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the transponder motion and treatment delivery were used
to calculate the motion-induced geometrical errors during
beam-on in the actual gated treatments and in simulated
non-gated standard treatments with CBCT-guided setup to
the mean transponder centroid position before each
fraction. The observed motion was used to reconstruct the
actually delivered CTV dose distribution with gating and
the would-be dose distribution without gating.
Results
Fig. 1 shows the internal tumor motion during a single
fraction. Due to drift and respiratory motion the mean (+/-
SD) geometric error during non-gated treatment at this
fraction (Fig1A) would have been 1.3mm (1.7) LR, 5.0mm
(7.7) CC, and -2.0mm (1.8) AP. The gated treatment,
including 5 couch shifts to counteract drift (Fig1B),
reduced the errors to 0.7mm (0.7) LR, 0.4mm (1.9) CC,
and -0.1mm (0.9) AP. Fig. 1C shows the CC geometrical
errors for all patients. The mean (range) number of couch
corrections for drifts during each gated fraction was 2.8
(0-7). The mean duty cycle during gated treatment was
60.8% (31.7-72.7%). As shown in Fig 2A, gating markedly
reduced the population based PTV margin needed for
intrafraction
motion.
Motion-including
dose-
reconstruction provided the CTV-DVHs of all fractions of
planned, actual gated delivered, and simulated non-gated
delivered doses. Mean CTV-DVHs are shown in Fig 2B. Note
the large DVH variation for non-gated treatments. The
mean (range) reduction in CTV D
95
relative to the planned
dose was 0.9 percent points (0.1-2.3) with gating and 6.8
percent points (0.9-29.6) without gating.
Conclusion
Gating based on internal motion monitoring markedly
reduced geometric and dosimetric errors in liver SBRT
compared to non-gated standard treatment. Results of the
full trial (15 patients) are expected for presentation at
ESTRO.
PV-0284 3D Performance Analysis of Cyberknife
Synchrony® Respiratory Tracking System
M.C. Sahin
1
, P. Hurmuz
1
, M. Yeginer
1
, G. Yazici
1
, G.
Ozyigit
1
1
Hacettepe University Faculty of Medicine, Radiation
Oncology, Ankara, Turkey
Purpose or Objective
Tumor movement is a challenging issue for the precise
delivery of radiation for thoracic tumors. The Synchrony
respiratory motion tracking system (RMTS) of Cyberknife®
robotic radiosurgery unit synchronizes radiation beam
delivery with the respiration induced tumor motion. This
study aims to investigate the performance of Synchrony
RMTS for different movement widths using polymer gel
dosimetry. To the best of our knowledge this is the first
study to make the three dimensional performance analysis
of Synchrony RMTS.
Material and Methods
The MultiPlan® treatment planning system (TPS) of
Cyberknife® was used to deliver 4 Gy to a tumor of 1X1X1
cm
3
. BrainLab Gating lung phantom was used to simulate
lung movements with three different amplitudes (1 cm, 2
cm and 3 cm). Three fiducials were inserted to the
phantom for tracking. Radiochromic film and polymer gel
dosimetry were used and measurements were compared
with the dose distributions acquired from the TPS. The
dose information of irradiated gel were read out using 1.5
T magnetic resonance imaging. The gamma index values
were analysed using the Ashland FilmQA Pro 3.0 software
for film dosimeters and Polygevero software for gel
dosimeters using the 3mm/3% criteria. PolyGevero gamma
index value of ≤1 is accepted as a passing criteria
according to the literature.
Results
The mean 3 mm/3% gamma index values of film dosimetry
were 92.6±1.94%, 91.0±4.00%, 90.3±2.04% for tumor
motions of 1 cm, 2 cm and 3 cm, respectively
(p<0.001). For polymer gel dosimetry, the mean gamma
index values calculated over almost three million points
were 0.56±0.10, 0.60±0.24 and 0.65±0.30 for tumor
motions of 1 cm, 2 cm and 3 cm, respectively (p<0.001).
Although the difference was statistically significant for 3
different amplitudes, the performance of the system was
within the acceptance limits