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S973
ESTRO 36 2017
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Material and Methods
Dose calculation was made using TPS OncentraBrachy-
v4.5.2 (Nucletron-Elekta) and verified with radiochromic
films Gafchromic EBT3. Tiny dimensions of MOSFET TN-
502RDM (1 mm x 1 mm x 3.5 mm) allow their introduction
into common needles used in HDR BT. Steel Trocar point
needles from Nucletron with diameter 1.9 mm and length
200 mm were used to place the source and the detectors
within a water phantom specifically designed for this
study.
Calibration factors, defined as the ratio between
measured dose and detector response (Gy/V), were
obtained for five detectors, using a calibration dose of 1
Gy. Calibration factor for each detector was calculated
averaging five repeated measures
.
Linearity of the response was evaluated until the detector
saturation. Temperature dependence within the range of
clinical interest, angular dependence and distance-to-
source dependence were also assessed. Each dependence
was evaluated for three detectors and each experimental
measurement was repeated three times. Mathematical
models were obtained for each dependence.
Results
Figure 1 shows a lineal behavior for three detectors
evaluated until 155Gy. Maximum variations of detector
response were 8.7% with distance-to-source (range of 7
cm), 10% with azimuthal angle and 7.6% with polar angle.
Temperature dependence was negligible for interest
range. Table 1 shows the results of the parameters
describing the mathematical models and their
uncertainties, as well as the goodness of fit.
Calibration factors measured were between 8.4-8.6 Gy/V.
Calibration factor reproducibility resulted in 2.1% for
intra-detector analysis and 0.9% for inter-detector
analysis. Finally, estimated global uncertainty associated
to our MOSFET measurements is 4.2% .
Conclusion
MOSFET TN-502RDM detectors show a high linearity with
accumulated dose and a good inter-detector
reproducibility, even better than intra-detector.
However, their implementation in a IVD program of HDR
BT needs a developed methodology to minimize the
impact of the large angular and distance source-to-
detector dependence of the MOSFET over their response.
This task would be done applying the mathematical
models obtained in this study .
References
[1] Tanderup K, Beddar S, Andersen CE, Kertzscher G,
Cygler JE. In vivo dosimetry in brachytherapy. Med
Phys2013; 40: 1–15. doi: 10.1118/1.4810943
EP-1798 Highly conformal external beam modalities vs.
brachytherapy boost for rectal cancer patients
S. Devic
1
, U. Mwidu
2
, A. Alkafi
2
, B. Moftah
2
, S. Shakir
1
, H.
Hijazi
1
, C. Yeung
1
, T. Vuong
1
1
McGill University, Oncology, Montreal, Canada
2
King Faisal Specialist Hospital & Research Centre,
Biomedical Physics, Rityadh, Saudi Arabia
Purpose or Objective
In our institution high-dose rate endorectal brachytherapy
(HDREBT) is given either as pre-operative downsizing
modality or as a boost after external beam therapy (EBT)
in patients with rectal adenocarcinoma. In this work, we
compare dosimetry between HDREBT and highly conformal
external beams therapy (EBT) modalities as an alternative
modality.
Material and Methods
Ten male rectal cancer patients treated with HDRBT boost
using the Intra-cavitary Mold Applicator (IMA) have been
scanned prior to first treatment fraction firstly without
brachytherapy applicator (to be used for EBT planning)
Brachytherapy plans were generated using IPSA inverse
planning module of MasterPlan treatment planning system
which does not provide the impact of the midline shielding
used during HDRBT, while the CT data without applicator
were used for planning on Cyber Knife (CK, Accuray
MultiPlan) and Rapid Arc (RA, Varian Eclipse). In addition
to clinical target volume (CTV) various critical structures