ESTRO 36 Abstract Book

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S497 ESTRO 36
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by the 3 observers showed a median dice similarity
coefficient of 0.71, 0.69 and 0.72 respectively. For all 3
observers the mean hausdorff distance was small with
median (range) distances between PET and DW of 2.3 (1.5
– 6.8), 2.5 (1.6 – 6.9) and 2.0 (1.35 – 7.6) mm respectively.
Over all patients, the median 95th percentile distances
were 6.0 (3.0 – 13.4), 6.6 (4.0 – 24.0) and 5.3 (3.4 – 26.0)
mm.
Conclusion
Diffusion weighted imaging optimized for geometric
accuracy resulted in target volume delineation with good
interobserver agreement and a large similarity with PET.
PO-0900 Quantifying the Effect of MRI Geometrical
Distortions on Radiotherapy Treatment Planning Doses.
M. Adjeiwaah1, M. Bylund1, J. Lundman1, J. Jonsson1, T.
Nyholm1
1 Umeå University, Radiation Sciences, Umea, Sweden
Purpose or Objective
The use of MRI for Radiotherapy Treatment Planning (RTP)
is increasing and the proposed MR-only workflow could be
beneficial. One worry of an MR-only RTP is geometrical
distortions. There are at present few studies focusing on
the effect of MR geometrical distortions on planned doses
in an MR-only treatment planning and to our knowledge,
none fully takes into account both gradient non-linearities
and Patient-induced Susceptibility effects. This study
focused on quantifying the effect of gradient non-
linearities and Patient–induced Susceptibility effects on
dose distributions for Prostate Cancers.
Material and Methods
The deformation field was generated by adding measured
machine-specific and simulated patient-induced
susceptibility effect deformation fields for a 3T scanner as
shown in Fig. 1. Different bandwidths and simulated
gradient readouts in the anterior/posterior (A/P) and
right/left (R/L) directions were used. To isolate the effect
of the distortions, the deformation fields were applied to
17 Prostate Patient CT images and their corresponding
clinically delineated structures, giving a distorted CT
(dCT). VMAT optimized plans were generated for all
distorted cases and recalculated on the undistorted CT
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.
Eppinga 1, J.J.C. Verhoeff1, M. Philippens 1, E. Seravalli1
1UMC Utrecht, Department of Radiation Oncology,
Utrecht, The Netherlands
2European 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.