ESTRO 36 Abstract Book

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S158 ESTRO 36
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Conclusion
Motion induced errors in dose were accurately and
continuously reported by gamma evaluations within two
seconds of occurring. Such monitoring may improve
patient safety by treatment intervention in case of gross
treatment errors and may help to expedite clinical use of
tracking. While developed mainly with tumour tracking in
mind its use is also readily available for standard non-
tracking treatments.
OC-0305 Validation of Dynamic Treatment-Couch
Tracking for Prostate SBRT
S. Ehrbar 1, S. Schmid1, S. Klöck1, M. Guckenberger1, O.
Riesterer 1, S. Tanadini-Lang1
1 University Hospital Zürich, Department of Radiation
Oncology, Zurich, Switzerland
Purpose or Objective
In stereotactic body radiation therapy (SBRT) of prostatic
cancer, a high dose per fraction is applied to the treated
region with steep dose gradients. Intrafractional prostate
motion can occur unpredictably during the treatment and
lead to target miss. Missing the target results in high doses
to nearby organs which can cause complications. It is
essential for a prostate SBRT treatment to observe and
mitigate this motion. Dynamic treatment-couch tracking
is a real-time adaptive therapy technique, compensating
the prostate displacement by counter-movement with the
treatment couch. This work investigated the dosimetric
benefit of couch tracking for prostate SBRT treatments in
the presence of prostatic motion.
Material and Methods
Ten previously treated prostate cancer patients with one
index lesion were selected. Treatment target volumes
(prostate and index lesion), and organs at risk (OAR:
bladder, rectum and urethra) were delineated using the
patient’s treatment CT and MRI scans. SBRT treatment
plans with integrated boost were prepared with a
prescribed dose of 5x7 Gy to the prostate and 5x8 Gy to
the index lesion. The treatment plans were applied with a
linear accelerator to a phantom, which was either i) in
static position, ii) moved according to five prostate motion
curves without motion compensation or iii) with real-time
compensation using electromagnetic guided couch
tracking. Electromagnetic transponders were mounted on
the phantom surface and their geometrical position was
evaluated in the tracked and untracked situation.
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. Zhang1, I. Huth2, M. Wegner2, D.C. Weber1, A.J.
Lomax1
1Paul Scherrer Institute PSI, Center for Proton Therapy,
Villigen PSI, Switzerland
2Varian 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,