168 / 1082
S155
ESTRO 36 2017
_______________________________________________________________________________________________
Dosimetric analysis confirmed the ability of the model to
approximate the ground truth (mean differences of
0.49Gy, 0.12Gy and 0.38Gy for PTV D98, PTV D2 and spinal
cord, respectively). For the patient, variations between
the in-room inhale phase and the corresponding planning
phase were 3.1mm/4.9mm on the tumor/diaphragm. With
respect to the planning inhale CT, the model output CT
presented differences mainly on the diaphragm position
(Figure A). Dosimetric changes with respect to the planned
dose were 3.14Gy, 1.82Gy and 0.42Gy for tumor PTV D98,
PTV D2 and spinal cord, respectively (Figure B and C). The
delivered dose was higher than planned since less motion
was present in the MR images than the planning CT.
Conclusion
We provided a dosimetric evaluation based on a global
motion model for MRI-guidance. The proposed model built
on 4DCT was updated based on interleaved 2D MRI data
and validated using a digital phantom. Dosimetric
variations on tumor were observed in the patient study,
demonstrating the utility and importance of using motion
models for dose accumulation. Future work will include
improvements in the motion model for MRI-guidance and
its application to a larger number of patients.
OC-0303 Evaluation of lung anatomy vs. lung volume
reproducibility for scanned proton treatments under
ABC.
L.A. Den Otter
1
, E. Kaza
2
, R.G.J. Kierkels
1
, M.O. Leach
2
,
D.J. Collins
2
, J.A. Langendijk
1
, A.C. Knopf
1
1
UMCG University Medical Center Groningen,
Department of Radiation Oncology, Groningen, The
Netherlands
2
The Institute of Cancer Research and The Royal Marsden
Hospital, CR-UK Cancer Imaging Centre, London, United
Kingdom
Purpose or Objective
Proton therapy is a highly conformal way to treat cancer.
For the treatment of moving targets, scanned proton
therapy delivery is a challenge, as it is sensitive to motion.
The use of breath hold mitigates motion effects. Due to
the treatment delivery over several fractions with delivery
times extending the feasible breath hold duration, high
reproducibility of breath holds is required. Active
Breathing Control (ABC) is used to perform breath holds
with controlled volumes. We investigated whether the
lung anatomy is as reproducible as lung volumes under
ABC, to consider ABC for scanned proton treatments.
Material and Methods
For five representative volunteers (3 male, 2 female, age:
25-58, BMI: 19 – 29) MR imaging was performed during ABC
at two separate fractions. The image voxel size was
0.7x0.7x3.0 mm
3
. Each fraction consisted of four
subsequent breath holds, resulting in a total of eight MRIs
per volunteer. The interval between fractions was 1-4
weeks, keeping the same positioning. The intra-fraction
reproducibility of the lung anatomy during breath hold was
investigated, by comparing the MRI of the first breath hold
with the three other MRIs of the same session. The inter-
fraction anatomical reproducibility was investigated by
comparing the first breath hold MRI of the first session
with the four MRIs during the second session. To avoid any
influence of setup variation, first a global rigid image
registration was performed. Then the lung volume was
semi-automatically segmented to define a region of
interest for the deformable image registration (DIR). DIR
was performed using Mirada RTx v1.2 (Mirada Medical,
Ltd.), with a DIR grid resolution of 3.5x2x3 mm
3
. The
deformation vector fields were analyzed using MATLAB
v2014b. Magnitudes of the deformation vectors were
calculated and combined for all five volunteers. The lung
volumes were divided into six segments, to analyze the
anatomical displacements on a local level. A boxplot
showing the intra- and inter-fraction displacements with a
schematic view of the six segments can be seen in figure
1.
Results
The lung volumes for all breath holds varied by 2% within
and 7% between fractions. Looking at all five volunteers,
up to 2 mm median intra- and inter-fraction displacements
were found for all lung segments. The anatomical
reproducibility decreased towards the caudal regions.
Inter-fraction displacements were larger than intra-
fractional displacements. Maximum displacements (99.3%
of the magnitude vectors) reached 6 mm intra-fractionally
and did not exceed 8 mm inter-fractionally.
Conclusion
While the lung volume differences were insignificant,
relevant anatomical displacements were found. Moreover,
a trend of increased displacements over time could be
seen. ABC mitigates motion to some extent. Nevertheless,
the remaining reproducibility uncertainties need to be
considered during scanned proton therapy treatments. As
next step, we aim to include this knowledge in a model to
estimate their dosimetric influence for scanning proton
therapy.