ESTRO 36 Abstract Book

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% .

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

Conclusion MOSFET TN-502RDM detectors show a high linearity with accumulated dose and a good inter-detector reproducibility, even better than intra-detector.