ESTRO 36 Abstract Book

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S465 ESTRO 36 2017
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(NKI).
PO-0862 Correlation of Liver and Pancreas Tumor
motion with Normal Anatomical Stru ctures
R. Kaderka1, A. Paravati1, R. Sar kar1, J. Tran1, K. Fero1,
N. Panjwani1, D. Simpson1, J. Murphy1, T. Atwood 1
1 University of California San Diego, Department of
Radiation Medicine and Applied Sciences, San Diego, USA
Purpose or Objective
Target motion caused by respiration remains the central
challenge to delivering SBRT in the abdomen. For targets
in the pancreas and liver, SBRT oftentimes necessitates
placement of metal fiducials to determine tumor position
with fluoroscopy, due to difficulty in visualizing tumors on
non-contrast imaging. Metal fiducials have limitations in
that they represent an invasive procedure which can
introduce treatment delays. Furthermore, fiducials can
migrate from their intended position, and the metal can
introduce imaging artifacts which make tumor delineation
a challenge. We hypothesized that upper abdominal tumor
motion would correlate with the motion of nearby organs
and could thereby serve as a fiducial-less proxy for tumor
motion.
Material and Methods
Fifteen patients (12 with pancreas and 3 with liver tumors)
underwent a 4-dimensional (4D) CT simulation prior to
treatment with SBRT. 4D CT images were divided into 10
phases and normal tissues were contoured on a single 4D-
CT phase and propagated to the other phase s using
deformable image registration. As a means of quality
control for image registration and contour propagation the
liver was manually contoured on all phases for 5 patients
by physicians and compared to the automated contour
propagation using a Dice coefficient. Motion was defined
from the center-of-mass of each structure, and a patient-
specific linear tumor position prediction model based on
liver position was developed.
Results
We found a strong overlap of manually entered contours
and the automatically segmented contours with a mean
Dice-coefficient of 0.95 (standard deviation 0.01). The
linear models accurately predicted tumor motion with a
mean absolute error of 0.5 mm and no error greater than
3.0 mm. Mean absolute and maximum errors by direction
and tumor type are listed in the table below.
Left-right
direction
Anterior-
posterior
direction
Superior-
inferior
direction
Pancreas tumors
mean absolute
error (mm)
0.3 0.4 0.5
Pancreas tumors
maximum error
(mm)
1.0 1.7 2.6
Liver tumors mean
absolute error
(mm)
0.3 0.4 0.8
Liver tumors
maximum error
(mm)
1.4 2.7 3.0
Conclusion
This study demonstrates that normal organ motion could
serve as a fiducial-less proxy for tumor motion with SBRT
in the upper abdomen when on-site real-time 4D
volumetric imaging becomes available during treatment.
Deformable image registration has been demonstrated to
be a reliable and fast tool for segmentation of normal
organs. Moving this motion management approach into
clinic requires additional research to optimize 4D image
quality and understand inter-fraction reproducibility.
PO-0863 Suggestion of optimal planning target volume
margins for stereotactic body radiotherapy of the spine
S.H. Jeon 1, S.Y. Park1, J.H. Kim1, J.I. Kim1, J.M. Park1
1Seoul National University Hospital, Radiation Oncology,
Seoul, Korea Republic of
Purpose or Objective
To suggest an optimal planning target volume (PTV)
margin in stereotactic body radiotherapy (SBRT) of the
spine.
Material and Methods
From December 2014 to July 2016, 40 patients received 42
fractions of SBRT for spinal tumors to thoracic or
lumbosacral spines using a volumetric modulated arc
therapy technique and patient immobilization. Before
treatment, kilovoltage cone-beam CT (CBCT) images were
obtained for a 4 degrees of freedom (DoF) correction of
patients alignment (translation + yaw). After corrections
were made, additional CBCT was acquired just before
treatment delivery (pretreatment CBCT). Immediately
following SBRT, CBCT was acquired again (posttreatment
CBCT). Residual setup errors for pretreatment CBCT was
determined by a 6 DoF manual matching. Intrafraction
motions were calculated as differences in errors between
pretreatment and posttreatment CBCT. Three clinical
target volumes (CTVs) were generated by translating and
rotating original CTV by residual setup errors alone
(CTV_R), intrafraction motions alone (CTV_I), and residual
setup errors and intrafraction motions combined
(CTV_R+I), respectively. Adding various uniform margins
to original CTV generated PTVs. The impact of PTV
margins on CTV coverage was evaluated. A provisional
criterion of adequate CTV coverage was that PTV
encompasses at least 97% of CTV.
Results
Time interval between pre-treatment and post-treatment
CBCTs was 6.8±2.5 min (mean±2SD). The 2SD values of
lateral, vertial, longitudinal translations and pitch, roll,
and yaw were 0.7mm, 0.8mm, 1.1mm, 1.7°, 1.1°, and
1.6°for residual setup errors and 1.0mm, 0.9mm, 0.9mm,
1.1°, 0.8°, and 1.1°for intrafraction motions,
respectively. Without margins, PTV showed adequate
coverage for CTV_R, CTV_I, and CTV_R+I in 48% (20/42),
71% (30/42), and 48% (20/42) of fractions, respectively.
With 1-mm uniform margins, PTV was adequate for 95%
(40/42), 98% (41/42), and 100% (42/42) of fractions,
respectively. 2-mm uniform margin was adequate in all
fractions for all three CTVs.
Conclusion
With appropriate immobilizations and 4DoF corrections, a
uniform 1-mm PTV margin may ensure an adequate CTV
coverage in most treatment sessions of spine SBRT.
Combined with a shortened treatment time, the small
extent of intrafraction motions may obviate the need of
treatment interruption for additional intra-session image
guidance. Despite perfect 6 DoF patient alignment, 1-mm
PTV margin is still needed to address intrafraction
motions.
PO-0864 Accuracy of fiducial based correction of target
motion in prostate SBRT treatments
T. Viren1, M. Korhonen2, J. Seppälä1
1Kuopio University Hospital, Cancer Center, Kuopio,
Finland
2University of Eastern Finland, Department of Applied
Physics, Kuopio, Finland
Purpose or Objective
Robotic stereotactic body radiotherapy (SBRT)
incorporating a fiducial based motion tracing system has
enabled almost real-time correction of intra-fraction
motion of a prostate during SBRT treatments of prostate
cancer. However, the effect of number and positioning of
the fiducials and the amount of prostate movements on
the accuracy of the treatment has not been reported. The
aim of the present study was to investigate the accuracy
of the fiducial based correction of target motion in