The purpose of this study was to investigate the feasibility of using Monte Carlo methods to assist in the commissioning of electron beams for a medical linear accelerator. The EGS4/BEAM code system was used to model an installed linear accelerator at this institution. Following an initial tuning of the input parameters, dosimetry data normally measured during the machine commissioning was calculated using the Monte Carlo code. All commissioning data was calculated for 6- and 12-MeV electron beams, and a subset of the commissioning data was calculated for the 20-MeV electron beams. On central axis, calculated percentage depth dose, cross-beam profiles, cone-insert ratios, and air-gap factors were generally within 2% of Dmax or 1 mm of the measured commissioning data; however, calculated open-cone ratios were not within 2%, in most cases. Calculated off-axis dose profiles for small fields were generally within the 2% (1-mm) criteria; however, calculated dose profiles for larger (open cone) fields frequently failed the 2% (1-mm) criteria. The remaining discrepancies between Monte Carlo calculations and measurement could be due to flaws in the Monte Carlo code, inaccuracies in the simulation geometry, the approximation of the initial source configuration, or a combination of the above. Although agreement between Monte Carlo calculated and measured doses was impressive and similar to previously published comparisons, our results did not prove our hypothesis that Monte Carlo calculations can generate electron commissioning data that is accurate within 2% of Dmax or 0.1 cm over the entire range of clinical treatment parameters. Although we believe that this hypothesis can be proved, it remains a challenge for the medical physics community. We intend to pursue this further by developing systematic methods for isolating causes of these differences.
The sensitivity of electron-beam Monte Carlo dose calculations to scattering foil geometrical parameters is described. A method for resolving discrepancies between Monte Carlo calculation and measured data in a systematic manner is also described. As part of a project to investigate the utility of Monte Carlo methods for calculating data required for commissioning electron beams, a large discrepancy between measured and calculated 20 MeV cross-beam profiles for the largest field size was found. It was hypothesized that the discrepancy was due to incorrect input data and that better agreement between calculation and measurement could be achieved with small changes in the scattering foil system geometry. Four parameters describing the foil system were varied individually until better agreement between calculation and measurement was achieved, and the percentage change in the parameter was tabulated as an indication of the sensitivity of the model to that parameter. The accelerator model for the 20 MeV electron beam was most sensitive to the distance between the scattering foils and to a slightly lesser extent, to the width of the shaped secondary scattering foil. Changes to the primary or secondary foil thickness also significantly modified the falloff and bremsstrahlung component of depth dose, which was unacceptable for the present case. Therefore, the distance between the two scattering foils was changed in our calculations, which the manufacturer later confirmed was indeed the case. For 6 and 12 MeV electron beams, the change was not nearly as significant. It was concluded that Monte Carlo calculations for higher-energy beams and larger field sizes are most sensitive to the geometric configuration of the scattering foil system and should therefore be calculated first to help verify the accuracy of the geometric information.
An active-target Time Projection Chamber (ELITPC) is being developed at the University of Warsaw to investigate the photo-disintegration reaction 16 O(γ,α) 12 C at energies relevant for nuclear astrophysics (down to ∼ 1 MeV in the centre of mass). Selected results from ongoing R&D activities are presented in this paper.
Purpose: To evaluate accuracy of VW implementation by the Varian Eclipse AAA Calculation Algorithm Material and Methods: A 6MV Siemens Oncor Linac beam was used to deliver symmetric and asymmetric VW (15, 30, 45, 60) profiles for a range of field sizes (5×5cm2 to 15×15cm2). All measurements were performed in absolute dose mode using a 2D diode array (Mapcheck, SunNuclear FL), at 100cm SAD and depth of 5cm under water equivalent conditions. The Varian Eclipse TPS software v.8, AAA calculation algorithm was used to determine the VW fluence maps in a water equivalent phantom. All measured and calculated Linac central axis values were analyzed. A relative and absolute comparison was conducted for all calculated and measured fluence maps using the Mapcheck Software v.3.01.08.00. Theoretical calculations of the VW factors were performed and compared with the ones calculated based one measured data. The measured output factors were compared with the calculated ones using our PennMU software and the Eclipse TPS AAA calculation algorithm. Results and discussions: All measured and theoretical symmetric VW wedge factors and output factors were in agreement. Differences up to 82% were found for asymmetric VW output comparison between the Eclipse AAA algorithm calculated output factors and the measured ones. The differences increase with the degree of asymmetry of the field and the VW angle. The data analysis suggest that the Eclipse AAA TPS defines the wedge factor to the central axis of the beam rather then to the central of the machine (VW factor definition). The relative fluence map analysis showed that over 95% of the VW measured profile are within 3%/3mm (%Diff/DTA). Conclusion: We found that the VW clinical implementation for the Eclipse TPS AAA algorithm, v.8, is valid for symmetric fields only and large errors can result for asymmetrical fields.
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