Fast Monte Carlo Code Research Articles

Fast electron beam simulation and dose calculation in radiotherapy

A flexible multiple source model capable of fast reconstruction of clinical electron beams is presented in this paper. A source model considers multiple virtual sources emulating the effect of the accelerator head components. A reference configuration ( 10 MeV and 10×10 cm 2 field size) for a Siemens KD2 linear accelerator was simulated in detail using the GEANT3 Monte Carlo code. Our model allows the reconstruction of other electron energies and field sizes, as well as other beam configurations of similar accelerators, using only the reference beam data. Electron dose calculations with the reconstructed beams were performed in a water phantom and compared with experimental data. An agreement of 1–2%/1– 2 mm was obtained, comparable to the accuracy of full Monte Carlo accelerator simulation. The source model reduces accelerator simulation CPU time by a factor of 7500 relative to full Monte Carlo approaches. The developed model was then interfaced to DPM, a fast radiation transport Monte Carlo code for dose calculation. Dose distributions and dosimetric parameters computed in the presence of air gaps using this procedure agree within 1%/ 1 mm with experimental data. The obtained accuracy corresponds to a significant improvement when compared to the treatment planning system PLATO, currently used in clinical practice. Globally, a two order of magnitude efficiency gain was achieved with respect to the full Monte Carlo approach.

Monte Carlo calculations for assessment of radiation dose to patients with congenital heart defects and to staff during cardiac catheterizations.

Effective dose is an important quantity in relation to assessment of radiation risk. Organ and effective doses to paediatric patients undergoing diagnostic and therapeutic heart catheterization procedures can be assessed by combining relatively simple measurements, e.g. of dose-area product (DAP), and calculated dose conversion factors (DCF). This also holds for the radiation dose to the hospital staff, e.g. the cardiologist. Monte Carlo (MC) simulation of radiation transport in mathematical anthropomorphic phantoms is used to obtain the DCFs, which strongly depend on beam quality and geometrical parameters. The performance of a dedicated fast MC code (PCXMC) for patient dosimetry is compared with that of a more elaborate general purpose MC code (MCNP). Resulting organ doses sometimes may differ considerably, partly due to phantom differences. While MCNP uses separate male and female mathematical phantoms, PCXMC uses a hermaphrodite. However, both codes yield effective doses that agree rather well, so PCXMC can be used for convenience. The MCNP code is used to calculate the effective dose to the cardiologist exposed to radiation scattered from the patient. Without protective clothing, effective dose per procedure to the cardiologist is at least two orders of magnitude lower than that to the patient. The effectiveness of various types and thickness of protective clothing has been evaluated for one view of one cardiac catheterization. The results of the calculations do not contradict experimental studies from the literature. MC simulation may serve as a useful tool to improve the accuracy of estimating occupational effective dose from personal dose monitors.

Neutral beam heating in the START spherical tokamak

The tight aspect ratios (typically A≈1.4) and low magnetic field of spherical tokamak (ST) plasmas, when combined with densities approaching the Greenwald limit, provide a significant challenge for all currently available auxiliary heating and current drive schemes. NBI heating and current drive are difficult to interpret in sub-megampere machines, as in order to achieve suitable penetration into the plasma core, fast ions have to be highly suprathermal and, as a result of the low magnetic field, can be non-adiabatic (i.e. non-conserving of magnetic moment µ0). The physics of NBI heating in START is discussed. The neutral beam injector deployed on START was clearly successful, having been instrumental in producing a world record tokamak toroidal beta of ≈40%. A fast ion Monte Carlo code (LOCUST) is described that was developed to model non-adiabatic fast ion topologies together with a high level of charge exchange loss and cross-field transport (present in START due to an envelope of high density gas surrounding the plasma). Model predictions compare well with experimental data, collected using a scanning neutral particle analyser, bolometric instruments and equilibrium reconstruction using EFIT. In particular, beta calculations based upon reconstruction of the pressure profile (by combining measurements from Thomson scattering, charge exchange recombination spectroscopy and model predictions for the fast ion distribution function) agree well with beta values calculated using EFIT alone (the routine method for calculation of START beta). These results thus provide increased confidence in the ability of STs to sustain high beta high confinement H mode plasmas and in addition indicate that the injected fast ions in collisional START plasmas evolve mainly due to collisional and charge exchange processes, without driving any significant performance degrading fast particle MHD activity.

Film dosimetric verification of the Voxel Monte Carlo (VMC) algorithm with electron beam dose distributions

Monte Carlo (MC) methods have the potential to predict radiation-therapy doses more accurately than any conventional technique, but normal MC simulations are very time consuming. Therefore, a fast MC code (Voxel Monte Carlo; VMC) was developed especially for radiation therapy purposes and experiments with the comparable precision were performed to demonstrate its accuracy. In the present study the dose distributions measured with film dosimetry in a cylindrical phantom were compared with calculations derived by VMC. The phantom consisted of 18 circular shaped PMMA slabs with a diameter of 20 cm and a thickness of approx. 1 cm. The films were placed between the slabs, and the whole phantom was irradiated with electron beams of different energies (6MeV, 10MeV, 18MeV). The measured optical density distributions were then converted into dose distributions using characteristic curves of the film. Taking into account experimental uncertainties and statistical calculation fluctuations, agreement was found between measurements and VMC simulations with a maximal deviation of 3 mm on isodose curves for 18MeV.