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Analysis in the direction orthogonal to the applied
deformation showed minimal errors, as expected ( <E> =
0.32 mm, SD = 0.43 mm). Across 14 clinical MR-CT
registration datasets, mean magnitude registration errors
within the GTV varied from 0.4 to 5.4 mm (population
mean = 1.8 mm), indicating that MM-DIR errors can be
significant for RT planning.
Conclusion
Reference free localised registration quality assessment
offers clinicians a tool to judge registration reliability,
which could increase confidence in and clinical usage of
MM-DIR in radiotherapy. A software tool was developed
and validated to achieve this. A strong correlation was
found between detected and applied registration errors.
Mean GTV error is a potential indicator for clinical
acceptability of registrations.
OC-0157 Atlas-based segmentation of prostatic urethra
in the planning CT of prostate cancer
O. Acosta
1
, M. Le Dain
1
, C. Voisin
1
, R. Bastien
1
, C. Lafond
2
,
K. Gnep
2
, R. De Crevoisier
2
1
LTSI-INSERM UMR 1099, Université de Rennes 1, Rennes,
2
Centre Eugene Marquis, Radiotherapy, Rennes, France
Purpose or Objective
to the dose delivered mainly to the bladder) and likely
also to the urethra (obstructive symptoms). Identification
of urethra for dose assessment from planning CT scans is
however challenging as the organ lies inside the prostate
and is not visible. Moreover, the dose received by the
urethra may not be superposed to the dose received by
the whole prostate. In case of prostate IMRT, the goals of
this work were therefore: i) to propose an automatic
method for urethra segmentation from the planning CT
and ii) to quantify the dose received by the urethra.
Material and Methods
An original weighted multi atlas-based segmentation
method was devised standing on a global characterization
of the urethra wrt the surrounding organs. For building the
atlas a first set of CT scans (512×512 0.63×0.63 mm axial
pixels and 3 mm slices) from 80 patients treated for
localized prostate cancer with Iodine 125 brachytherapy
was used. All the patients had an urinary probe allowing
an ease manual urethra segmentation. Prostate, bladder
and urethra were delineated by a radiation oncologist. An
average patient, in terms of prostate volume, was
selected as common reference system where all the
patients were rigidly aligned. Each segmented urethra was
characterized by its central line, the relative bladder
position and prostate characteristics (height, excentricity
and volume). An in-house demons based registration using
prostate contours and Laplacian maps was performed to
propagate urethra delineation to the test patients. The n-
most similar individuals were selected and final
segmentation was obtained by a weighted vote. Leave-one
out cross validation of the atlas for urethra segmentation
was first performed on the training data set. Mean
Centerline Dispersion (MCD) and Hausdorff Distance (HD)
were used for accuracy assessment. The method was then
applied to a second set of 95 patients having received 78
Gy by IMRT for prostate cancer. Target volume and organs
at risks (bladder, prostate) were delineated on computed
tomography (CT) slices according to the French GETUG
group recommendations. Then, the urethra was
segmented using the proposed approach and dose was
measured inside the resulting segmentation and compared
to the dose to the prostate.
Results
From the training data set, the number of most similar
atlases was optimized to 10 in the leave one out scheme.
Average MCD of 2.3 mm and HD of 3.5 mm were thereby
obtained. In the testing data base dose received by the
segmented urethra were significantly higher than the
whole prostate in a range of dose from 74 Gy to 79 Gy
(Wilcoxon test p<0.01).
Conclusion
An accurate atlas based segmentation method was
proposed allowing assessment of dose within prostatic
urethra. Dose in this organ was significantly higher than
the whole prostate, mainly in the highest dose range.
Results open the way to further NTCP studies relating
urinary toxicity such as obstructive symptoms to the
urethra dose.
OC-0158 a priori scatter correction of cone-beam CT
projections in photon vs. proton therapy gantries
A.G. Andersen
1
, Y. Park
2
, O. Casares-Magaz
1
, U. Elstrøm
1
,
J. Petersen
1
, B. Winey
2
, L. Dong
3
, L. Muren
1
1
Aarhus University Hospital, Department of Medical
Physics, Aarhus V, Denmark
2
Massachusetts General Hospital, Department of
Radiation Oncology, Boston- Massachusetts, USA
3
Scripps Proton Therapy Center, Department of Medical
Physics, San Diego- California, USA
Purpose or Objective
Cone-beam (CB) CT is becoming available on proton
therapy gantries, to allow image/dose-guidance and
adaptation for protons. To use these techniques clinically,
the challenges related to image quality and Hounsfield
Unit accuracy need to be solved. Algorithms for scatter
correction have been developed, and have been explored
for CBCT systems on photon therapy gantries but so far not