ESTRO 36 Abstract Book

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S484 ESTRO 36
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Results
Figure 2 illustrates that proxy-free and common external
breathing signal-driven PB-reconstructed 4DCT data are
comparable both in terms of image quality and
represented motion amount. In detail, the considered
proxy-free datasets contained approximately 5% more
artifacts than the PB data sets. Differences of represented
tumor mass center motion as well as the amount of e.g.
diaphragm motion between end-inhalation and -
exhalation were negligible (max. 1 voxel).
Conclusion
We presented a novel approach for proxy-free slow-pitch
helical 4DCT reconstruction and illustrated its feasibility.
Although the proxy-free reconstructed images contain
slightly more motion artifacts, we consider the approach
to be helpful especially in the case of corrupted breathing
signals recordings (no need for re-scanning the patient).
PO-0883 Clinical Implementation Model-Based CT to
Replace 4DCT for Lung Cancer Treatment Planning
D. Low1, D. O'Connell1, L. Yang1, J. Lewis1, P. Lee1
1 UCLA Medical Center, Department of Medical Physics, Los
Angeles, USA
Purpose or Objective
To implement motion-model based CT into clinical
practice, replacing 4DCT for breathing motion
management treatment planning.
Material and Methods
A breathing motion model that employs a mathematical
motion equation, two real-time breathing surrogates,
breathing amplitude and breathing rate, and employing
multiple fast helical, low-dose CT scanning has been
introduced into clinical practice. The imaging process uses
a bellows-based system to monitor the breathin g cycle,
which is defined as the amplitude and rate of the bellows
signal. The fast helical CT scans are reg istered to
determine the lung tissue positions, correlate d to the
breathing amplitude and rate on a slice-by-s lice basis. A
published motion equation is employed to characterize
the motion for each voxel. The motion model is employed
to reconstruct the original fast helical CT scans and the
original and reconstructed scans compared to determine
the overall model motion prediction accuracy. Eight
amplitude-based CT images are constructed and sent to
the treatment planning system, along with a three-
dimensional motion model accuracy (defined as the 75th
percentile motion error in each voxel) map. The patients
still undergo a commercial 4DCT protocol to provide a
comparison between the current standard of care and the
model-based process. Comparisons between the
commercial and model-based approaches have been
conducted on 19 patients to evaluate the magnitude of
sorting artifacts in each process on a scale of 1-4, 1 having
no artifacts and 4 having severe artifacts. The average CT
noise for both protocols was described by examining a
region of interest in the liver.
Results
Mean tumor displacement was 11.5 +/- 6.9 mm and the
mean motion model error was 1.77 +/- 0.79 mm. The mean
artifact severity ratings for the 4DCT and model-based CT
approaches were 2.2 and 1.2, respectively. There were
three instances of grade 4 artifacts and no instances of
grade 3 or worse artifacts for the 4D and model-based
approaches, respectively. The average CT noise was
reduced from 57.7 HU to 11.6 HU.
Conclusion
The model-based approach provides the clinic with motion
artifact free images that have lower noise and whose
geometry accurately reflects the tumor and other lung
tissues during the CT scanning session. We are still limited
by the treatment planning system's input requirements for
a series of breathing-phase defined images. Work is
ongoing to develop treatment planning protocols that
better match the data resulting from the model-based
approach.
PO-0884 Availability of MRI improves interobserver
variation in CT-based pancreatic tumor delineation
O.J. Gurney-Champion1, E. Versteijne1, A. Van der
Horst 1, E. Lens1, H. Rütten2, H.D. Heerkens3, G.M.R.M.
Paardekooper4, M. Berbee5, C.R.N. Rasch1, J. Stoker6,
M.R.W. Engelbrecht6, M. Van Herk7, A.J. Nederveen6, R.
Klaassen8, H.W.M. Van Laarhoven8, G. Van Tienhoven1, A.
Bel1
1Academic Medical Center, Department of Radiation
Oncology, Amsterdam, The Netherlands
2Radboud University Medical Center, Department of
Radiation Oncology, Nijmegen, The Netherlands
3University Medical Center Utrecht, Department of
Radiotherapy, Utrecht, The Netherlands
4Isala Clinics Zwolle, Department of Radiotherapy,
Zwolle, The Netherlands
5MAASTRO Clinic, Department of Radiation Oncology,
Maastricht, The Netherlands
6Academic Medical Center, Department of Radiology,
Amsterdam, The Netherlands
7University of Manchester and Christie NHS trust, Faculty
of Biology- Medicine & Health- Division of Molecular &
Clinical Cancer Sciences, Manchester, United Kingdom
8Academic Medical Center, Department of Medical
Oncology, Amsterdam, The Netherlands
Purpose or Objective
To assess whether the availability of magnetic resonance
images (MRIs) alongside the planning CT scan for target
volume delineation in pancreatic cancer patients
decreases interobserver variation.
Material and Methods
Eight observers (radiation oncologists) from six institutions
delineated gross tumor volume (GTV) on contrast-
enhanced (CE) 3DCT and internal GTV (iGTV) on 4DCT for
four pancreatic cancer patients. At least six weeks after
submitting these delineations, the observers were asked
to repeat the delineations, now with MRIs available in a
separate window (3DCT+MRI and 4DCT+MRI). The MRI
included plain and CE T1-weighted gradient echo, T2-
weighted turbo spin echo, and diffusion-weighted
imaging. Interobserver variation in volumes of (i)GTVs was
analyzed. Also, the generalized conformity index (CIgen), a