion of anatomic structure below the practical dose calculation resolution produces deviations in the resulting dose distributions. METHODS: EGSnrc calculated dose distributions from the BRANCH lung model of Part I are compared and contrasted to dose distributions from a CT representation of the same BRANCH model for three different phases of the respiration cycle. RESULTS: The exclusion of branching structures below a CT resolution of 1 x 1 x 2 mm3 resulted in a deviation in dose. The deviation in dose was as high as 14% but was localized around the branching structures. There was no significant variation in the dose deviation as a function of either field size or lung density. CONCLUSIONS: The exclusion of explicit branching structures of the lung in a CT representation creates localized deviations in dose. To ensure accurate dose calculations, CT resolution must be increased

An enhanced HOWFARLESS option for DOSXYZnrc simulations of slab geometries.

The Monte Carlo code DOSXYZnrc is a valuable instrument for calculating absorbed dose within a three-dimensional Cartesian geometry. DOSXYZnrc includes several variance reduction techniques used to increase the efficiency of the Monte Carlo calculation. One such technique is HOWFARLESS which is used to increase the efficiency of beam commissioning calculations in homogeneous phantoms. The authors present an enhanced version of HOWFARLESS which extends the application to include phantoms inhomogeneous in one dimension. When the enhanced HOWFARLESS was used, efficiency increases as high as 14 times were observed without any loss in dose accuracy. The efficiency gains of an enhanced HOWFARLESS simulation was found to be dependent on both slab geometry and slab density. As the number of two-dimensional voxel layers per slab increases, so does the efficiency gain. Also, as the mass density of a slab is decreased, the efficiency gains increase.

Increasing the speed of DOSXYZnrc Monte Carlo simulations through the introduction of nonvoxelated geometries.

This article presents a method for increasing the speed of DOSXYZnrc Monte Carlo simulations through the introduction of nonvoxelated geometries defined in any coordinate system. Nonvoxelated geometries are used to isolate regions of uniform density and composition from the scoring grid. Particle transport within these geometric regions is not restricted by the boundary constraints of the scoring grid. This allows for larger particle steps, which in turn reduces the calculation time. A water tank phantom, water-lung interface phantom, cylindrical calibration phantom, and CT phantom were each used to test the application of the nonvoxelated approach. Each phantom was simulated using both the original DOSXYZnrc code and the new nonvoxelated code. The equivalence between the original and nonvoxelated simulations were quantified using a chi2 analysis. To within the statistical uncertainty, the voxelated and nonvoxelated simulations were found to give nearly identical results, regardless of boundary crossing algorithm. The speed increase was found to be a function of both voxel dimension and field size. Using nonvoxelated geometries and the EXACT boundary crossing algorithm, the speed increase was as high as 9.0, 5.1, 5.7, and 1.3 times faster for the water tank, water-lung interface, cylindrical calibration, and CT phantoms, respectively. If the PRESTA-I boundary crossing algorithm was used, the calculation speed increase was up to 6.0, 2.7, 3.3, and 1.2 times faster. These results clearly show that the nonvoxelated technique greatly increases simulation speed without any loss in dose accuracy.