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Exact Simulation of Varian Clinac 2100C/D with Use of Phase Space file and Representation of Appropriate Source Model for Clinical Applications

Ezzati, Ahadollah | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44462 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Sohrabpour, Mostafa; Rabi Mahdavi, Saeed
  7. Abstract:
  8. MC Simulation is considered to be one of the most accurate methods for transport of radiation in various media. Computational speed is the limiting factor to apply the MC method in clinical settings. One of the methods to increase the speed in MC simulations is the use of phase space file (PSF). PSF is generated by transporting the particles through the linear accelerator head. The characteristics of these particles crossing a reference plane are stored in the PSF file. The PSF can be used in subsequent simulations as a radiation source. The use of PSF is effective but has a drawback of having latent variance. Latent variance is a problem inherent in using phase space files. Latent variance reduction methods and source models can be used to reduce this latent variance. Source models that were derived from those phase space files compromise the correlation between the physical quantities to some extent. Using source models can save both disc space and computing time. In this study, azimuthal particle redistribution (APR), and azimuthal particle rotational splitting (APRS) methods implemented in MCNPX2.4 source code. The efficiency of these methods were calculated and compared from two tallies of f6 and *f8. The APRS was found to be more efficient than the APR method in f6 tally. Using *f8 tally both methods had nearly the same efficiency. Latent variance reduction factors were obtained for 6, 10 and 18 MV photons as well. The APRS relative efficiency contours have been obtained. These contours reveal that by increasing the photon energies, the contours depth and the surrounding areas were further increased. The relative efficiency contours indicated that variance reduction factor is position and energy dependent. The out of field voxels relative efficiency contours showed that latent variance reduction methods increased the Monte Carlo (MC) simulation efficiency in the out of field voxels. The APR and APRS average variance reduction factors had differences less than 0.6% for splitting number of 1000.A comprehensive source model(SM) has been developed for azimuthally symmetric photon beams. This SM is unique in that it accounts for a higher level of energy dependence on the particle’s direction and it is independent from accelerator components, unlike the other published SM based articles. The model can be applied to any arbitrary azimuthally symmetric beam and has source biasing capabilities that significantly increase the simulation speed. This SM was benchmarked against the original PSF and measurements. Agreement was within 2% of the maximum dose at 100 cm SSD and 2.5% of the maximum dose at 200 cm SSD for beam profiles at depth of 3.5 cm and 15 cm with respect to the original PSF. Differences between the source model and the PSF in the out-of-field regions were less than 0.5% of the profile maximum value at 100 cm SSD. Differences between measured and calculated points were less than 2% of the maximum dose or 2 mm distance to agreement (DTA) at 100 cm SSD. Finally to quantify the DNA damage induced in a clinical megavoltage photon beam at various depths in and out of the field of ELEKTA linear accelerator 10 MV photon beam was simulated. There was an observable difference in the energy spectra for photons and electrons for points in the primary radiation field and those points out-of-field. In the out-of-field region, the mean energy for the photon and electron spectra decreased by a factor of about six and three from the in-field mean energy, respectively. Despite the differences in both the photon and electron spectra, these changes did not correlate with a change in RBE in a clinical MV photon beam as the electron spectra were dominated by electrons with energies greater than 20 keV
  9. Keywords:
  10. Monte Carlo Method ; Varian Accelerator ; Treatment Planning ; Phase Space File ; Source Model ; DNA Damage

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