ESTRO 36 Abstract Book

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S148 ESTRO 36
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Conclusion
A-priori information from 4DMRI provides a breathing
phase-specific approximation of OOPM and can be used to
correct OOPM in slice-to-slice registrations. Such
procedure significantly improved GTV position estimation
when relevant OOPM is observed, i.e. on the axial slice.
The corrected IPM represents a more accurate
approximation of the motion field that would be measured
if full 3D volumes were acquired and registered in real-
time to the planning data. Future work should focus on
robustness to inter-fraction variations in patients’ data.
[1]Mutic et al 2014 Semin Radiat Oncol
[2]Paganelli et al 2015 MICCAI
[3]Zachiu et al 2015 PMB
PV-0283 Gated liver SBRT based on internal
electromagnetic motion monitoring
E. Worm1, M. Høyer2,3 , R. Hansen1, L.P. Larsen4, B.
Weber1, C. Grau1,3 , P. Poulsen1,3
1 Aarhus University Hospital, Department of Oncology,
Aarhus, Denmark
2 Aarhus University Hospital, The Danish Centre for
Particle Therapy, Aarhus, Denmark
3 Aarhus University, Institute of Clinical Medicine,
Aarhus, Denmark
4 Aarhus University Hospital, Department of Radiology,
Aarhus, Denmark
Purpose or Objective
To present our results with the new technique of
respiratory gated liver SBRT based on internal
electromagnetic motion monitoring. The study presents
the geometric and dosimetric improvements in treatment
accuracy of the gating compared to standard CBCT-guided
non-gated treatment.
Material and Methods
Thirteen patients with primary liver cancer or metastases
had three electromagnetic transponders (Calypso)
implanted near the target and received three-fraction
gated liver SBRT at a TrueBeam Linac. The PTV was
created by a 5mm axial and 7mm (n=10) or 10mm (n=3)
cranio-caudal (CC) expansion of the CTV as defined on an
exhale breath-hold CT. A mean homogenous dose between
45 and 61.8Gy was prescribed to the CTV using 7-field
IMRT or 3D conformal planning. The PTV was covered with
67% of the prescribed dose. Treatment was delivered in
free-breathing but gated to the exhale breathing phase
according to the continuously monitored (25Hz)
transponder centroid position. Gate ON windows were set
to +/- 3mm LR/AP and +/-4 mm CC around the exhale
position of the transponders. The couch was adjusted
remotely if baseline drifts above ~1mm of the exhale
transponder position occurred. Post-treatment, log files of
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 D95 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.