ESTRO 36 Abstract Book

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S81 ESTRO 36
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Conclusion
We have shown that an a priori scatter correction
algorithm for CB projections improves CBCT image quality
on both photon- and proton therapy gantries, potentially
opening for CBCT-based image/dose-guided proton
therapy.
OC-0159 Dual energy CBCT increases soft tissue CNR
ratio and image quality compared to standard CBCT in
IGRT
M. Skaarup1, D. Kovacs 1, M.C. Aznar1, J.P. Bangsgaard1,
J.S. Rydhög1, L.A. Rechner1
1 The Finsen Center - Rigshospitalet, Clinic of Oncology,
Copenhagen, Denmark
Purpose or Objective
We investigate a method for enhancing soft tissue contrast
to noise ratio (CNR) and clinical image quality of cone-
beam computed tomography (CBCT) by using a dual energy
CBCT protocol.
Material and Methods
Nine patients were scanned using a standard CBCT
protocol of either 100 or 125 kVp and a DE-CBCT protocol
of two separate scans of 80 and 140 kVp respectively.
Other scan parameters were identical and total radiation
dose was kept at a similar level for both protocols. Virtual
monochromatic dual energy (VMDE) images were
reconstructed using a linear mix of the 80 and 140 kVp
scan.
The weight, with which the two images were combined,
was calculated based on known attenuation coefficients of
two basis materials at a specific monochromatic energy. A
linear combination of these can be used to express the
attenuation coefficients of the 80 and 140 kVp scan at that
same monochromatic energy. To find the optimal virtual
reconstruction energy for soft tissue imaging, multiple
reconstructions were done for energies in the range 40-
180 keV.
CNR measurements were performed on both standard
CBCT and VMDE images for a number of different tissue
combinations, e.g. contrast between tumour-fat, tumour-
surrounding tissue, muscle-fat, rectum-surrounding
tissue, parotid-fat, seminal vesicle-surrounding tissue and
lung-heart (see figure 1 for an example). In addition, 5
experienced observers conducted a blinded ranking
between VMDE images (reconstructed at 55, 65, 75 and
100 keV) and the standard CBCT images, i.e. five image
series per patient. For each combination of image series
the observers were asked to compare the images side-by-
side, focusing on soft tissue image quality as well as
presence of metal artefacts. To eliminate left-right bias,
each combination was shown twice.
Results
VMDE images reconstructed at energies in the range 60 to
70 keV showed improved CNR for all soft tissue regions
when compared to the standard CBCT. On average, the
reconstruction energy corresponding to the maximum CNR
improvement is 65.5 ± 2.4 keV. The increase in maximum
CNR varied from 29% to 78%.
The clinical observer comparison gave a series of rankings
for each image series for each patient (see table 1). Using
signed rank Wilcoxon comparison test, the observers found
the VMDE images at 65 keV preferable to the standard
CBCT image. The p-value was found to be < 0.01, where p
< 0.05 is considered significant. An estimate of inter-
observer variability test was done with Fleiss’ kappa and
found to be moderate with a κ-value of 0.47, where values
above 0.4 is considered acceptable and 1 is perfect
agreement. Occasionally, an observer ranked the 75 keV
reconstruction as the most preferable image while the
overall preferred image was the 65 keV reconstruction.
Except in the case of patient one where the standard CBCT
image was ranked the highest of all.
Conclusion
VMDE images can increase soft tissue contrast and improve
clinical image quality for image-guided radiotherapy
compared to the standard CBCT protocol.
OC-0160 Radiomics Features Harmonization for CT
and CBCT in Rectal Cancer
R. Luo1, J. Wang1, H. Zhong1, J. Gan1, P. Hu1, L. Shen1,
W. Hu1, Z. Zhang 1
1Fudan University Cancer Hospital, Radiation Oncology,
Shanghai, China
Purpose or Objective
Inter-scanner variability can lead to degradation of
radiomics model quality. Therefore, feature
harmonization is necessary for consistent findings in
radiomics studies, especially for multi-institution studies.
The purpose of this study is to establish harmonization