ESTRO 38 Abstract book

S917 ESTRO 38

resolution of the depth, the measurement was carried out by stacking the TLD sheet. Results The PDD showed consistent data between the ionization chamber and the TLD sheet, but the deviation was large in the TLD sheet. Due to the movement of the build-up region into the couch, the surface dose increased. This increase in surface dose could be evaluated in both the ionization chamber and the TLD sheet. The surface dose increased by approximately 80% by passing through the carbon couch. By sandwiching a 5 cm thick Styrofoam plate between the phantom and the carbon couch, it was possible to reduce the surface dose by 11.9% at 6 MV and 18.1% at 10 MV respectively. In these measurements, the TLD sheet was able to evaluate the dose closest to the surface of the phantom. The TLD sheet was able to evaluate the depth dose with the same variance as the Gafchromic film, and it was an advantage that the depth resolution in the stack measurement was higher. Although it takes time and effort to reduce the measurement interval in the depth direction using the parallel plate ionization chamber, it has been possible to quickly and conveniently measure by stacked method using the TLD sheet. Conclusion We evaluated the dose in the build-up region using the TLD sheet which is a new material, and its usefulness was suggested. The TLD sheet was particularly useful for dose assessment on the surface of the phantom. In the TLD sheet, there is a fluctuation in the intensity of the signal depending on the position in the sheet, and the correction for this is currently under consideration. It was shown that the dose near the surface of the phantom is greatly increased by passing through the carbon couch but the surface dose can be reduced by placing a low density material between the phantom and the couch. EP-1704 A 2D reusable OSL-film for class solution specific QA of large modulated fields in the PART trial R. De Roover 1 , C. Berghen 1 , G. De Meerleer 1 , T. Depuydt 1 , W. Crijns 1 1 Catholic University of Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective The PART trial (NCT03079323) evaluates the effect of prophylactic para-aortic lymph node irradiation in addition to prostate(bed) and pelvic lymph node radiotherapy in histopathologically proven pelvic lymph node metastatic (pN1) prostate cancer. With treatment fields up to 35 cm, the inclusion of the para-aortic lymph nodes challenges the current in phantom patient-specific quality assurance (QA). As state-of-the art reusable dosimeters have a maximum field-of-view (FOV) of 25 cm for single shot measurements. Additionally, these dosimeters consist typically of an array of detectors with a resolution between 0.5 and 1.0 cm. Radiochromic films offer a sub-mm resolution, but these dosimeters are for single use, increasing the cost and excluding a dosimeter specific calibration. A large-sized (43 x 36 cm²) reusable optical-stimulated luminescence (OSL)-film may measure such large fields with sub-mm resolution in a single shot measurement. The potential of OSL-film as an absolute infield dosimeter in intensity modulated radiotherapy was previously reported. But the film’s out-of-field-low- energy dependent overresponse requires additional modelling. This study investigates the use of such a modelling in a class solution specific calibration for the PART trial. Material and Methods Ten patients currently included in the PART trial were selected for OSL measurements. Prescription was 70 Gy to the prostate bed in 35 fractions or 67.25 Gy to the prostate in 25 fractions and 45 Gy to the pelvic and para-aortic lymph nodes in 25 fractions. Treatment was delivered on

Conclusion The synthetic ruby system which we developed have high temporal resolution, thus, it is possible to measure the total exposure time to read the rise time and the fall time on the graph. The MU value increased linearly with the number of photons which was same as the reference dosimeter, the synthetic ruby system have a potential for the real-time exposure dosimetry system on the radiation therapy. However, we have to discuss the influence of the developed system in the exposure filed, pursuit of the precision, the possibility of multipoint measurement, the size and the covering material of dosimeter. EP-1703 Dose Evaluation of Build-up Region of Photon Beam using Thermoluminescence Dosimeter (TLD) Sheet K. Sasaki 1 , Y. Shiota 2 , M. Miura 2 1 Gunma Prefectural College of Health Sciences, Graduate School of Radiological Technology, Maebashi, Japan ; 2 Iwata City Hospital, Medical Physics, Iwata, Japan Purpose or Objective Although there are various devices for measuring high- energy radiation, radiographic films, Radiochromic films and parallel plate ionization chambers are generally used to obtain a dose of surface or build-up region with high accuracy. In this study, a new thermoluminescence dosimeter sheet (TLD) sheet dosimeter with effective atomic number close to water and high measurement flexibility was used to evaluate the dose in the build-up region after passing through the carbon couch. In addition, reduction of surface dose by sandwiching low-density material was also studied. Material and Methods Percent depth dose (PDD) after passing through the carbon couch was measured using 6 MV, 10 MV photon from Novalis-TX (Varian Medical systems). The PDD from the phantom surface to the depth of 5 cm was measured by NACP-02 parallel plate ionization chamber (IBA), Gafchromic film EBT3 (Ashland) and TLD sheet (Toyo Medic). Changes in surface dose by sandwiching the Styrofoam between the phantom and the couch were also measured. The chemical formula of the TLD sheet is LiB 3 O 5 and its thickness is 150 µm, about half that of Gafchromic film. In order to utilize this thinness as the