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S154
ESTRO 36 2017
_______________________________________________________________________________________________
Nanotechnology- Center for Nanomedicine and
Theranostics, Lyngby, Denmark
4
Aarhus University Hospital, Department of Oncology,
Aarhus, Denmark
Purpose or Objective
The purpose of this study was to estimate the intra- and
inter-breath-hold tumour position uncertainty in voluntary
deep-inspiration breath-hold (DIBH) radiotherapy for
patients with locally advanced non-small cell lung cancer.
Material and Methods
Patients had liquid fiducial markers injected in
mediastinal lymph nodes, and, if possible, in the primary
tumours. Treatment was delivered during DIBH. Anterior
and lateral fluoroscopic movies were acquired in free
breathing (FB) and visually guided DIBH at three fractions
(start, middle and end) during radiotherapy (33 fractions,
2 Gy per fraction) of nine patients with locally advanced
non-small cell lung cancer. Fluoroscopies were acquired
post treatment for two perpendicular gantry angles
(Figure 1). Marker excursions in free breathing and DIBH,
inter-breath-hold position uncertainty, systematic and
random errors during DIBH in each of the three cardinal
directions were investigated using an image based
tracking algorithm, defining the marker template as one
of the images from the middle of the first DIBH
fluoroscopy.
The mean marke r position during each DIBH, relative to a
template frame for the first fluroscopy, was regarded as
each fractions and markers uncertainty during the DIBH. A
systematic error for the patient group was calculated as
the standard deviation (SD) of all these mean marker
positions. The standard deviation of the markers position
within each DIBH was used to quantify the intra-breath-
hold uncertainty (Figure 1). A root mean square (RMS) of
the intra-DIBH SD was calculated to estimate random
errors.
Results
A reduction of 2-6 mm in marker excursion in DIBH
compared to FB was observed in the three cardinal
directions (anterior-posterior (AP), left-right (LR) and
cranio-caudal (CC)). Fourier transformation of the motion
trajectories indicated that the lymph node motion during
DIBH mainly originated from cardiac motion. The
systematic errors during DIBH were 0.5 mm (AP), 0.5 mm
(LR) and 0.8 mm (CC). The random errors during DIBH were
0.3 mm (AP), 0.3 mm (LR), and 0.4 mm (CC). The standard
deviation of the inter-breath-hold shift was 0.8 mm (AP),
0.6 mm (LR), and 1.0 mm (CC) (Figure 2).
Conclusion
Our study showed that the motion of lung tumours could
be substantially reduced, but not eliminated, using
visually guided DIBH radiotherapy. Intra- and inter-breath-
hold position uncertainty of the tumour and lymph nodes
were mostly less than 2 mm for visually guided DIBH
radiotherapy of non-small cell lung cancer.
OC-0302 Dosimetric evaluation of a global motion
model for MRI-guided radiotherapy
C. Paganelli
1
, S. Albertini
1
, F. Iudicello
1
, B. Whelan
2
, J.
Kipritidis
2
, D. Lee
2
, P. Greer
3
, G. Baroni
1
, P. Keall
2
, M.
Riboldi
1
1
Politecnico di Milano, Dipartimento di Elettronica-
Informazione e Bioingegneria, Milano, Italy
2
University of Sydney, Radiation Physics Laboratory-
Sydney Medical School, Sydney, Australia
3
Calvary Mater Newcastle, Department of Radiation
Oncology, Newcastle, Australia
Purpose or Objective
MRI-Linac therapy will enable real time adaption of
radiotherapy and is being actively developed by several
academic and commercial groups. To acquire images of
high spatial and temporal resolution, interleaved 2D
imaging is typically used. However, to enable closed loop
adaptive radiotherapy, accumulated 3D dose is required.
A possible way to bridge the gap between 2D and 3D
images is via patient-specific motion models. To date, no
dosimetric evaluation of a global motion model based on
interleaved MRI images has been reported. In this work,
we present the use of a global motion model to
compensate for geometric changes during treatment and
to evaluate dosimetric variations between the delivered
and planned dose distributions.
Material and Methods
4DCT and interleaved sagittal/coronal cine-MRI from a
diagnostic scanner (1.5T) were acquired for a lung cancer
patient. A global motion model was built on the 4DCT
dataset using principal component analysis, and updated
through the use of surrogates derived from in-room cine-
MRI data (tumor, diaphragm and lung vessel motion). An
ITV-based IMRT treatment plan (60Gy in 30 fractions) was
developed on the 4DCT and applied to the model output
for dose evaluation. Validation of the motion model was
performed on a CT/MRI XCAT phantom (1mm resolution),
in which the ground truth CT output of the in-room
scenario was available at the time sample of the simulated
cine-MRI. Analysis of different surrogates as well as their
sagittal/coronal motion components were performed in
terms of both geometric and dosimetric variations.
Results
Based on the phantom data, the accuracy of the motion
model was 1.2mm/1.6mm on tumor/diaphragm.