(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: The purpose of the study was to investigate the dependence of inhomogeneity correction factors (ICFs) on tissue phantom ratio (TPR20,10) the so-called quality index (QI).
Methods: Water phantoms containing regions of the lung (0.26 g/cm³), adipose tissue (0.92 g/cm³) and bone (1.85 g/cm³) were constructed in Eclipse TPS. ICFs calculations were performed with the Anisotropic Analytical Algorithm (AAA) for several field sizes and for points lying at several depths inside of and below different thicknesses and densities of the inhomogeneities for a range of QI as TPR20,10 (6 MV)= 0.67±k*0.01 and TPR20,10 (15 MV)= 0.76±k*0.01, where k = -3, -2, -1, 0, 1, 2, 3. ICFs were also measured in a CIRS thorax phantom for 10x10 cm² field size, for 6 MV and 6 MV FFF generated in a TrueBeam accelerator.
Results: Calculations at 5 cm depth below inhomogeneities (where CPE exists) showed a small difference (up to 5.0%) on the range of QI for both nominal energies. However, 6.0% variations in QI lead to changes of ICFs of 10.0% (6 MV) and 13.8% (15 MV) at 1 cm below lung slab for a 5x5 cm² field size. For adipose slab, less than 1.0% difference was found for both energies. 2.0% (6 MV) and 2.4% (15 MV) differences were found for bone slab. These differences of ICFs decreased when calculated inside of inhomogeneities. ICFs decreased with increasing field size. Measurements with the CIRS phantom also demonstrated differences of ICFs consistently up to 6.0% between the 6 MV and 6 MV FFF beams.
Conclusion: Small changes in ICFs were found in inhomogeneous phantoms in the regions where charged particle equilibrium (CPE) exists. For regions, where there is no CPE, the dependence of the ICFs on QI is complicated. Measurements result carried out with the CIRS phantom were consistent with calculations.
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