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S73
ESTRO 36
_______________________________________________________________________________________________
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
(LAIC
Thick
) and 4 of type PTW-34080 (LAIC
Thin
) 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 LAIC
Thin
. 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 LAIC
Thick
. For LAIC
Thin
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. Wohlfahrt
1,2
, C. Möhler
3,4
, W. Enghardt
1,2,5,6
, S.
Greilich
3,4
, C. Richter
1,2,5,6
1
OncoRay - 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
2
Helmholtz-Zentrum Dresden - Rossendorf, Institute of
Radiooncology, Dresden, Germany
3
German Cancer Research Center DKFZ, Division of
Medical Physics in Radiation Oncology, Heidelberg,
Germany
4
National Center for Radiation Research in Oncology
NCRO, Heidelberg Institute for Radiation Oncology HIRO,
Heidelberg, Germany
5
Department of Radiation Research in Oncology, Faculty
of Medicine and University Hospital Carl Gustav Carus-
Technische Universität Dresden, Dresden, Germany
6
German 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