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Parameterization of the Photon Beam Dosimetry

S Lebron

S Lebron*, J Li, G Yan, D Kahler, C Liu, Univ Florida, Gainesville, FL

SU-E-T-15 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: In radiation therapy, obtaining accurate data for photon beam dosimetric quantities such as fractionated depth doses (FDDs), profiles and output factors (OFs) is important since it affects the accuracy of dose calculation for patient treatment. The purpose of this study is to develop methods to parameterize all photon dosimetry parameters for a specific linear accelerator model and to create an expert data set in order to (1) minimize the interpersonal measurement uncertainties, (2) automate the quality assurance program, (3) minimize data collection during machine commissioning and (4) implement analytical de-convolution method to remove the detector volume averaging effect.

Methods: OFs, FDDs and profiles for different field sizes and depths were measured in an Elekta linear accelerator using a cylindrical 3D scanner (SunNuclear). All data were smoothed for the analysis and profile data were also centered and symmetrized. The OFs were analyzed using a polynomial equation. The FDDs were analyzed using two methods: one consisted of a ratio of two exponential functions to parameterize the whole curve and the other dividing the curve into two parts characterized by Gaussian and polynomial functions. For profile modeling, half side of the profile was divided into four regions described by four different Gaussian equations. This set of equations was used to model the photon beam dosimetry for various field sizes and depths.

Results: The differences between measured and modeled data were less than: 0.002% for OFs, 0.1% and 3.4% for FDDs for the first and second method respectively and 0.1% for the profiles. The calculated data generated an R² =1 for the OFs, R² >0.993 for the FDDs, and R² >0.99 for the profiles.

Conclusion: This novel analytically calculated model proved to be accurate in calculating the FDDs, profiles and OFs for different field sizes and depths.

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