ESTRO 36 Abstract Book

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S521 ESTRO 36
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Conclusion
The advanced schematic 3D mapping diagram provides
precise topographic and quantitative 3D information on
extent of disease and for CTV for BT, using repetitive MRI.
There is a significant correlation of GTVD with HRCTV and
IRCTV in regard to volumes and dimensions. This new tool
may also be used for BT CTV definition based on GE and
CT/US.
PO-0941 Verifying the treatment planning system in
individualized HDR brachytherapy of cervical cancer
M. Van den Bosch1, B. Vanneste1, R. Voncken1, L. Lutgens1
1 MAASTRO Clinic, Department of Radiation Oncology,
Maastricht, The Netherlands
Purpose or Objective
In state of the art high-dose-rate (HDR) brachytherapy of
the cervical cancer interstitial needles are regularly
placed in addition to the standard applicators to increase
the possibility for dose optimization, i.e. higher tumour
coverage and/or sparing of OAR’s. The use of these
needles enables more individualized treatment plans.
Consequently dose distributions and dose plans have
become highly individualized. As a result, the main output
parameter of the planning system, the total reference air
kerma (TRAK), is more difficult to verify. In this study, it
is investigated whether the high risk clinical target volume
(HR-CTV) can be used to predict the TRAK.
Material and Methods
26 treatment plans of 10 cervical cancer patients were
included in this study. In all patients the titanium Varian
Fletcher applicator was inserted. The number of
interstitial PEEK Varian needles was determined by the
radiation oncologist at the time of the application. T2-
weigthed MR scans were acquired in treatment position
and used for delineation of the HR-CTV, intermediate risk-
CTV (IR-CTV) and organs at risk (OARs). Contouring was
done by the responsible radiation oncologist whereas a
treatment plan was made by the radiation therapist using
BrachyVision (algorithm: TG-43). The calculated TRAK
values of each plan were rescaled to a source strength of
10 Ci and to a fraction dose of 7 Gy (prescribed to the HR-
CTV).
Results
The number of needles varied from 0 to 8 (average 3.8
needles per application). The rescaled TRAK and mean
volume of the HR-CTV was 0.37 cGy m 2 (range: 0.23-0.50
cGy m2) and 26.8 cc (range: 8.0-59.1 cc), respectively. In
general, the TRAK value increased with volume. In figure
1a the TRAK values are plotted against the HR-CTV. The
relation between these parameters can be described by a
linear equation (see figure 1b). When setting an upper and
lower limit of two standard deviations a 95% confidence
interval can be derived and outliers can be identified. The
higher TRAK value of these outliers suggest the volume
that received the prescribed dose is much larger than the
HR-CTV. This was true for these plans: due to excessive
reduction of the HR-CTV, a higher dose in the IR-CTV was
desired and planned in the direction of the uterus top.
Conclusion
The HR-CTV can be used to predict the TRAK value.
Outliers may indicate abnormalities in treatment planning
and further inspection of their dose distributions is
required. In this study, the deviations in the dose
distributions of the outliers were accepted, since they
resulted in an improved individualized treatment plan.
Using this relationship, the quality assurance of the
treatment plan can be improved.
Poster: Brachytherapy: Physics
PO-0942 Real time in vivo dosimetry for cervix HDR
brachytherapy - feasibility study using a MOSFET
J. Mason 1, P. Bownes1
1Leeds Cancer Centre, Medical Physics & Engineering,
Leeds, United Kingdom
Purpose or Objective
Implementation of in vivo dosimetry (IVD) in
brachytherapy is partly limited by lack of c ommercially
available devices that support IVD. In this study a
modified rectal retractor and MOSFET were used to
investigate the feasibility of real time IVD for cervix
brachytherapy with simulated treatment plans delivered
in a water phantom.