ESTRO 36 Abstract Book

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S493 ESTRO 36 2017
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images. Plans optimized on the realistically distorted data
and undistorted data were compared based on their DVH
and the two one- sided equivalence test (TOST).
Results
Increasing the bandwidth reduced the distortions. Moving
from 122 to 244 Hz/Pixel decreased the maximum
distortions by 43% and reduced the absolute difference in
doses to the PTV between dCT and CT plans from 0.417 ±
0.241 Gy to 0.129 ± 0.286 Gy in the R/L gradient readout
direction. However, this increase in bandwidth did not
significantly affect the difference in doses in the A/P
readout direction: 0.347 ± 0.150 Gy and 0.362 ± 0.240 Gy
respectively. We found a difference of 1.2% and 1.9%
between dCT and undistorted plans for gradient readout
in R/L and A/P directions for the rectal volume receiving
more than 69 Gy. The equivalence test on the two plans
showed the 90% Confidence Interval all lied within the
equivalence intervals (-0.6, 0.6) Gy for difference in PTV
mean doses and (-1, 1) % for difference in the relative
volume of the PTV and Rectum with a 0.05 significance.
Conclusion
By combining measured Machine-specific and si mulating
Patient-induced Susceptibility effects w e have
successfully investigated their combined effect on dose
distributions for Prostate cancer treatment plans. Our
results showed that dose errors due to disturbed Patient
outline and shifts due to Patient-induced Susceptibility
effects at Prostate/Rectum interfaces caused by gas in the
Rectum were small. The smallest effect was found for high
bandwidth and readout in the R/L direction. Equivalence
tests showed equivalence within our investigated
equivalence intervals at 0.05 alpha level for all studied
dose distribution quality indicators.
PO-0901 Is MRI in immobilization mask nec essary for
brain metastasis patients?
A. Van Lier1, A. De Boer1, M. Kramer1, G. Fa netti2, W.
Eppinga1, J.J.C. Verhoeff1, M. Philippens1, E. Seravalli1
1 UMC Utrecht, Department of Radiation Oncology,
Utrecht, The Netherlands
2 European Institute of Oncology, Department of
Radiation Oncology, Milan, Italy
Purpose or Objective
To investigate the necessity of performing MRI in
treatment position (ie. with immobilization mask) for
brain metastasis patients.
Material and Methods
Ten patients who were referred for brain metastasis
radiosurgery were analysed in this study. A planning CT (1
mm slice thickness), a contrast-enhanced T1 3D MRI scan
(1.5T, 1 mm isotropic voxel size, surface coils) with
patient immobilized in a 3-point thermoplastic shell
(mask-MR) and a contrast-enhanced T1 3D MRI scan (1.5T,
1 mm isotropic voxel size, multi-channel head coil)
without immobilization mask (no mask-MR) were acquired.
First, a clinician stated which of the MRI scans had superior
quality, to assure that the no-mask MR had at least the
same image quality compared to the clinically used mask-
MR. Then, the two MRIs were registered independently to
the planning CT by a normalized mutual information
algorithm which was restricted to rigid registration. The
GTV was delineated by 3 clinicians on 1) mask-MR and 2)
no mask-MR. The brain stem, chiasm and right eye were
delineated by one clinician. Furthermore, 8 well-defined
landmarks were marked by an observer in both scans.
Residual registration errors were estimated for both MRIs
by measuring the absolute coordinate differences in the
three orthogonal directions between the set of landmarks
on both imaging series after registration. Moreover, the
absolute differences in the centres-of-gravity coordinates
of GTV (median of 3 observers), brain stem, chiasm and
right eye on mask-MR and no mask-MR were compared.
Results
The no mask-MR image quality was found to be superior in
9 of the 10 patients. The average coordinate difference
between mask-MR and no mask-MR for all landmarks along
the three orthogonal directions were within 0.5 mm (table
1). Similar results were found for the coordinates of the
centre-of-gravity of all delineated OARs and GTV.
Deviations in OAR registration > 1mm could be attributed
to variations in delineation (figure 1). Only in one case, a
registration error was observed. All GTV deviations were
within 1mm.
Conclusion
The registration of MRIs obtained with or without
immobilization mask to a planning-CT generally differs
less than the MRI resolution (1 mm isotropic). Therefore,
immobilization of the head during MRI for patients
undergoing radiotherapy of brain metastasis is not
necessary.
However, to guarantee high accuracy of image registration
when omitting an immobilization device during MRI, more
attention should be paid to the quality of MR-CT fusion.
Furthermore, consecutive MR images should be matched
separately to CT, to correct for intra-scan motion.
We foresee two benefits of scanning without mask. Firstly,
the patient comfort during the MRI scan sessions will be
improved. Secondly, omission of the immobilization mask
permits the use of a multi-channel head coil which results
in higher image quality. Moreover, using a head coil allows
for introduction of MRI techniques which require high
signal-to-noise ratios or acceleration (e.g. DWI and FLAIR).
PO-0902 Identifying the dominant prostate cancer focal
lesion using 3D image texture analysis