ESTRO 36 Abstract Book

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S493 ESTRO 36
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Oncology, Dresden, Germany
7 Institute of Radiooncology, Helmholtz-Zentrum
Dresden-Rossendorf, Dresden, Germany
Purpose or Objective
In neuro-oncology, 3 Tesla (3T) MRI is the current clinical
standard for tumor localization, radiotherapy volume
delineation and stereotactic (radio)surgery, sometimes
complemented by amino acid PET imaging. With superior
SNR and image resolution, anatomical 7T MRI can visualize
micro-vascularization in glioblastomas potentially
allowing improved target volume delineation. However,
concerns about geometrical distortion (GD) with
increasing field strength (B 0) are detrimental for
applications of 7T MR in image-guided interventions. For
high-precision treatment strategies, the spatial integrity
of anatomical images needs to be warranted within ±1mm.
The aim of the study was to evaluate B0- and sequence-
related GD in clinically relevant 7T sequences and
compare it to equivalent 3T sequences and CT images
Material and Methods
To quantify B0- and sequence-related GD in T1-GRE, T1-
TFE, T2-TSE, T2-TSE FLAIR on 7T and 3T sequences, a
dedicated anthropomorphic head-phantom (CIRS Model
603A) was used. The phantom is composed of bone-/soft-
tissue equivalent materials and contains a 3D grid (3mm
rods spaced 15mm apart). System-based distortion
correction methods were applied to restore the gradient
uniformity of 3T and 7T. For all CT and MR images, 436
points of interests (POIs) were defined by manual
reconstruction of the 3D grid points in the respective
images. GD was assessed in 3 ways. Firstly, global GD was
estimated by the mean absolute difference (MADglobal)
between the measured and the true Euclidian distances of
all unique combinations of POIs, independent of location
within the phantom. Secondly, local GD was assessed by
MADlocal between the measured and the true Euclidian
distances of all POIs relative to the magnetic field
isocenter. Thirdly, a distortion map was created by
evaluating 3D displacement vectors for each individual
grid point
Results
MADglobal in 3T and 7T images ranged from 0.19−0.75mm
and 0.27−1.91mm, respectively, and was more
pronounced than in CT images. CT was not entirely free of
GD with MADglobal ranging from 0.14−0.64mm. B0-related
GD was larger in 7T than in 3T MRI with MADlocal ranging
from 0.21-1.81mm and 0.11-0.73mm, respectively
(p<0.05). MADlocal increased with increasing distance from
the magnetic isocenter and largest GDs were noted at the
level of the skull (Fig. 1). MADlocal was <1mm for all
sequences up to 68.7mm from the isocenter. Sequence-
related GD at 7T was prominent in T1-TFE and significantly
differed from other 7T sequences (p<0.001). Figure 2
shows an anisotropic distribution of GD in T1-TFE with
increasing GD along the frequency-encoding direction
Conclusion
System-related GD was present in all 3T and 7T MR images
but remained within the 2mm tolerance limit. Near the
magnetic isocenter, 7T anatomical images showed no
difference in geometric reliability to 3T MR images.
Careful selection of 7T sequences and judicious use of GD
correction methods can warrant the geometrical quality
required for incorporation of 7T MR into image-guided
interventions
PO-0895 MRI-based analysis of volumetric changes of
healthy brain tissue in glioma patients after photon RT
A. Gommlich 1,2,3, H. Wahl4, F. Raschke2, M.
Baumann1,2,3,5,6, M. Krause1,2,3,5,6, E.G.C. Troost1,2,3,5,6
1Institute of Radiooncology, Helmholtz-Zentrum
Dresden-Rossendorf, Dresden, Germany
2OncoRay - Center for Radiation Research in Oncology,
Translational and Clinical Radiation Oncology, Dresden,
Germany
3NCT - National Center for Tumor Diseases- partnersite
Dresden- Germany, partnersite, Dresden, Germany
4Institute of Neuroradiology- University Hospital Carl
Gustav Carus, Institute of Neuroradiology, Dresden,
Germany
5University Hospital Carl Gustav Carus, Department of
Radiotherapy and Radiation Oncology, Dresden, Germany
6DKTK - German Cancer Consortium, Partnersite,
Dresden, Germany
Purpose or Objective
State-of-the-art Linac-based photon beam irradiation
achieves highly conformal target volume coverage in
glioma patients, but is also known to cause side-effects to
surrounding tissues and organs. Apart from subjective
measures (e.g., questionnaires, function tests) objective
means to quantify tissue damage, e.g., anatomical or
functional magnetic resonance imaging (MRI) are urgently
needed to compare different treatment techniques and
beam qualities (e.g., protons vs. photons) and to develop
predictive measures for optimal sparing of normal brain
tissue. As initial part of our program for dose-dependent
spatial mapping of structural and functional radiation
induced brain damage, we assessed here a retrospectively
collected MRI-dataset in order to potentially detect
volumetric changes of the healthy brain tissue (gray and
white matter) in the non-affected hemisphere of glioma
patients treated with photon irradiation.