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ESTRO 36 2017
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images showed maximal 95th percentile distortions of
0.39, 0.32, 0.28, and 0.25 pixels for the neck, lungs,
thorax with the lungs excluded, and pelvic region,
respectively. In order to accommodate other field
strengths and bandwidths, normalized displacement
values were also simulated for these body regions.
Table 1: Simulated displacement values normalized to
field strength and bandwidth [pixels * BW / B0]
Conclusion
The 95th percentile of the patient-induced susceptibility
distortions can be kept below 0.5 pixels for a 3 T system
and 440 Hz bandwidth. With the provided normalized
data, distortions for other field strengths and bandwidths
can be calculated. The developed simulation software can
also be used to quickly and easily estimate the
susceptibility-based distortions from a given series of
patient CT images that are converted into susceptibility
values, or directly from a susceptibility map.
PO-0904 Development of an MRI-protocol for
radiotherapy treatment guidance in gastric cancer
V.W.J. Van Pelt
1
, M.F. Kruis
1
, T. Van de Lindt
1
, L.C. Ter
Beek
2
, M. Verheij
1
, U.A. Van der Heide
1
1
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiation Oncology, Amsterdam, The
Netherlands
2
Netherlands Cancer Institute Antoni van Leeuwenhoek
Hospital, Radiology, Amsterdam, The Netherlands
Purpose or Objective
Because of the superior soft-tissue contrast of MRI,
integration of MRI in pre-operative radiotherapy (RT) for
gastric cancer, is expected to improve the identification
of shape and position of the target volume. MRI of the
stomach is technically challenging due to respiratory,
cardiac and bowel motion. In this study we therefore
developed a scan protocol consisting of anatomical and
functional sequences for staging and target delineation
(TD), for treatment planning (TP) including motion
modeling and for intra-fraction motion monitoring (MM).
Material and Methods
For staging and TD we compared high resolution (HR) T2-
weighted (T2w) turbo spin echo (TSE) MRI, applying either
navigator or respiratory sensor triggering during the
exhale position of the diaphragm to reduce motion
artifacts. For TP, the feasibility of a fast 3D HR mDixon
with a large Field of View (FoV) within one exhale breath-
hold (BH) was evaluated. For motion modeling, a 4D T2w
MRI with retrospective self-sorting reconstruction was
tested for robustness
[1]
. For intra-fraction MM, 2D T1w
dynamic turbo field echo (TFE), fast field echo (FFE) and
TSE Cine-MRI with a refocusing pulse were compared. For
staging and treatment response monitoring, a single-shot
echo planar Diffusion Weighted Imaging (DWI) was tested
using b-values of 0, 200 and 800 s/mm², applying either
free-breathing (FB), BH, navigator or respiratory
triggering. For Dynamic contrast enhanced (DCE) MRI, FB
T1w spoiled gradient echo, 4D mDixon and 4D THRIVE with
keyhole technique were compared. Subtraction images
were reconstructed to show the uptake of intravenous
contrast agent.
The sequences were tested on healthy volunteers and one
patient using a 3T MR system (Ingenia; Philips Healthcare,
The Netherlands) and reviewed by two MR-experts and one
radiologist. Pineapple juice was given orally to distend the
stomach and suppress signal from the stomach filling.
Gadolinium was used as intravenous contrast agent for the
patient only.
Results
Visual inspection showed that for TD and staging, T2w
exhale respiratory navigator triggered, rather than a
respiratory sensor, provides excellent contrast with
limited motion artifacts. For TP, mDixon with a large FoV,
a high signal to noise ratio (SNR) and HR in one BH is
feasible. For motion modeling, 4D T2w MRI resulted in a
good slice ordering, high SNR and HR. For MM, TSE Cine-
MRI gave a good SNR and HR without artifacts. For staging
and treatment response monitoring, FB DWI with an
increased number of averages gave the best result, only
limited motion and susceptibility artifacts were visible. FB
4D THRIVE DCE resulted in a good temporal resolution and
limited motion artifacts.
(Figure 1)
Conclusion
We developed a comprehensive imaging protocol for the
entire RT guidance treatment chain. The complex motion
artifacts were reduced by applying either navigator
triggering or BH techniques. The new gastric cancer
protocol looks therefore very promising and will be used
for MR-based delineation for RT.
[1] van de Lindt T, et al. ESTRO 35 2016 Abstract-book:PV-
0325; 171-172
PO-0905 (Semi-)Automatic contouring strategies for
rectal boost treatment on the MR-Linac
C.N. Nomden
1
, M.P.W. Intven
1
, A.N.T.J. Kotte
1
, I.H.
Kiekebosch
1
, S. Mook
1
, I.M. Jürgenliemk-Schulz
1
, G.G.
Sikkes
1
, L.T.C. Meijers
1
, E.N. De Groot
1
, G.H. Bol
1
, B. Van
Asselen
1
, L.G.W. Kerkmeijer
1
, B.W. Raaymakers
1
1
UMC Utrecht, Radiation Oncology, Utrecht, The
Netherlands
Purpose or Objective
The MR-Linac enables online treatment adaptations in
response to changes in anatomy. This stresses the need for
fast contouring strategies for target and OARs.
Unfortunately, manual delineation in an online workflow
is time consuming and therefore suboptimal. The purpose
of this study was to investigate whether automatic and
semi-automatic contouring strategies result in clinical
acceptable contours for an online workflow on the MR-
Linac.
Material and Methods
Fifteen patients with early staged rectal cancer were
scanned at an 1.5T MRI for five consecutive days. The scan
consisted of a T2 weighted MRI; voxelsize 0.63x0.63mm,
slice thickness 4 mm and a total number of 30 slices. For
each scan the following contours were delineated by an
experienced radiation oncologist (manual contours): GTV,
mesorectum, bladder, rectum, sphincter, gynecological
volume (in one contour: vagina, cervix and uterus), left
and right femur. The manual contours of the first day were
used as input for the automatic/semi-automatic
contouring strategies. Automatic contouring software
(ADMIRE research v1.13.5 Elekta AB, Stockholm, Sweden)
was used for MR based deformable registration and