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S478
ESTRO 36 2017
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Conclusion
For the first time, it has been possible to quantitatively
demonstrate that accumulated delivered dose to the
rectal wall is more strongly correlated with rectal bleeding
and proctitis in prostate radiotherapy than planned dose.
The results support the hypothesis that incorporating
delivered dose into multi-variable predictive models could
improve toxicity outcomes.
Poster: Physics track: CT Imaging for treatment
preparation
PO-0881 4DMRI for RT planning; novel precise
amplitude binning in the presence of irregular breathing
I. Bones
1
, O.J. Gurney-Champion
2
, A. Van der Horst
1
, A.
Bel
1
, T. Alderliesten
1
, G. Van Tienhoven
1
, K. Ziemons
3
, Z.
Van Kesteren
1
1
Academic Medical Centre, Radiotherapy, Amsterdam,
The Netherlands
2
Academic Medical Centre, Radiotherapy and Radiology,
Amsterdam, The Netherlands
3
FH Aachen University of Applied Sciences, Medical
Physics, Jülich, Germany
Purpose or Objective
Irregular breathing, often the case in clinical practice,
introduces the need for proper outlier handling for 4DMRI
reconstruction. Discarding outliers may lead to
underestimation of the respiratory-induced organ motion.
Our study aimed to develop and evaluate an amplitude
binning strategy that reduces reconstruction artefacts
while improving precision in the presence of irregular
breathing.
Material and Methods
Twelve volunteers and 2 abdominal cancer patients were
scanned with our 4DMRI sequence. In this 6 minute scan,
11 2D coronal slices were acquired repetitively (60 times)
during free breathing, using a T2W TSE sequence
(resolution: 1.3x1.6x5.0 mm
3
). Prior to each slice
acquisition, the position of the diaphragm was assessed
using a 1D acquisition.
Subsequently, the 2D slices were binned in 10 equidistant
bins according to the 1D diaphragm position (amplitude
binning). To account for outliers, we developed a strategy
that sets the inclusion range such that 95% of the
diaphragm positions are included, while the peak-to-peak
range is minimized (denoted Min95). We compared this
with two frequently used strategies (Fig.1): one that
selects the maximum inhale and exhale position as range
(MaxIE), not discarding outliers, and one that selects the
mean inhale and exhale position as inclusion range
(MeanIE).
The strategies were evaluated based on the following
parameters:
•
* Data included (DI); the fraction of data used
for reconstruction after exclusion of outliers.
•
* Reconstruction completeness (RC); the
fraction of the 110 (11 slices x 10 bins)
bin/slice combinations in the 4D data set that
are filled.
•
* Intra-bin variation (IBV); the standard error of
the mean diaphragm position inside a bin/slice
combination.
•
* Peak-to-peak range (PP);
•
* Image smoothness (S); assessed by quantifying
how well a parabola fits the diaphragm shape
in a sagittal plane of the reconstructed 4DMRI,
per bin (S = R
2
adj
averaged over all bins). S
ranges from 0 (discontinuous diaphragm shape;
artefacts) to 1 (smooth shape; no artefacts).
A low DI indicates underestimation of motion amplitude.
A low IBV indicates high binning precision. Low RC, low S
and high IBV result in image artefacts, e.g. discontinuities
between reconstructed slices.
A paired Wilcoxon’s signed rank test was used to test
differences in parameters between binning strategies.
Results
Excluding only 5% of images during amplitude binning, the
developed Min95 strategy outperformed the MaxIE
strategy with a 9.5% higher mean RC, 5.6 mm lower mean
PP and virtually the same mean IBV and S (all significant,
Table 1).
The MeanIE strategy with a mean DI of 76.4%, severely
underestimated the motion amplitude even though it had
a higher S, higher RC and lower IBV compared to MaxIE.
The Min95 strategy outperformed the MeanIE strategy with
an 18.6% higher mean DI.