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S142
ESTRO 36 2017
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OC-0277 Assessment of the implant geometry in
interstitial brachytherapy by a hybrid tracking system
N. Pallast
1
, M. Kellermeier
1
, K. Kallis
1
, B. Steinmetz
1
, V.
Strnad
1
, C. Bert
1
1
Universitätsklinikum Erlangen- Friedrich-Alexander-
Universität Erlangen-Nürnberg, Department of Radiation
Oncology, Erlangen, Germany
Purpose or Objective
Electromagnetic tracking (EMT) is a promising g approach
to measure variations of the implant geometry in
interstitial brachytherapy. The coordinate system for EMT
data measurements is usually decoupled from the one of
computed tomography (CT) used for treatment planning.
Therefore, an optical tracking system (OTS) is introduced
to associate EMT and CT coordinate systems. The accuracy
of this hybrid tracking system was investigated in phantom
studies and the system is currently used in a clinical
feasibility study.
Material and Methods
EMT data providing the implant geometry were measured
by an implant sensor integrated in the cable of an
afterloader prototype (Flexitron, Elekta, The
Netherlands). Breathing motion was compensated by
three additional fiducial sensors on the chest.
Simultaneously, an OTS (Polaris, NDI, Canada) collected
data of eight infrared (IR) markers of which three were
attached to the EMT fiducial sensors and five to the skin.
A reproducible marker position was ensured by adhesive
pads glued on the patient prior the first measurement.
To align both coordinate tracking systems, a
transformation between the OTS and EMT (
OTS
T
EMT
) was
estimated by a so-called hand eye calibration. An
additional registration from the OTS to the CT (
CT
T
OTS
) was
determined to associate their coordinate systems. Finally,
both transformations were combined to get a direct
relation
of
EMT-
and
CT-derived
data.
The resulting calibration error of the
OTS
T
EMT
transformation was evaluated by measuring different
poses of an in-house developed calibration tool. This tool
was also used to assess the registration error of
CT
T
OTS
. To
prove the overall accuracy of this approach, another in-
house developed phantom was used, equipped with eleven
catheters and eight IR-markers. The feasibility of a daily
patient data acquisition was examined in an institutional
review board-approved study by using the afterloader
prototype and the OTS in addition to regular iBT
treatments.
Results
Based on different poses of the calibration tool, the root
mean square errors for
OTS
T
EMT
and
CT
T
OTS
were 0.56 mm
and 0.49 mm, respectively. The overall accuracy of
CT
T
EMT
resulted in 0.74 mm. The determined transformations
were applied to the phantom measurements and showed
a mean deviation to the CT data of 0.92 mm. Currently,
65 catheters from four patients were tracked by the EMT
system in combination with the OTS. The deviations of the
implant geometry, determined by this hybrid tracking
approach, are comparable to the previous results,
obtained using only the EMT procedure.
Conclusion
The novel hybrid tracking system allows direct mapping of
EMT and CT data. So far, the system was successfully used
to measure the implant of four patients. The clinical study
is ongoing.
OC-0278 Red-emitting inorganic scintillation detectors
to verify HDR brachytherapy treatments in real time
G. Kertzscher
1
, S. Beddar
1
1
The University of Texas MD Anderson Cancer Center,
Department of Radiation Physics, Houston, USA
Purpose or Objective
Treatment verification during brachytherapy (BT) is
presently limited because only few detectors can measure
accurate and precise dose rates in the steep gradients that
are characteristic for HDR BT. Red-emitting inorganic
scintillation detectors (ISDs) are promising for BT because
they can generate large signal intensities. Furthermore,
they facilitate efficient background suppression because
of the small overlap with the Cerenkov and fluorescence
light contamination induced in the fiber-optic cable (the
stem signal) primarily emitted in blue/green regions. The
purpose of this study was to assess the suitability of red-
emitting ISDs for real-time verification during BT.
Material and Methods
We investigated ISDs based on the 5 inorganic scintillators
ruby, Y
2
O
3
:Eu, YVO
4
:Eu, Y
2
O
2
S:Eu and Gd
2
O
2
S:Eu, of which
the first was rigid and the others in powder form. The ISDs
were compared with plastic scintillation detectors (PSDs)
based on the organic scintillator BCF-12. Each detector
consisted of a 1 mm-diameter scintillator that was coupled
to a 1 mm-diameter and 15 m-long fiber-optic cable.
Optical filters were placed between the ISD volume and
the fiber-optic cable to prevent the stem signal from
striking the scintillator and inducing photoluminescence.
The fiber-optic cable was coupled to a charge-coupled
device camera or a spectrometer to measure signal
intensities and emission spectra, respectively. The
detectors were exposed to an
192
Ir HDR BT source in
experiments dedicated to determine their scintillation
intensities, the influence of the stem signal and
photoluminescence, and time-dependent luminescence
properties.
Results
Figure 1 shows the emission spectra of all detectors (left)
and that the scintillation intensities were up to 19, 44, 16,
54 and 130 times larger than that of the PSD (right). Figure
2 shows time dependent luminescence properties of the
ISDs. The Y
2
O
2
S:Eu and Gd
2
O
2
S:Eu ISDs are not
recommended because their accuracy was compromised
by their time dependence. The scintillation of the ruby,
Y
2
O
3
:Eu and YVO
4
:Eu ISDs changed by +1.6%, -2.8% and
+1.1%, respectively, during 20 Gy, which is the dose that
the ISD inserted in urethra could absorb during a typical