170 / 1082
S157
ESTRO 36 2017
_______________________________________________________________________________________________
Radiation dose was measured within the phantom by a
biplanar diode array. The dosimetric performance of
couch tracking was compared to no compensation: Gamma
agreement and other dose parameters were evaluated
within the biplanar array, as well as target- and organ-
specifically.
Results
The root-mean-square error of the motion traces (range:
0.8-4.4 mm) was substantially reduced with couch
tracking (0.2-0.4 mm). Spikes (>1 mm) in the compensated
motion curve were only observed at steep gradients (>7.5
mm/s). The dose measurements with the phantom showed
on the 1%/1 mm level significantly better gamma
agreement with tracked motion (range: 83.4%-100%) than
with untracked motion (28.9%-99.7%). Also with the
2%/2 mm criterion, gamma agreement was significantly
superior for the tracked motion (98.4%-100%) compared to
the untracked (52.3%-100%) (see Fig. 1). Also the organ
specific evaluation resulted in significantly better target
coverage with tracking, however the dose to the rectum
and bladder showed a dependency on the motion
direction.
Conclusion
Couch tracking was able to mitigate the prostate motion
and improved the dosimetric accuracy of prostate SBRT.
The treatment couch was able to compensate the
prostatic motion with only some minor residual motion at
steep motion gradients. Therefore, couch tracking
combined with electromagnetic position monitoring for
prostate SBRT is feasible and improves the accuracy in
treatment delivery when prostate motion is present.
OC-0306 Is re-gating a robust motion mitigation
approach independent of PBS scanning scenario?
Y. Zhang
1
, I. Huth
2
, M. Wegner
2
, D.C. Weber
1
, A.J.
Lomax
1
1
Paul Scherrer Institute PSI, Center for Proton Therapy,
Villigen PSI, Switzerland
2
Varian Medical Systems, Particle Therapy GmbH,
Troisdorf, Germany
Purpose or Objective
Different scanned proton therapy (PBS) systems provide
different scanning dynamics, directly changing the
temporal interference between pencil beam delivery and
tumour motion. With this study, we have systematically
evaluated interplay effects, and compared motion
mitigation performance, for different PBS scanning
delivery scenarios.
Material and Methods
Using 6 4DCT(MRI) datasets of liver tumours, with irregular
motions >10mm (CTV: 100-400cc; period: 5.3/6.3s), 4D
treatments assuming different prescription doses
(2/12Gy), field directions (AP/LR) and mitigation
approaches (3 gating windows (GW) with or without 1-9
layered (LS) or volumetric (VS) rescanning) were simulated
for 8 PBS scanning scenarios (see table). As such,
simulations were performed for spot or raster scanning
(positioning with/without beam pulse between successive
spots), for constant and varied beam current, and for
cyclotron or synchrotron beam dynamics (e.g. continuous
or pulsed beam). The resulting 4D plans were compared
using dose-volume metrics (D5-D95, V95) for the CTV, as
well as total predicted treatment time (TT).
Results
Independent of the delivery scenario and prescribed dose,
neither gating alone nor rescanning alone could mitigate
motion effects completely, with residual interplay effects
(D
5-95
) of more than 10-20% being observed for GW=5mm
w.o. rescanning (shown by purple error-bars). Moreover,
the D
5-95
of synchrotron based simulations were found to
be >5% higher than for cyclotron scenarios. However, with
re-gating (re-scanning + gating, shown by green and blue
error-bars), motion mitigation performance was found to
be similar effective (close to static reference) for all
scanning dynamics and rescan modes, with the main
difference being only in treatment efficiency. Without any
mitigation, mean TT’s for the 2Gy/12Gy plans were 2x/3x
longer for synchrotron than for cyclotron scenarios. For re-
gating (GW5+LS5), mean TT’s of synchrotron based plans
were on average 2.5x higher when combined with LS and
3.5x higher when combined with VS. Moreover, the
advantage of varying beam current has been
demonstrated by the approximately constant TT as a
function of prescription dose. In addition, for the high
dose scenario, variations caused by differences in
geometry, motion amplitude, field direction and starting
phase, are smaller for varying beam current scenarios in
comparison to corresponding constant scenarios.