ESTRO 36 Abstract Book

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S156 ESTRO 36 2017
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OC-0304 Real-time gamma evaluations of motion
induced dose errors as QA of liver SBRT tumour
tracking
T. Ravkilde1, S. Skouboe1 , R. Hansen1, E.S. Worm1, P.R.
Poulse n1
1 Aarhus University Hospital, Department of Oncology,
Aarhus N, Denmark
Purpose or Objective
Organ motion during radiotherapy can lead to serious
deterioration of the intended dose distribution. As modern
radiotherapy shifts increasingly towards escalated doses,
steeper dose gradients and hypofractionation, the
demands on accurate delivery increase concurrently. A
large body of studies show that tumour tracking can be
applied to mitigate the effects of motion and restore dose
fidelity, yet clinical introduction seems reluctant. In this
study we report on a method for continuous evaluation of
the tracking dose delivery that conforms to common dose
analysis practice and can be acted upon in real time.
Material and Methods
Experiments were performed on a TrueBeam linear
accelerator (Varian Medical Systems) with target motion
being recorded by an electromagnetic transponder system
(Calypso, Varian Medical Systems). A HexaMotion motion
stage (Scandidos) reproduced the liver motion traces for
five different liver SBRT patients as previously measured
using intrafraction kV imaging. VMAT SBRT treatment
plans were delivered to the moving phantom with MLC
tracking, without tracking (simulating the actual delivery)
as well as to a static phantom for reference (planned
delivery). Temporally resolved dose distributions were
measured at 72 Hz using a Delta4 dosimeter (Scandidos).
Accelerator parameters (monitor units, gantry angle, MLC
leaf positions, etc.) were streamed at 21 Hz to prototype
software that performed continuous reconstruction of the
dose in real time by a simplified non-voxel based 4D pencil
beam convolution algorithm. Also in real time, but on a
separate thread, 3%/3mm gamma evaluations were
calculated continuously throughout beam delivery to
quantify the deviation from the planned intent. After
experiments, the time-resolved gamma tests were
compared with the same quantities from the measured
data.
Results
The motion induced gamma errors were well
reconstructed both spatially (Figure 1) and temporally
(Figure 2). In 95% of the time both actual and planned
doses were reconstructed within 100 ms. The median time
for reconstruction was 65 ms, which translates into a
typical frequency of about 15 Hz. Asynchronously, but also
continuously, 95% of gamma evaluations were performed
within 1.5 s with the median being at 1.2 s. Over all
experiments the root-mean-square difference between
reconstructed and measured gamma failure rates was
2.9%.
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. Ehrbar1, S. Schmid1, S. Klöck1, M. Guckenberger 1, O.
Riesterer1, S. Tanadini-Lang1
1University 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.