ESTRO 36 Abstract Book

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S480 ESTRO 36 2017
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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
Horst1 , 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 Tienhoven 1, A.
Bel1
1 Academic Medical Center, Department of Radiation
Oncology, Amsterdam, The Netherlands
2 Radboud University Medical Center, Department of
Radiation Oncology, Nijmegen, The Netherlands
3 University Medical Center Utrecht, Department of
Radiotherapy, Utrecht, The Netherlands
4 Isala Clinics Zwolle, Department of Radiotherapy,
Zwolle, The Netherlands
5 MAASTRO Clinic, Department of Radiation Oncology,
Maastricht, The Netherlands
6 Academic Medical Center, Department of Radiology,
Amsterdam, The Netherlands
7 University of Manchester and Christie NHS trust, Faculty
of Biology- Medicine & Health- Division of Molecular &
Clinical Cancer Sciences, Manchester, United Kingdom
8 Academic 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
measure of overlap of the delineated volumes (1=full
overlap, 0=no overlap), was calculated. Furthermore, the
local observer variation was calculated for approx. 32,000
points on the median delineated surface (i.e. the surface
of the volume that ≥50% of the observers included in their
delineation). Local observer variation was defined as the
standard deviation (SD) over the perpendicular distances
between delineated surfaces at that point and is also
known as local SD. The overall observer variation was
defined as the root-mean-square of all local SDs. These
parameters were compared between CT-only and CT+MRI
delineations, for 3DCT and 4DCT (Wilcoxon signed-rank
test; significance level α=0.05).
Results
Delineations differed substantially between observers in
both CT and CT+MRI, as illustrated for the GTV in the
figure. For both GTV and iGTV, the mean volume on
CT+MRI was 32% smaller than on CT only (p<0.0005)
(Table). Although smaller volumes were delineated on
CT+MRI, the CIgen was similar in both studies (CT+MRI:
0.33, CT: 0.32). Furthermore, CT+MRI showed smaller
overall observer variations (average SD=5.9 mm) in six out
of eight delineated structures compared to CT only
(average SD=7.2 mm). The median volumes from the
(i)GTV on CT+MRI were included for 97% and 92% in the
median volumes from GTV and iGTV on CT, respectively.
Finally, iGTV delineation on 4DCT increased uncertainty
with and without MRI, compared to GTV delineation on
3DCT.
Both CT and CT+MRI delineations had regions of large local
observer variation (SD>0.8) close to biliary stents and
enlarged lymph nodes. This was largely due to ambiguous
instructions (near stents) and poor protocol compliance
(near lymph nodes).
Figure: GTV delineations by the eight observers (each a
different color) for 3DCT+MRI and 3DCT.