Abstract
Purpose: PTW's Octavius 1000 SRS array performs IMRT QA measurements with liquid filled ionization chambers to allow closer detector spacing and higher resolution. Increased density of liquid relative to air, leads to increased ion recombination effects, which are proportional to dose/pulse experienced at the detectors. During IMRT QA, dose/per pulse will vary across the array, leading to varying collection efficiencies. Magnitudes of recombination effects were investigated and a method was developed to correct them in QA measurements. Methods: The OCTAVIUS 1000 SRS was used to measure different IMRT plans using varying dose/pulse and results were normalized to air filled chamber measurements to obtain collection efficiencies. Regression analysis was performed to relate collection efficiency to dose/pulse. Matlab software was developed to correct IMRT QA measurements using these correlations. Measurements were performed on a Truebeam equipped with high-definition MLC, using 6MV and 10FFF RapidArc plans. Plans were measured in the OCTAVIUS 4D system, which measures dose in time intervals. Dose/time was converted to dose/pulse using pulse repetition information. Collection efficiency to dose/pulse correlations were used to correct measured dose. 3D gamma analysis was performed on plans with and without corrections. Results: Liquid filled ionization chamber collection efficiency correlated linearly to dose/pulse with slopes of 2.15%/(mGy/pulse) for 6MV and 5.74%/(mGy/pulse) for 10FFF. After corrections, average gamma pass rates improved by [0.02%,0.06%,0.24%] for 6MV and [0.19%,0.54%,2.01%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Maximum changes in gamma pass rates were [0.1%,0.3%,0.5%] for 6MV and [0.4%,0.8%,5.1%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Conclusion: OCTAVIUS 1000 SRS ion recombination effects have little effect on 6MV measurements. However, the effect is larger for higher dose/pulse unflattened beams when using tighter gamma tolerances, as is common for SRS/SBRT. This researched was supported by PTW.
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