In this paper, a new full rate, full diversity 2x2 STBC with Linear complexity in the receiver is introduced for keyhole channels, where the rank-deficiency of the channel matrix degrades system performance. This code is optimized based on the known criteria for these types of the channels. Simulation results demonstrate that the proposed code outperforms some of the well-recognized STBCs when BPSK and 4-QAM constellation are utilized.

Background: Monte Carlo (MC) modeling of a Linear accelerator is a prerequisite for Monte Carlo dose calculations in external beam radiotherapy. In this study, a simple and efficient model was developed for Elekta SL-25 Linear accelerator using MCNP4C Monte Carlo code. Materials and Methods: The head of Elekta SL-25 linac was simulated for 6 and 18 MV photon beams using MCNP4C MC code. Energy spectra and fluence distribution of photons crossing the phase space plane were calculated. A simple point source model was developed based on calculated photon spectra and spatial distribution. Using this model, percent depth doses (PDDs), and beam profiles were calculated for different field sizes. The results of MC calculations were compared with measurements. Results: There was a good agreement between MC calculations and measurement for descending part of PDD curves. However, comparing calculated PDDs with measurement showed up to 10% differences for build up region of PDD curves for both energies. For beam profiles, there was 2% difference in flat region and up to 15% difference was seen for out of field region. These results were acceptable according to the recommended criteria. Using this model, the run time was decreased 24 times in comparison to original full Monte Carlo method. Conclusion: Our study showed the presented model to be accurate and effective for MC calculations in radiotherapy treatment planning. Also, it substantially lowers MC runtime for radiotherapy purposes.  

Introduction: Previous investigations have shown that Monte Carlo methods are suitable for the simulation of the transportations photon beam in medical Linear accelerators. The simulated beams could be used for the measurement of the dose distribution in phantoms and patients' body. Angular, energy and radial distributions are the most important data of photon beams and can be used for clinical applications of simulation. The phase-space data of the Elekta SL75/25 Linac was simulated for MCNP4C code. In this investigation, we have presented detailed data of the angular, energy and radial distributions at four scoring plate perpendicular to the central axis of the photon beam.Materials and Methods: Firstly, we simulated the linac head geometry and verified its' accuracy. The mean energy and radial intensity distribution of the linac electron beam was determined by the comparison of the simulated and measured percentage depth doses (PDDs) and beam profile curves. Four scoring plates located 0.1cm under the target, the primary collimator, the flattening filter and the secondary collimator were simulated as concentric circles starting at 0.1cm radius and with 0.1cm intervals up to 3.5cm. Thereafter, bremsstrahlung radiations were simulated and transported in the linac head down to the water phantom and the angular, energy and radial distributions at the four scoring plate were scored.Results: Comparison of our results with the previous reports from the EGS4 code indicated that the simulated photon spectra resulted from the MCNP4C code is a little more at high energies. The mean energy was 2.18MeV having being in good agreement with previous investigations. At all the scoring planes, the beam becomes softer as we get away from the central axis of the beam. The maximum and minimum variation happens for the target and the secondary collimators with a value of 1.24MeV and 0.13 MeV respectively. The head structures of the linac, altogether, caused a beam hardening ofO.61MeV, but among all the components of the head structures, the flattening filter has most effect in this regard. The radial distributions of the primary collimator and flattening filter indicate a difference of 0.6% and 0.05% from that of the central bin respectively. The photon fluence is reduced to 50.04% after crossing the flattening filter with the majority of them being at low energies.Conclusion: Higher energy noted from the use of MCNP4C code is due to the energy physics cut-off card used, causing a shift in the mean energy of the primary electrons toward the higher energies and consequently increasing the relevant photon fluence. The primary collimators and the flattening filter have the most effect on the fluence uniformity and the energy fluence uniformity respectively. Passing the radiation through the head structures causes the beam hardening with the flattening having the most effect.