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S943
ESTRO 36 2017
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To study the performance, image quality and clinical
potential of a novel fast MV-CBCT panel mounted on a
straight-through linac with 6X-FFF (filter-free) beam.
Material and Methods
A prototype MV imager is mounted in-line with the linac
at 154 cm source-to-imager distance. MV-CBCT imaging is
performed with 28 cm fixed width (projected at isocenter-
100 cm) and variable field length (up to 26 cm max) FOV,
200º arc, 15 sec rotation time, at 5MU and 10MU dose
levels. Images of a Catphan® 604 phantom are analyzed in
terms of noise, uniformity, spatial and contrast resolution,
and contrast linearity. The results are compare with those
from Truebeam kV-CBCT imager, and potentially, a
Tomotherapy MV-CT imager, all benchmarked to
diagnostic CT scanner images. Imaging dose will also be
assessed for completeness of testing of the MV-CBCT
system.
Results
The uniformity and noise of the MV-CBCT was acceptable
but not as good relative to diagnostic CT and kV-CBCT,
with variation of 32HU from the center to the
periphery. Spatial resolution is shown in Fig. 1 with
3lp/mm for 5MU and 4lp/mm for 10MU dose levels, as
compared to 7lp/mm for both kV-CBCT and diagnostic
CT. The low contrast resolution of the MV-CBCT was >>
1%, compared to 1.0% and 0.5% for kV-CBCT and diagnostic
CT. Contrast linearity and sensitometry is shown in Fig. 2,
with the MV-CBCT (and 10MU dose level) being the
modality that deviates especially for higher density
objects like Acrylic, Delrin and Teflon (green data
points). Additional tests with varying field length (small,
medium and max length) for the MV-CBCT did not show
any correlation and/or improvement to the image quality
results. Imaging dose will be assessed and presented
based on absolute beam output calibration with a 6cc ion
chamber and OSLDs. The Tomotherapy data and analysis
is underway for comparisons.
Conclusion
Preliminary image quality tests of a prototype MV-CBCT
imaging system that utilizes a 6X-FFF beam and a fast
acquisition mode revealed acceptable performance, but
as expected, worse than those of kV-CBCT and diagnostic
CT. The fast acquisition may potentially be beneficial for
motion management treatments such as DIBH for breast
and lung tumors. The MV-CBCT also has superior contrast
linearity for higher density and metallic materials because
of reduced beam hardening, and could be utilized to
supplement diagnostic CT images for treatment planning
in such cases.
EP-1739 The feasibility of atlas-based automatic
segmentation of MRI for prostate radiotherapy planning
C. Morris
1
, M.J. Gooding
2
, A. Henry
3
, R. Speight
4
1
University of Leeds, Department of Medicine, Leeds,
United Kingdom
2
Mirada Medical Ltd, Mirada Medical Ltd, Oxford, United
Kingdom
3
St James Institute of Oncology, Department of Clinical
Oncology, Leeds, United Kingdom
4
St James Institute of Oncology, Medical Physics and
Engineering, Leeds, United Kingdom
Purpose or Objective
Atlas-based autosegmentation is an established tool for
segmenting structures for CT-planned prostate
radiotherapy. MRI is being increasingly integrated into the
planning process. The aim of this study was to assess the