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Validation of the daily quality assurance tool tomodiagnostics for helical tomotherapy

LELIE, Steven
Industrieel ingenieur nucleaire technologie


Abstract :
Abstract presented for the annual symposium of the Belgian society of clinical physics in
Validation of the daily quality assurance tool TomoDiagnostics for helical
Lelie S. (1), Van de Vondel I. (2), Tournel K. (2), Duchateau M. (2), Verellen D.(2) ,
Schroeyers W. (1)
(1) XIOS Hogeschool Limburg
(2) Radiotherapie, Oncologisch Centrum, Universitair Ziekenhuis Brussel
Introduction: Due to a particular method of dose delivery helical tomotherapy introduced
some new quality assurance aspects. Traditional methods cannot always be applied in this
setting and the development of new schedules is necessary. Some of the factors, such as
machine output, should be checked daily. Unfortunately the IMRT measurements, used to
check dose output, take about 50 minutes, which is a lot of time. That's why at the UZ
Brussel a method was developed that uses the MVCT-detector of the machine, as well as
the dose chamber output. It compares these values to reference values and takes about 200
seconds. The aim of this study was to relate these values to the actual IMRT measurements
on phantom, making it possible to reduce QA time significantly.
Method and materials: Both IMRT and detector measurements were performed during a
period of six weeks. The IMRT measurement consists of 4 measurements with an A1SL
ionization chamber (Standard Imaging, Madison, US) on a planned tomotherapy cheese
phantom (Gammex RMI, Middleton, US), which include cylindrical off and on axis
targets. Secondly the detector measurement was performed and a linear regression was
calculated between the 5 parameters received from the procedure and the date from the
IMRT measurements.
Results: After 10 datasets were fed to the regression, the model was able to predict future
measurements with a precision of 0.5% (measured over model ratio).
Conclusion: Using detector information, daily QA on tomotherapy can be reduced from
50 minutes to 200 seconds.
Abstracts vi
Abstract English
Background and Purpose: Helical tomotherapy introduced, due to its particular method
of dose delivery, some new quality assurance (QA) aspects because traditional methods
cannot always be used in this setup. The delivered dose distribution in helical tomotherapy
is highly dependent on the output and the shape of the beam ('cone profile') as the system
does not use the traditional flattening filter. Small variations in these two parameters can
have huge influences on IMRT-treatment making daily dose monitoring top priority, but
very time consuming using conventional methods. This study represents the validation of
an in-house developed tool 'TomoDiagnostics' which is a completely automatic daily QA
tool which is fully automated and independent of other dosimetric tools. This method
should reduce the daily QA time from 50 minutes to 5 minutes.
Materials and Methods: The 'TomoDiagnostics' software package was developed to
work independently on the system and to retrieve the integrated MVCT detector and dose
chamber data directly without the need for pre-processing. The system will daily compare
cone profile and the three system dose chambers to a fixed reference. It will also analyze
these signals to calculate target and magnetron condition. To validate the link between
output, cone profile and daily patient QA, a dedicated IMRT plan containing disjunctive
target volumes on- and off-axis of linac rotation was used and a regression model was built
to relate the TomoDiagnostics data with the ionization chamber measurements for the
IMRT plan.
Results: The results show that the in-house tool can be used to monitor output and cone
shape for a tomotherapy unit in a fast and reliable way. Furthermore the tool has the ability
to separate the effects of cone profile and output in one single measurement with two main
parameters. Using a 4D linear regression the software is capable of predicting the IMRT
ionization chamber measurements within 0.5%. Also the system is capable of predicting
system related problems such as target failure prematurely.
Conclusion: The in-house developed software tool based on detector and dose chamber
monitoring can be used to monitor output and cone profile variations in one procedure. By
using a regression the tool can also replace the IMRT QA measurements provided that an
extensive QA ionization chamber measurement is made once a week. This allows that the
weekly QA time will be reduced from 250 minutes to 70 minutes.

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If you want to cite this thesis in your own thesis, paper, or report, use this format (APA):

LELIE, S. (2008). Validation of the daily quality assurance tool tomodiagnostics for helical tomotherapy. Unpublished thesis, Xios, IWT.
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