ESTRO 36 Abstract Book

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S899 ESTRO 36 2017
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Conclusion
Large differences in proton SPR estimation were found
between DECT and SECT, although these were within the
uncertainties which are currently used for dose
calculation in particle therapy. These differences indicate
that DECT will allow for reduction of treatment margins,
resulting in better dose conformity. We are currently
performing proton treatment planning for the patients
comparing DECT- and SECT-based proton SPRs to
investigate the dose difference in the tumour and in the
surrounding healthy tissues, as well as potential impact on
the range uncertainty margins used in proton treatment
planning.
EP-1673 Electron-density assessment using dual-energy
CT: accuracy and robustness
C. Möhler1,2, P. Wohlfahrt3,4, C. Richter3,4,5,6, S. Greilich1,2
1German Cancer Research Center DKFZ, Division of
Medical Physics in Radiation Oncology, Heidelberg,
Germany
2National Center for Radiation Research in Oncology
NCRO, Heidelberg Institute for Radiation Oncology HIRO,
Heidelberg, Germany
3OncoRay - National Center for Radiation Research in
Oncology, Faculty of Medicine and University Hospital
Carl Gustav Carus- Technische Universität Dresden-
Helmholtz-Zentrum Dresden - Rossendorf, Dresden,
Germany
4Helmholtz-Zentrum Dresden-Rossendorf, Institute of
Radiooncology, Dresden, Germany
5Department of Radiation Research in Oncology, Faculty
of Medicine and University Hospital Carl Gustav Carus-
Technische Universität Dresden, Dresden, Germany
6German Cancer Consortium DKTK, Dresden, Germany
Purpose or Objective
Current treatment planning for essentially every external
radiation therapy (photons, electrons, protons, heavier
ions) is not able to account for patient-specific tissue
variability or non-tissue materials (e.g. implants, contrast
agent) which can lead to considerable differences in dose
distributions (figure 1). This is due to the conversion of CT
numbers to electron density or stopping power using a
heuristic Hounsfield look-up table. In contrast, dual-
energy CT (DECT) allows for a patient-specific
determination of electron density – the only (most
important) parameter influencing photon (ion) dose
distributions. Among the many algorithms proposed for
this purpose, a trend towards increased complexity is
observed, which is not necessarily accompanied by
increased accuracy and might at the same time militate
against clinical implementation. Here, we therefore
investigated the performance of a seemingly simple
linear-superposition method (Saito, 2012, Hünemohr et
al., 2014).
Material and Methods
Key feature of the studied approach is a parameterization
of the electron density, given by 'alpha blending” of the
two DECT images. The blending parameter can be
obtained by empirical calibration using a set of bone tissue
surrogates and a linear relationship between relative
photon absorption cross sections of the higher and lower
voltage spectrum. First, this linear relation was analyzed
to quantify the purely methodological uncertainty (i.e.
with ideal CT numbers as input), based on calculated
spectral-weighted cross sections from the NIST XCOM
database for tabulated reference tissues (Woodard and
White, 1986). A clear separation from CT-related sources
of uncertainty (e.g. noise, beam hardening) is hereby
crucial for a conclusive assessment of accuracy. Secondly,
we tested the proposed calibration method on published