ESTRO 36 Abstract Book

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S470 ESTRO 36
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S.H. Jeon1, S.Y. Park1, J.H. Kim1, J.I. Kim1, J.M. Park1
1 Seoul 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. Viren 1, M. Korhonen2, J. Seppälä1
1 Kuopio University Hospital, Cancer Center, Kuopio,
Finland
2 University 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
prostate SBRT treatments and to evaluate the effect of
fiducial number and positioning to the accuracy of the
fiducial tracking.
Material and Methods
CT image was acquired from custom-made phantom
incorporating different fiducial configurations (Fig 1).
Subsequently, typical prostate SBRT treatment plan
(5x7.25Gy) was calculated in the phantom using treatment
planning software (Ray Tracing algorithm, Multiplan,
Accuray, USA). To measure the dose distribution within
the phantom calibrated Gafchormic films (4 x 4 inch,
Gafchromic EBT 3, RPD Inc., USA) were placed inside the
phantom. A prostate treatment was irradiated in three
different phantom positions: no movement, typical
clinical prostate movements, and maximum movements
allowed by the automatic fiducial tracing system (Fig
1).The phantom movements were conducted using
Robochouch (Accuray, USA).To mimic the suboptimal
positioning of the fiducials the measurements were
repeated with four different seed configurations (optimal,
typical clinical case, clinical case with three fiducials,
clinical case with two fiducially). Measurements were
conducted in coronal and sagittal planes. Finally, the films
were scanned (Perfection V700, Epson, USA) 72 hours after
the irradiation and the measured and calculated dose
distributions were compared using gamma-analysis
(5%/2mm threshold).
Figure 1. A) Custom made phantom used to measure
prostate SBRT treatment plans. B) The directions of the
prostate movements and rotations. C) Typical clinical and
maximum intra-fraction prostate movements used in the
present study
Results
The accuracy of the automatic correction of intra-fraction
motion of the target was clinically acceptable when three
or four seed configuration was used in the motion tracking
(Table 1). No significant changes in gamma pass rates were
detected when the amount of phantom movement was
increased. Clinically unacceptable gamma pass rates were
detected only when two fiducials where used in tracking.
Table 1. Gamma pass rates of measured and calculated
treatment plan comparisons for different fiducial
configurations and phantom movements.
Conclusion
Automatic correction of the target movement was
reasonably accurate for clinical use when three or four