(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: FFF beams show differential characteristics that have to be account during commissioning process.High dose rate and recombination effects in ionization chambers are a challenging effect in FFF beams. In this work we compare and analyze data set for 6 and 10 MV FFF beams for two one year delay installed systems.
Methods: Commissioning data sets for 6 MV and 10 MV conventional and FFF beams from twoLinac from two independent institutions were acquired using iba ionization chambers in institution A and PTW ionization chambers in institution B, polarized at +300 V and +400 V respectively, expecting different saturation and recombination effects when measuring high dose rate modes. Recombination factors, PDD, profiles for different depths and field sizes were acquired. A comparison of the measurements was performed using point differences and gamma function. A comparison with manufacture reference data is also presented.We studied the possibility to correct PDD and profiles using a correction factor dependant on point dose rate in order to obtain better quality measurements.
Results: Measurements of ksat is lower for PTW semiflex (max 1.009 for 10 MV FFF), and a ksat dependency on dose rate on PDD measurements (ksat 1.002 at 20 cm depth and ksat 1.009 at dosemax, whereas CC13 shows ksat values of 1.029 and 1.013 respectively).We obtain good agreement beyond dmax better than 0.3% for both conventional and FFF beams, while in buildup zone an agreement better than 3% or 1 mm DTA is achieved. Same behaviour is observed in dose profiles.If we apply a correction to PDD curve using a ksat dose rate function we obtain better agreement reducing the differences in shallow region.
Conclusion: Reproducibility of commissioning measurements between different systems is excellent, and dosimetry data could be improved if a dose rate dependence of correction factors is observed.
Keywords
Taxonomy
TH- External beam- photons: Small/nonstandard field experimental dosimetry
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