204 / 1082
S191
ESTRO 36 2017
_______________________________________________________________________________________________
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 Heerden
1
, N. Van Wieringen
1
, C. Koedooder
1
,
C.R.N. Rasch
1
, B.R. Pieters
1
, A. Bel
1
1
Academic 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 cm
3
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.
Material and Methods
Seven patients were studied, treated with MRI-guided PDR
BT (two times 24 x 0.75 Gy, given in two applications BT1
and BT2). DIR was performed using the Feature-Based
Deformable Registration (FBDR) tool, connected to a
research version of Oncentra®Brachy (Elekta
Brachytherapy, Veenendaal, the Netherlands). The
delineated rectums were converted to 3D surface meshes,
and a mapping was established to propagate elements on
the surface of rectum
BT1
to the surface of rectum
BT2
. The
transformation vectors were used to deform the BT1 dose
distribution. Next, the BT1 and BT2 doses were summed
voxel-by-voxel. To investigate the dose warping
uncertainty a physically realistic model (PRM) describing
rectal deformation was used. In this model the central
axes of rectum
BT1
and rectum
BT2
were constructed. The
axes were assumed to be fixed in length. For both
rectum
BT1
and rectum
BT2
, orthogonal planes were
reconstructed at 5 evenly spaced positions on the axis
(Fig. A). 100 points were evenly distributed over the
intersection curve of each plane with the rectal wall. It is
assumed that the most dorsal point of the rectum is fixed
and also that the rectal wall only stretches
perpendicularly to the central axis. For point pairs on
rectum
BT1
and rectum
BT2
that were at the same location
according to the PRM, the dose for BT1 and BT2 was added
(D
PRM
) and compared as a 'ground truth” to the DIR
accumulated dose (D
DIR
) in the BT2 point. For BT, the high
dose regions in the OAR are most relevant and points
within the 2 cm
3
volume receiving the highest dose should
be correctly identified. We therefore evaluated the
percentage of points where D
PRM
and D
DIR
were both >D
2cm3
.
Results
Over all patients, D
DIR
varied between 1.1-44.4Gy
EQD2
and
D
PRM
varied between 1.1-40.1Gy
EQD2
(α/β=3Gy for late OAR
toxicity, T
1/2
=1.5 hours). For point pairs, the absolute
difference between D
DIR
and D
PRM
was 0-8.3Gy
EQD2
(Fig. B).
The 2 cm
3
volumes receiving the highest dose according to
the two models have an overlap of 66% (Fig. C).
Conclusion
With the rectal model it is feasible to quantify dose
warping uncertainties, which could be as high as 8.3
Gy
EQD2
. Most points (>66%) in high dose regions were
correctly identified as part of D
2cm3
.