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performance of this approach is evaluated by simulating
brachytherapy procedures using data of 10 patients
diagnosed with prostate cancer.
Material and Methods
Throughout HDR prostate brachytherapy, unpredictable
anatomy movements may cause errors in dose delivery and
potentially, this may result in failure to reach clinical
constraints (e.g. for single fraction monotherapy: D95%
PTV>19 Gy, D10% urethra<21 Gy, D1cc bladder<12 Gy and
D1cc rectum<12 Gy). In this study, a novel adaptive dose
planning pipeline for MR-guided HDR prostate
brachytherapy using a single needle robotic implant
device is proposed to address this issue (Figure 1a). The
dose plan (needle track positions, source positions and
dwell times) and needle insertion sequence are updated
after each needle insertion and retraction with MR–based
feedback on anatomy movements (cf. Figure 1b). The
pipeline was assessed on moving anatomy by simulating
MR-guided HDR prostate brachytherapy with varying
number of needle insertions (from 2 to 14) for 10 patients.
The initial anatomy of the patients was obtained using the
delineations of the prostate tumor and the OAR considered
(urethra, bladder and rectum) on MR images. Each needle
insertion and retraction induced anatomy movements
which were simulated in 2 steps: (1) a typical 3D rotation
of the prostate was imposed (2) a regularization of the
movement in space was then applied. The initial and final
dose parameters were compared in the situations with and
without update of dose plan and needle insertion
sequence.
Results
The computation time for re-planning was less than 90
seconds with a desktop PC. The actual delivered dose
improved with vs. without update of dose plan and needle
insertion sequence: On average, the dose coverage of the
PTV was higher in the situation with vs. without update
(Figure 1c). Moreover, the difference increased with the
number of needle insertions. The dose received by the PTV
in the situation with re-planning was not significantly
different compared to the initial dose plan. Finally, the
dose to the OAR’s was not significantly different between
the initial dose plan and the dose delivered in the situation
with and without update.
Conclusion
This study proposes a new adaptive workflow with
feedback on the anatomy movements for MR-guided HDR
prostate brachytherapy with a single needle robotic
implant device. The assessment of the pipeline showed
that the errors in the dose delivered due to movement of
anatomy can be compensated by updating the dose plan
and the needle insertion sequence based on MRI.
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