Program Information
Simplified Pencil Beam Kernel For Unfiltered Energies
M Erturk1*, C Kocar2 , M Tombakoglu2 , S Gurdalli1 , (1) MNT Saglik Hizmetleri, Istanbul, ,(2) Hacettepe University, Ankara,
Presentations
SU-E-T-698 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:Aim of the study was to evaluate pencil beam kernel, with minimum kernel parameters, for unfiltered photon energies by using measured broad beam data.
Methods:Double exponential function (A_z*exp(-a_z*r)+B_z(-b_z*r), which required 4 parameters for each depth, was chosen as pencil beam kernel function. When first term was assumed as primary dose kernel and the second exponential term was assumed as scattered dose kernel only Bz, which was magnitude of the scattered dose component relative to primary dose component, was leaved as a function of depth. Profile at air was used as intensity data for each depth, so depth dependency of intensity was also neglected. MATLAB optimization toolbox (“fminsearch”) was used to minimize differences between computed and measured profiles. Penalty function was created with global gamma analysis by using 1% - 1mm criteria. Interested dose point threshold was set as 5% of the maximum dose. Profiles of 10x10 field at five standard depths were used as comparison data. Work flow started with suggestion of kernel parameters, then dose distribution of five standard depths was computed and compared with measured data by using gamma analysis, this calculation was iterated until convergence.
Results:After convergence, it was seen that Bz could be described as a first order polynomial of depth (R^2=0.985 when parameters were fitted with relevant depth). Then B_z was defined as (b0*r+b1) and kernel was reoptimized. All points on 5x5 and 10x10 field profiles at five depths were in the criteria of 1% - 1mm global gamma analysis. For 20x20 there were no any failed points when criteria was %2-2mm for threshold %20 of the macximum dose.
Conclusion: It is possible to reduce pencil beam kernel parametres for unfiltered photon energies. Improvement required for large fields.
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