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ESTRO 36
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OC-0156 Automated reference-free local error
assessment in clinical multimodal deformable image
registration
M. Nix
1
, R. Speight
1
, R. Prestwich
2
1
St James' Institute of Oncology, Radiotherapy Physics,
Leeds, United Kingdom
2
St James' Institute of Oncology, Clinical Oncology,
Leeds, United Kingdom
Purpose or Objective
Multimodal deformable image registration (MM-DIR), for
MR-CT fusion in RT planning, is a difficult problem.
Algorithms in commercial applications can leave
significant residual errors and performance can vary
considerably through a 3D image set. Currently, quality
assessment relies on clinical judgement or time-
consuming landmarking approaches for quantitative
comparison. Due to the variability of MM-DIR performance,
a pre-clinical commissioning approach cannot be relied
upon to quality assure clinical performance. The primary
objective was to develop and validate an automated
method for localised error assessment of clinical
multimodal deformable image registrations, without
reference data. This should aid clinical judgement of
registration reliability across the volumetric data and
hence increase clinical confidence in MM-DIR fusion for RT
planning.
Material and Methods
A computational method for determining the local
reliability of a given clinical registration has been
developed. Two registration assessment algorithms, using
blockwise mutual information (BMI) and pseudo-modal
cross correlation (pmCC) respectively, have been
implemented and compared. Error information is
presented as a quantitative 3D ‘iso-error’ map, showing
areas of a registered dataset where errors are greater than
a certain magnitude and may not be reliable, e.g. for
contouring tumour or organ at risk volumes. The
developed software was validated using a ‘gold-standard’
rigidly-registered image set, derived from immobilised
MR, registered to immobilised CT, which was deformed
with known rotations, translations and more complex
deformation fields. Detected and applied errors were
compared across the dataset. Mean errors within the GTVs
of 14 head and neck MR-CT registrations were analysed
using the BMI method and used to identify cases where the
registration may be clinically unacceptable.
Results
Both algorithms consistently detected applied errors
larger than 2 mm. Errors detected using the BMI method,
following intentional rotation of gold-standard pre-
registered clinical MR data, were strongly correlated with
applied errors, in magnitude and direction (Pearson’s r >
0.96).
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