Program Information
Predicting Photon Percentage Depth Dose From Profile Due to Changes in Beam Quality
S Lebron*, D Kahler , C Liu , University of Florida, Gainesville, FL
Presentations
SU-G-TeP1-10 (Sunday, July 31, 2016) 4:00 PM - 4:30 PM Room: ePoster Theater
Purpose:To predict photon percentage depth dose (PDD) from profile due to a change in flattened (FF) and flattening-filter-free (FFF) beam quality.
Methods: 6MV photon beam PDDs and profiles in a 3D water tank (3DW) and profiles in an ionization chamber array (ICP) were collected for different field sizes and depths with FF and FFF beams in a Versa HD (Elekta Ltd.). The energy was adjusted by changing the bending magnet current (BMC) ±15% from the clinical beam (6MV) in 5% increments. For baseline establishment, PDDs(depth≥3cm) were parameterized with bi-exponential functions and the PDD 20 to 10cm ratios (PDD₂₀,₁₀) were calculated. Then, the FF profile at 10cm from the central axis (Pr₁₀) and the slope of the FFF central linear region (SFFF) were calculated. Calibration curves were established: (1) change in Pr₁₀ and SFFF as functions of the change in PDD₂₀,₁₀ and (2) change in PDD(depth=3, 15 and 30cm) as function of the change in PDD₂₀,₁₀. The differences between Pr₁₀ and SFFF from baseline were calculated and, from calibration curves, changes in PDD₂₀,₁₀ and PDD(depth=3, 15 and 30cm) were obtained. Then, absolute PDD(depth=3, 15 and 30cm) values were input into a least-square-optimization algorithm to calculate the bi-exponential function’s optimal coefficients and generate the PDD(depths≥3cm).
Results:The change in PDD₂₀,₁₀ relative to baseline increased (<±4%) with BMC. Pr₁₀ increased (±6%) and SFFF decreased (±11%) with BMC. Relative differences between measured and calculated (i.e. PDD calculation from Pr₁₀ and SFFF) PDDs were less than 1%. Results apply to FF and FFF beams measured in 3DW and ICP.
Conclusion:Pr₁₀ and SFFF are more sensitive than PDD to changes in beam energy and PDD information can be accurately generated from them. With known 3DW and ICP profile relationship, ICP can be used to obtain PDD for current photon beam.
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