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S973

ESTRO 36 2017

_______________________________________________________________________________________________

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