973 / 1096
S957
ESTRO 36
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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
feasibility of MRI-based atlas-based autosegmentation for
organs-at-risk (OAR) and prostate target volumes, and to
compare the segmentation accuracy with CT-based
autosegmentation.
Material and Methods
Images were retrospectively selected from 6 prostate
patients who received whole field T2 weighted 3D SPACE
MRI and CT in the radiotherapy treatment position (at the
Northern Centre for Cancer Care, Newcastle). Organs at
risk (Bladder, rectum, seminal vesicles, left and right hips,
penile bulb) and the prostate were manually delineated
on the CT and MRI separately. A ‘leave one out’ approach
was used to automatically segment structures onto the
remaining images separately for CT and MRI. Contour
comparison was performed using the DICE index and mean
distance to conformity (MDC) positional metrics. MDC,
DICE and absolute volume were used to assess the
performance of the contouring by comparing the
automatic to the manual contours. A paired t test was
used to determine the statistical significance between MRI
and CT.
Results
The volume analysis (data not presented) showed that
manual and automatic contouring on MRI gave smaller
contours than CT (significantly so for the hips, prostate
and seminal vesicles). The positional analysis results are
shown in table 1. MRI autocontouring was more accurate
than CT for the bladder (MDC significantly so) and the
prostate/penile bulb (although not significantly). There
was little difference in accuracy between CT/MRI
autocontouring for both hips, rectum and seminal vesicles.
Conclusion
Accurate atlas-based automatic segmentation of
structures for prostate radiotherapy is feasible using T1-
MRI; segmentation of the penile bulb and seminal vesicles
was found to be poor. Comparison with CT-based
automatic segmentation suggests that the process is
equally or more accurate using MRI. Although this study
was on a small sample size these results support further
translation of MRI-based segmentation methodology into
clinical practice.
EP-1740 Nationwide audit of multileaf collimators
performance
K. Chelminski
1
, W. Bulski
1
1
The Maria Sklodowska-Curie Memorial Cancer Center,
Medical Physics Department, Warsaw, Poland
Purpose or Objective
The delivery of accurate intensity-modulated radiation
therapy (IMRT) or stereotactic radiotherapy depends on a
multitude of steps in the treatment delivery process. The
proper intensity modulation depends on the proper
functioning of a multileaf collimators (MLC). The aim of
this audit was the control of the proper collimator leafs
positioning.
Material and Methods
The methodology of the audit of small field output
performance was established within the framework of the
CRP E2.40.16 project "Development of Quality Audits for
Radiotherapy Dosimetry for Complex Treatment