Use of a 2-Dimensional Ionization Chamber Array to Measure Head Leakage of a Varian Truebeam^®Linear Accelerator

The purpose of this work is to evaluate the use of a two-dimensional (2D) planar ion chamber array to characterize leakage radiation from the head of the linear accelerator. Ion chamber arrays provide a benefit over a singular ion chamber measurement as they allow for the measurement of a larger area in order to isolate the point of maximum leakage dose and the small size of each individual ion chamber minimizes volume-averaging effects. A Varian Truebeam® undergoing acceptance testing was used for all measurements. The gantry was wrapped in Portal Pack for Localization (PPL) radiographic film in order to isolate the location of maximum leakage. A calibration curve was developed and used to determine dose-to-film. An Ion Chamber Profiler (IC Profiler™) manufactured by Sun Nuclear Corporation was used to confirm measurements by the PPL film. All measurements were normalized to leakage at 100 cm from the target relative to the central axis. Three points were investigated with the IC Profiler, including the top of the gantry, the Varian logo, and the side of the gantry. For the three locations, respectively, the PPL film and the IC profiler were measured 0.142% and 0.131%, 0.036% and 0.030%, and 0.014% and 0.019%. The good agreement between the PPL film and the IC Profiler provides confidence in the use of a more efficient and accurate ion chamber array for head leakage measurements.

Simulation method for measurement of the cross-section of the^(14)N(n,a)^(11)B reaction using a gridded ionization chamber

A simulation method for measurement of the cross-section of the^(14)N(n,a)^(11)B reaction with gas and solid samples using a gridded ionization chamber(GIC)has been established.Using the simulation,the experimental spectra of both^(14)N(n,a)^(11)B events and background from other reactions can be predicted,and the experimental scheme can be optimized.According to the simulation results,the optimal experimental parameters,including the pressure of the working gas and the compositions of the working gas and the sample,can be determined.In addition,the simulation results can be used to determine the valid event area and calculate the detection efficiency for valid events.A measurement of the cross-sections of the^(14)N(n,a)^(11)B reaction at E_(n)=4.25,4.50,4.75,5.00,5.25,and 5.50 MeV,based on the 4.5-MV Van de Graff accelerator at Peking University(PKU)using a GIC as the detector for the outgoing a particles,has been performed.The good agreement of the spectra from the simulation and experiment demonstrated the universality of this simulation method,which can be used to accurately measure neutroninduced light-charged particle emission reactions.

Verification of an independent dose calculation method for portal-specific QA of proton and carbon ion beams

Objective:To verify the accuracy of an independent dose calculation method,as incorporated into an in-house developed treatment planning system(TPS),for performing quality assurance of dose distributions delivered to a water phantom planned by a clinical TPS.Methods:A Monte Carlo based track repeating algorithm was incorporated into an in-house treatment planning system for proton and carbon ion beams.Calculations were performed in a flat water phantom for both a traditional pencil beam algorithm and a new Monte Carlo algorithm,and then compared to measurements made at multiple depths with a 2D ionization array for 44 patient portals.The comparisons utilized a Gamma analysis.Results:A total of 124 measurements were performed for proton and carbon ion patient portals.Using a small Gamma criteria of 2%/2 mm,an average of 93%and 97%of measurement points passed for each portal for pencil beam and Monte Carlo calculations,respectively.The passing rate was substantially higher for Monte Carlo calculations than for pencil beam calculations for portals that used a range shifter.Conclusions:The implemented independent method has been verified against measurements.The high passing rate with small tolerances leads to the possibility of reducing the number of required quality assurance measurements.