ESTRO 36 Abstract Book

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S73 ESTRO 36
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1 Medizinische Universität Wien Medical University of
Vienna, Department of Radiation Oncology and Christian
Doppler Laboratory for Medical Radiation Research for
Radiation Oncology, Vienna, Austria
2 EBG MedAustron GmbH, Medical Physics, Wiener
Neustadt, Austria
Purpose or Objective
A large area ionization chamber (LAIC) can be used to
measure output factors of narrow beams. In principle,
dose area product measurements are an alternative to
central-axis point dose measurements. Using an LAIC
requires detailed information on the uniformity of the
signal response across its sensitive area.
Material and Methods
8 LAICs (sensitive area with nominal diameter of 81.6mm)
were investigated in this study, 4 of type PTW-34070
(LAICThick) and 4 of type PTW-34080 (LAICThin) with water-
equivalent entrance window thicknesses of 4mm and
0.7mm, respectively. Measurements were performed in an
X-ray unit (YXLON) using peak voltages of 100-200kVp and
a collimated beam of 3.1mm FWHM. The LAICs were
mounted on the moving mechanism of an MP3-P (PTW) and
moved with a step size of 5mm to measure the chamber’s
response at lateral positions. To account for beam
positions where only a fraction of the beam overlapped
with the sensitive area of the LAIC, a corrected response
was calculated as the basis for determining relative
response as a function of radial distance from the
centre. The impact of a heterogeneous LAIC response,
based on the obtained response maps was henceforth
investigated for small field photon beams (as small as
6x6mm²) and proton pencil beams (FWHM=8mm).
Results
A pronounced heterogeneity of the spatial responses was
observed in both the thick and thin window LAICs. These
heterogeneities could be calculated as a function of the
radial coordinate as there was no pronounced angular
dependency. All 4 LAIC Thick followed a monotonously
increasing response towards the chamber centre, while
the absolute response values varied up to 1.5%, excluding
the 2mm borders of the LAICs. In contrast the LAIC Thin
trends were not uniform and responses varied by up to 10%
(Fig 1). Investigating absolute dosimetry for a proton
pencil beam the signal varies with a systematic offset
between 2.4% and 4.1% for LAIC Thick and between -9.5% and
9.4% for LAICThin. For relative dosimetry (e.g. depth-dose
profiles) the increase of beam size with increasing depth
was investigated as the influencing factor. Systematic
response variation by 0.4% and 1% at the most were found
for the investigated LAICs. The systematic offset for
absolute dose measurements for decreasing photon field
size showed that for 6x6mm² field sizes the response was
systematically 2.5-4.5% higher for LAICThick. For LAICThin the
response varies even over a range of 20%. The entrance
window thickness was evaluated to be constant within
measurement uncertainty by performing measurement at
multiple peak voltages.
Conclusion
This study highlights the need for chamber-depended
response maps when using LAICs for absolute and relative
dosimetry with proton pencil beams or small photon
beams.
OC-0150 Dual-energy CT-based proton treatment
planning to assess patient-specific range uncertainties
P. Wohlfahrt1,2, C. Möhler3,4, W. Enghardt1,2,5,6, S.
Greilich3,4, C. Richter1,2,5,6
1OncoRay - 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
2Helmholtz-Zentrum Dresden - Rossendorf, Institute of
Radiooncology, Dresden, Germany
3German Cancer Research Center DKFZ, Division of
Medical Physics in Radiation Oncology, Heidelberg,
Germany
4National Center for Radiation Research in Oncology
NCRO, Heidelberg Institute for Radiation Oncology HIRO,
Heidelberg, 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
To reduce range uncertainties in particle therapy arising
from a generic heuristic conversion (HLUT) of CT numbers
in stopping-power ratios (SPRs), an accurate patient-
specific SPR prediction is desirable. Treatment planning
based on dual-energy CT (DECT) can account for tissue
diversity and potentially contribute to shrink clinical
safety margins. Consequently, in this study dose
distributions derived from both a clinical HLUT and a
patient-specific DECT-based SPR prediction are compared