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ESTRO 36
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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.
Cardenal Carro
2
, J.T. Anchuelo Latorre
2
, M. Ferri Molina
2
,
A. Kannemann
2
, D. Guirado
3
, P.J. Prada
2
1
Hospital Universitario Marqués de Valdecilla,
Radiophysics, Santander, Spain
2
Hospital Universitario Marqués de Valdecilla, Radiation
Oncology, Santander, Spain
3
Hospital Universitario San Cecilio, Radiophysics,
Granada, Spain
Purpose or Objective
In vivo dosimetry (IVD) applied to HDR BT treatments
allows to monitor real dose delivered to clinically relevant
areas. MOSFET detectors are the most suitable devices for
this task because of their tiny dimensions, which enables
their introduction into identical needles to those used in
treatments. However, these type of detectors show
responses depending on source-to-detector angle and
distance. Mathematical models describing these
dependences can be obtained from a correct detector
characterization. Applying these models on the
measurements should minimize the impact of those
dependences, improving precision and accuracy. The
purpose of this study was to evaluate clinical data of IVD
applied to HDR prostate BT using MOSFET TN-502RDM from
Best Medical Canada with the Ir-192 Vr2 source contained
in Flexitron aferloader (Nucletron-Elekta) and
mathematical models describing those dependences
obtained in a previous characterization work.
Material and Methods
Clinical data were taken from five patients suffering from
prostate cancer. One to three measuring points were
taken for each patient, where the MOSFET were
positioned. Anatomical areas measured were
neurovascular bundles, bulbourethral area and
periurethral area. Nine measuring points were taken and
evaluated.
Real time ultrasound image guided technique was used to
implant the treatment needles. An additional needle was
needed for each measuring point. Oncentra Prostate
4.2.2.4. was used to calculate the treatment planning
following a standard procedure. Subsequently,
coordinates of measuring points and dwell positions were
taken as well as dose contribution of each dwell position
to each measuring point.
After irradiation, mathematical models were applied on
measured dose. Table 1 shows the three models
considered, their parameters and the goodness of fit. TPS
dose, direct MOSFET measured dose and calculated dose
after applying the mathematical models on direct MOSFET
dose were evaluated.
Results
Figure 1 shows the results normalized to TPS d ose. All
measurements suffer an approach to TP S dose after
applying the mathematical models. The ave rage value of
percentage difference between TPS dose and direct
measured dose was 15% while the average percentage
difference after applying the mathematical models
decreases to 9% without any point exceeding 20%.
Estimated global uncertainty associated to these
corrected measurements were into the range 3.7-4.3%.