ESTRO 2023 - Abstract Book

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ESTRO 2023

through the origin since Zeff of its inhomogeneities (e.g. PTFE, see Tab. 1) are near water. Though the iLUT is markedly different, the resulting dose difference was only between 0.7 and 1.3 % (head, pelvis). For PACA, different LUTs resulted in changes of plan metrics of generally less than 1.0 %, though in some cases it could be up to 1.3 % (median PTV dose) for targets, and for organs at risk 1.5 % in high dose regions and several % in low dose regions. Conclusion Inhomogeneity phantoms for RTP should comprise both water- and bone-like materials, Zeff being the decisive parameter. As bone surrogate aqus. of CaCl2 seem well suitable. PTFE and other materials with water-like Zeff, though included in commercial phantoms, are definitely not suitable as bone surrogate. However, for photon plans the conversion LUT has rather limited clinical relevance, especially compared to changes in tube voltage, and measurements should always stand the comparison to standard values. Literature [1] Thomas, Br J Radiol (1999)

[2] Smilowitz et al., JACMP (2015) [3] PACA-Study, to be published [4] Liu et al., Med Phys (2021) [5] ICRU Report 42, Bethesda (1987)

Tab. 1 Phantom materials

Fig. 1 Hounsfield to electron density conversion