Monte Carlo (MC) calculations used to require long CPU times, which made it prohibitive for routine clinical use. Several variance reduction techniques (VRTs) are included in the BEAMnrc MC code to significantly improve the efficiency of photon beams for which the computation time constitutes a problematic parameter. This work focused on the optimization of VRTs parameters to improve the efficiency by BEAMnrc code. This work deals with the use of different VRTs parameters alone or combined, such as directional bremsstrahlung splitting (DBS), uniform bremsstrahlung splitting (UBS), photon and electron splitting, Russian roulette (RR), range rejection, and bremsstrahlung cross section enhancement (BCSE) included in BEAMnrc code. We found that the relative improvement in efficiency due to UBS without RR, ICM_SPLIT (e-/e+), BCSE, and DBS is over 85, 130, 3489, and 4652 times higher in comparison to analog simulation for total photon fluence efficiency in 12 MV photon beam, respectively. VRTs combined in parallel simulation lead to enhance the dose efficiency by 19321 times compared to analog simulation. It is demonstrated that it is possible to further improve the efficiency by optimizing VRTs parameters combined and at the same time preserving its accuracy in parallel simulation of a 12 MV photon beam. It is demonstrated that it is possible to further improve the efficiency by optimizing VRTs parameters combined and at the same time preserving its accuracy in parallel simulation of a 12 MV photon beam.

We used GATE simulation to study the effect of the coincidence time window (CTW) along with the block gap and the intercrystal gap on the count rate performance and the spatial resolution of the Biograph™ mCT 20 Excel. We ran simulations on our local cluster to reduce computation time. The task was split into several jobs that were then triggered simultaneously on the cluster nodes. The BiographTM mCT 20 Excel was validated using the NEMA NU 2-2012 protocol. Our results showed good agreement with experimental data. The simulated sensitivity, peak true count rate, peak noise equivalent count rate (NECR), and scatter fraction showed agreement within 3.62%, 5.77%, 0.6%, and 2.69%, respectively. In addition, the spatial resolution agreed within <0.51 mm. The results showed that a decrease in the coincidence time window and the block gap and an increase in the intercrystal gap increase the count rate performance and improve the spatial resolution. The results also showed that decreasing the coincidence time window increased the NECR by 27.37%. Changing the intercrystal gap from 0 to 0.2 mm and the block gap and from 4 to 0.4 mm increased the NECR by 5.53% and improved the spatial resolution at 1 cm by 2.91% and that at 10 cm by 3.85%. The coincidence time window, crystal gap, and block gap are important parameters with respect to improving the spatial resolution.

Purpose: In the last years, some studies investigated dosimetric benefits of a free flattening filter for the photon mode in the radiotherapy field. This study aims to provide a theoretical study published and analysis of basic dosimetric properties for a Saturne 43 Linac head to implement free flattening filter beams clinically.Material and Methods: This is the first Monte Carlo study for the head of Saturne 43 with replacement flattening filter mode investigating beam dosimetric characteristics, including central axis absorbed doses, beam profiles and photon energy spectra. The later ones were analyzed for flattening filter and replacement flattening filter beams using BEAMnrc and DOSXYZnrc Monte Carlo codes for 10 × 10 cm2, 5 × 5 cm2 and 2 × 2 cm2 square field sizes.Results: A 3.94-fold increase of dose rate and electron contaminating increased by 246.4 % with the replacement flattening filter mode for field size of 10 × 10 cm2. Reduction was made by replacement flattening filter beam in the peripheral dose up to 30%, and the time was reduced more than 50 %.Conclusion: Results obtained from our study revealed that some characteristic dosimetries such as the maximum increase in depth dose rate, decrease in out-of-depth dose, and reducing time can be beneficial for the unflattened beam to be used in the radiotherapy for the advanced techniques.

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