MCNP simulation of a Theratron 780 radiotherapy unit

Characterization of Gamma Knife Perfexion™ source based on Monte Carlo simulation

This study is aimed at characterizing a single Cobalt-60 source capsule of the Gamma Knife Perfexion™ unit using the BEAMnrc Monte Carlo code. The Gamma Knife Perfexion™ source capsule was modeled using the BEAMnrc user code according to the manufacturer's technical details. The modeled parts include the source, the area around the source, and the capsule. The cylindrical source is 1 mm in diameter and 17 mm in length, with a physical density (ρ) of 8.9 × 10³ kg/m³. The simulation parameters were an electron cutoff energy (ECUT) of 0.7 meV and photon cutoff energy (PCUT) of 0.01 meV. Energy fluence was calculated on a 0.25 cm diameter scoring plane located 3.1 cm from the source. Simulations were performed with and without the encapsulation of the source to investigate its effect on the spectrum and fluence of emitted gamma rays. The results showed that the influence of source encapsulation on the gamma rays is an increase in the relative number of particles in each energy bin of aggregate gamma rays by 92.36% at 0.23 meV energy and 66.12% at 1.10 meV energy. The secondary gamma rays were found to increase by 94.17% at 0.23 meV energy and 63.74% at 1.10 meV energy. The encapsulation of the source attenuated the gamma rays, which altered the spectrum. The mean energy of the beam increased, thereby exhibiting a beam-hardening effect.

Effects of Alignment of Adjustable Collimator on Dosimetric Parameters of a Telecobalt Machine

The objectives of this study was to investigate the most appropriate hinge point for the alignment of adjustable collimators/trimmer bars in a telecobalt machine for obtaining acceptable dosimetric parameters of a telecobalt machine. Variations of relative output of the telecobalt machine with selection of different hinge points were also investigated. MCNP code was used for the present the study. A water phantom of dimension 50 × 50 × 40 cm3 having voxels each of volume 0.72 cm3 was used in our study for generating beam profiles and depth dose curves. When hinge points are selected at the periphery of the source bottom and the source top, flatness, symmetry and penumbra were found to be well within the recommended tolerance limits whereas values are far beyond with the hinge points selected at the centre of source bottom or the source top. Moreover, it was observed that the relative output of a telecobalt machine with hinge points at centre of the source bottom and the source top are appreciably lower than that of at periphery of source bottom particularly for smaller field sizes. This effect is due to the blockage of the part of the source volume in the radiation field. Therefore, hinge point for the alignment of adjustable collimators/trimmer bars should be selected either at periphery of source bottom or the source top for obtaining clinically acceptable flatness, symmetry and penumbra. However, selecting hinge point at the periphery of the source bottom for the alignment of the adjustable collimators/trimmer bars would be more appropriate as height of the source will vary depending on activity of the source used in the source capsule for given specific activity.

Considerations of MCNP Monte Carlo code to be used as a radiotherapy treatment planning tool

The present work has simulated the photon and electron transport in a Theratron 780® (MDS Nordion)⁶⁰Co radiotherapy unit, using the Monte Carlo transport code, MCNP (Monte Carlo N-Particle). This project explains mainly the different methodologies carried out to speedup calculations in order to apply this code efficiently in radiotherapy treatment planning.

Characterization of a 60Co unit at a secondary standard dosimetry laboratory: Monte Carlo simulations compared to measurements and results from the literature

PURPOSE

To compare a Monte Carlo (MC) characterization of a 60Co unit at the Swedish Secondary Standard Dosimetry Laboratory (SSDL) with the results of both measurements and literature with the aims of (1) resolving a change in the ratio of air-kerma free in air Kair and absorbed dose to water Dw in a water phantom noted experimentally after a source exchange in the laboratory and (2) reviewing results from the literature on similar MC simulations. Although their use in radiotherapy is decreasing, the characteristics of 60Co beams are of interest since 60Co beams are utilized in calibrating ionization chambers for the absolute dosimetry of radiotherapy beams and as reference radiation quality in evaluating the energy dependence of radiation detectors and in studies on radiobiological effectiveness.

METHODS

The BEAMnrc MC code was used with a detailed geometrical model of the treatment head and two models of the 60Co source representing the sources used before and after source exchange, respectively. The active diameters of the 60Co sources were 1.5 cm in pellet form and 2.0 cm in sintered form. Measurements were performed on the actual unit at the Swedish SSDL.

RESULTS

Agreement was obtained between the MC and the measured results within the estimated uncertainties for beam profiles, water depth-dose curve, relative air-kerma output factors, and for the ratios of Kair/Dw before and after source exchange. The on-axis energy distribution of the photon fluence free in air for the unit loaded with its present (1.5 cm in diameter) source agreed closely with the results from the literature in which a source of the same make and active diameter, inside a different treatment head, was simulated. The spectrum for the larger (2.0 cm in diameter) source was in close agreement with another published spectrum, also modeling a 60Co source with an active diameter of 2.0 cm inside a different treatment head.

CONCLUSIONS

The reduction in the value of Kair/Dw following source exchange was explained by the spectral differences between the two sources that were larger in the free in-air geometry used for Kair calibrations than at 5 g/cm2 depth in the water phantom used for Dw calibrations. Literature review revealed differences between published in-air 60Co spectra derived for sources of different active diameters, and investigators in need of an accurately determined 60Co in-air spectrum should be aware of differences due to source active diameter.

Monte Carlo simulation of a multi-leaf collimator design for telecobalt machine using BEAMnrc code

This investigation aims to design a practical multi-leaf collimator (MLC) system for the cobalt teletherapy machine and check its radiation properties using the Monte Carlo (MC) method. The cobalt machine was modeled using the BEAMnrc Omega-Beam MC system, which could be freely downloaded from the website of the National Research Council (NRC), Canada. Comparison with standard depth dose data tables and the theoretically modeled beam showed good agreement within 2%. An MLC design with low melting point alloy (LMPA) was tested for leakage properties of leaves. The LMPA leaves with a width of 7 mm and height of 6 cm, with tongue and groove of size 2 mm wide by 4 cm height, produced only 4% extra leakage compared to 10 cm height tungsten leaves. With finite (60)Co source size, the interleaf leakage was insignificant. This analysis helped to design a prototype MLC as an accessory mount on a cobalt machine. The complete details of the simulation process and analysis of results are discussed.