ESTRO 36 Abstract Book

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S192 ESTRO 36
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Conclusion
Overall, good agreement is found between ACE and MC
dose calculations in front of the eye plaques in water. The
consistent difference of ~3-4% observed for all
comparisons with MC simulations is potentially due to
differences in the MC simulation codes used to generate
the data, and scaling of the ACE dose distribution in water
to match TG-43 data in OcB. Updated seed models will be
used to investigate this discrepancy. The good level of
agreement indicates that further investigation of ACE in
applications involving a virtual, voxelized eye phantom,
and patient CT datasets, is warranted.
OC-0359 Microdosimetric evaluation of intermediate-
energy brachytherapy sources using Geant4-DNA
G. Famulari1, P. Pater1, S.A. Enger1,2
1 McGill University, Medical Physics Unit, Montreal,
Canada
2 McGill University Health Centre, Department of
Radiation Oncology, Montreal, Canada
Purpose or Objective
Recent interest in alternative radionuclides for use in high
dose rate brachytherapy (Se-75, Yb-169, Gd-153) with
average energies lower than Ir-192 has triggered the
investigation of the microdosimetric properties of these
radionuclides. A combination of Monte Carlo Track
Structure (MCTS) simulations and track sampling
algorithms was used to predict the clinical relative
biological effectiveness (RBE) for fractionated
radiotherapy at relevant doses and dose rates. Previous
studies have concluded that the dose mean lineal energy
in nanometre-sized volumes is approximately proportional
to the α-ratio derived from the linear-quadratic (LQ)
relation in fractionated radiotherapy in both low-LET and
high-LET radiation.
Material and Methods
Photon sources were modelled as point sources located in
the centre of a spherical water phantom with a radius of
40 cm using the Geant4 toolkit. The kinetic energy of all
primary, scattered and fluorescence photons interacting
in a scoring volume were tallied at various depths from the
point source. Electron tracks were generated by sampling
the photon interaction spectrum, and tracking all the
interactions following the initial Compton or photoelectric
interaction using the event-by-event capabilities of
Geant4-DNA. The lineal energy spectra were obtained
through random sampling of interaction points and
overlaying scoring volumes within the associated volume
of the tracks.
Results
For low-LET radiation, the dose mean lineal energy ratio
was approximately equal to the α-ratio in the LQ relation
for a volume of about 30 nm (Fig 1). The weighting factors
(often denoted clinical RBE) predicted were 1.05, 1.10,
1.14, 1.19 and 1.18 for Ir-192, Se-75, Yb-169, Gd-153, and
I-125, respectively (Fig 2). The radionuclides Se-75, Yb-
169, and Gd-153 are 5-14 % more biologically effective
than current Ir-192 sources. There is little variation in the
radiation quality with depth from the source.
Fig 1: Dose mean lineal energy ratios between Co-60 and
100 kVp Fig 2: Dose mean lineal energy ratios as a
function of scoring diameter
X-rays as a function of scoring diameter. The dotted line
corresponds for various brachytherapy sources.
to α-ratio of 1.20.
Conclusion
Currently, the International Commission on Radiation
Protection (ICRP) assigns a radiation weighting factor of
unity for all photon emitting sources, equating the RBE of
high and low energy photon sources. However, the clinical
RBE for lower energy brachytherapy sources are
considerably above unity and should be taken into account
during the treatment planning process, to ensure that the
equivalent dose delivered to the tumour is similar for
different sources.
OC-0360 Dose warping uncertainties for the
cumulative rectal wall dose from brachytherapy in
cervical cancer
L.E. Van Heerden1, N. Van Wieringen1, C. Koedooder1,
C.R.N. Rasch1, B.R. Pieters1, A. Bel 1
1Academic Medical Center, Radiation Oncology,
Amsterdam, The Netherlands
Purpose or Objective
Brachytherapy (BT) is part of radiotherapy for women with
locally advanced cervical cancer; nowadays, BT is
commonly given in multiple applications to the tumour
area. In clinical practice, the 2 cm3 receiving the highest
dose (D 2cm3) in the rectum is calculated by assuming that
the high dose volumes overlap for each treatment. To
account for rectal deformation due to differences in filling
and/or the presence of air, many authors state it is
preferable to sum the 3D dose distributions using dose
warping after deformable image registration (DIR).
However, little is known about the reliability of DIR for
dose warping. The purpose of this study is to quantify the
dose warping uncertainty in the rectum using a physically
realistic model, which describes rectal deformation.