ESTRO 36 Abstract Book

S411 ESTRO 36 _______________________________________________________________________________________________

Faculty of Science, Serdang, Malaysia 3 University of Malaya, Clinical Oncology Unit- Faculty of Medicine, Kuala Lumpur, Malaysia 4 University of Malaya, Department of Physics- Faculty of Science, Kuala Lumpur, Malaysia 5 Malaysia Nuclear Agency, Medical Physics Group, Kajang, Malaysia 6 University of Surrey, Department of Physics, Guildford, United Kingdom Purpose or Objective The dosimetry characteristics of the fabricated Germanium doped cylindrical fibre (CF) and flat fibres (FF) were evaluated including linearity, repeatability, energy dependence, dose rate dependence, angular dependence and fading. The TL kinetic parameters of Ge-doped CF and FF have been investigated using the computerized glow curve deconvolution analysis. Material and Methods A screening process was carried out for the optical fibres with 6 MV photon beam. Optical fibres with sensitivity out of ±5% mean sensitivity value were discarded in order to ensure the selected dosimeters have constant response. The dosimetry characteristics tests were performed using 6 MV and 10 MV photon beams. The dosimeters were irradiated with doses ranging from 1 Gy to 3 Gy with several dose rates (100 MU/min up to 500 MU/min). Fabricated perspex phantoms were used to study the angular dependency of the optical fibres. Fading rate was studied for 55 days post irradiation. The glow curves were analyzed with a curve fitting computer program known as WinGCF. The glow curves were deconvoluted into five individual peaks in order to figure out the kinetic parameters such as the maximum peak temperature ( T max ), peak integral ( PI ), activation energy ( E a ) and frequency factor. Results The screening result revealed that the coefficient of variation was observed to be less than ±10% for CF and FF. Both CF and FF were found to be linear with r 2 more than 0.99 over the entire dose range explored for both 6 MV and 10 MV photon beams. These fibres provide consistent reading within ±5% over five repeated measurements. The signal lost was higher in FF (57%) compare to CF (38%) after 55 days of irradiation. Both fibres also offer dose rate- and angular independence. The glow curve for both CF and FF consist of 5 individual glow peaks. The peak height increased with increasing irradiation dose. The T max of the glow peaks (P1 to P5) is consistent over the dose range used . Peak 1 has the lowest E a which lies between 0.544 to 0.636 eV and 0.632 to 0.720 eV for CF and FF respectively, indicating the shallow electron traps. The results also revealed that the PI for both of the fibres will increase as the dose increase. The Ge-doped CF and FF demonstrated a constant glow curves shape with increasing dose. As the dose increases, area under the glow curve increases, suggesting an increasing number of electrons released from its traps. Conclusion CF demonstrated greater TL signal compared to FF. In order to employ these optical fibres in absorbed dose measurement, correction factors for energy dependence and fading should be applied. The Ge-doped CF and FF demonstrated the second-order kinetic model to the high temperature half of the curve is slightly broader than the low temperature half which suggest the possibility of strong electron retrapping. The evaluation on Ge doped optical fibres showed a highly favourable TL characteristics exhibited by CF and FF indicate a great potential in radiotherapy postal dose audit. PO-0777 Importance of dosimetry formalism for cells irradiation on a SARRP and consequences for RBE M. Dos Santos 1 , V. Paget 2 , M. Ben Kacem 2 , F. Trompier 3 , M. Benadjaoud 1 , A. François 2 , O. Guipaud 2 , M.

detector was positioned in the dose minimum below. Finally, the detector in air instead of water with a PMMA cap fitted on top. A range of typically used detectors were analyzed, namely a microDiamond, a PinPoint ionization chamber, an EDGE diode, as well as three shielded and three unshielded diode detectors. EBT3 Gafchromic film served as reference. Measurements were carried out on a PRIMUS linac at a photon beam quality of 6 MV with field sizes between 0.8 and 10 cm. Responses in the blocked field and the PMMA setup were combined to calculate the response in the open square fields. The results were interpolated to a general matrix from which responses in any field could be calculated. Examples of such fields were measured for comparison.

Results A higher detector overresponse and increasing detector to detector differences were observed when the primary beam was blocked out, whereas almost identical response was seen for all detectors in the primary beam. A combination of the responses in those two setups in a detector-dependent ratio reproduced the values obtained in the open field geometry with less than 1% deviation for all detectors studied and all quadratic field sizes. For rectangular and offset fields the agreement is still almost within 1 %. Only when the detector was close to the field edge larger deviations occurred (fig. 2).

Conclusion Detector responses in open fields could be calculated from the response to scatter and in the primary beam with 1% agreement in all studied square fields and for all studied detectors. The calculation was extended to rectangular and non-symmetric fields yielding results in agreement with the measurements for a wide range of fields. This method suggests a way to calculate correction factors for arbitrary fields. PO-0776 Thermoluminescence Characteristics of Fabricated Ge Doped Optical Fibre for Radiotherapy Dosimetry M.S. Ahmad Fadzil 1 , N. Tamchek 2 , N.M. Ung 3 , A. Ariffin 4 , N. Abdullah 5 , D.A. Bradley 6 , N. Mohd Noor 1 1 Universiti Putra Malaysia, Department of Imaging- Faculty of Medicine and Health Sciences, Serdang, Malaysia 2 Universiti Putra Malaysia, Department of Physics-