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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 m
2
) 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. Bownes
1
1
Leeds 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.
Material and Methods
A decommissioned rectal retractor was modified by
drilling a small hole to allow a microMOSFET to be
inserted. The MOSFET was commissioned measuring
energy dependence and angular dependence of response
for the range of source-MOSFET positions expected in
cervix brachytherapy treatments. Standard and conformal
cervix plans covering the range of applicator sizes and
geometries used in clinical treatments were delivered in a
water phantom. The MOSFET was monitored during
treatment delivery and measured doses compared to
treatment planning system (TPS) calculated doses for the
total plan and for ring and inter-uterine tube (IUT)
individually.
Results
Corrections were applied for energy dependence response
(6% variation between 1 and 8 cm source-MOSFET
positions) and angular dependence of response (up to 8%
under response for the largest polar angle of 170°). Total
plan measurements agreed with TPS calculated doses
within 3.1% - 7.8% for 30° and 60° applicators but
measured 16% -24% high for 45° applicators (k=2
uncertainty was estimated as 14% for total plan
measurements). Separate analysis of ring and IUT
measurements similarly showed good agreement for all
cases except the 45° IUT for which measurements were on
average 55.3% higher than expected. For the 45° IUT the
MOSFET position is directly in line with the source cable
and longitudinal source axis based on the source positions
assumed by the TPS (see figure). A combination of a small
rotation of the source relative to the IUT axis and
deviation of the actual source position from the centre of
the IUT could explain the measurement difference. To
verify this, treatments for the 45° applicator were re-
measured with the MOSFET taped to the outside of the
rectal retractor in a position that was not aligned to the
IUT and measured doses agreed within 8%.
Conclusion
In vivo dosimetry for cervix brachytherapy would be
feasible if commercial rectal retractors were designed to
allow a dosimeter to be inserted. However it is important
to avoid dosimeter positions aligned with the source
longitudinal axis as this is a region of high dose
uncertainty.
PO-0943 Evaluation of a recent in vivo dosimetry
methodology for HDR prostate BT using MOSFET
detectors
R. Fabregat Borrás
1
, S. Ruiz-Arrebola
1
, E. Rodriguez
Serafín
1
, M. Fernández Montes
1
, A. García Blanco
2
, J.