ESTRO 36 Abstract Book

S401 ESTRO 36 2017 _______________________________________________________________________________________________

largest for the small fields of 0.5x0.5 and 1x1 cm 2 while negligible for field sizes larger than 4x4 cm 2 . The OF measured by the MP512 detector with an air gap of 0.5 and 1.2 mm show a good agreement with OF measured using EBT3 film and MO Skin for 6 and 10 MV, respectively. Similar results were observed for the PDD measurements in field sizes of 5x5 and 10x10 cm 2 . The PDD for a 2x2 cm 2 was within ±3% of the EBT3 for both photon energies. The PDD measured with MP512 is within ±1.6% and ±1.5% of that measured using a Markus ionization chamber (IC) for 6 and 10 MV fields respectively. The PDD measured by electron beams demonstrated no significant effect with increasing air gap above the MP512 for all energies. The results for both 0.5mm and 2.6mm gap are within ±3% of similar measurements made using the Markus IC. Conclusion The MP512 response with different air gaps immediately above the detector in solid water phantom have been investigated in clinical photon and electron fields. The results confirm that the MP512 monolithic diode array is suitable for QA of small fields in a phantom. The study shows that the air gap size has a significant effect on small field photon dosimetry performance of the MP512 consistent with a loss of electronic equilibrium. The small air gap of 0.5 mm and 1.2 mm is the best air gap for small field dosimetry in 6 and 10 MV photon beams respectively. The effect of air gap on electron beam dosimetry using the MP512 was demonstrated to be not significant due to the electronic equilibrium conditions always being fully established. PO-0767 Revisiting EPID design for modern radiotherapy requirements P. Vial 1,2 , S. Blake 2,3 , Z. Cheng 2,3 , S. Deshpande 1,4 , S. Atakaramians 5 , M. Lu 6 , S. Meikle 7 , P. Greer 8,9 , Z. Kuncic 2 1 Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute, Department of Medical Physics, Liverpool BC, Australia 2 University of Sydney, Institute of Medical Physics- School of Physics, Sydney, Australia 3 Ingham Institute, Medical Physics, Liverpool, Australia 4 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 5 University of Sydney, Institute of Photonics and Optical Science- School of Physics, Sydney, Australia 6 Perkin Elmer, Medical Imaging, Santa Clara, USA 7 University of Sydney, Faculty of Health Sciences & Brain and Mind Centre, Sydney, Australia 8 University of Newcastle, School of Mathematical and Physicsal Sciences, Newcastle, Australia 9 Calvary Mater Newcastle Hospital, Radiation Oncology, Newcastle, Australia Purpose or Objective New methods of treatment verification that are in keeping with advances in radiotherapy technology are desirable. The availability of kilovoltage in-room imaging for example has led to a general trend away from the poorer contrast megavoltage (MV) imaging for patient-set-up. The widespread use of intensity-modulated radiotherapy (IMRT) also reduces the utility of treatment beams as a source of imaging for treatment verification. At the same time there has been a steady increase in the use of electronic portal imaging devices (EPIDs) for dose verification. There is however emerging evidence of new roles for MV imaging in real-time target tracking. In this work we address the issue of EPID detector specifications in light of changing clinical requirements. We present a general overview of the detector development work our group has undertaken to design an EPID that better supports applications relevant to current and future clinical practice. Material and Methods Prototype EPID technologies developed by our group include: a direct detector EPID where the metal/phosphor

Conclusion The results of our study show that 3D printing technology can be used to fabricate patient-specific, large scale phantoms that could be used for a variety of research, dosimetric, and quality assurance purposes. PO-0766 The effect of air gaps on Magic Plate (MP512) for small field dosimetry K. Utitsarn 1 , N. Stansook 1 , Z. Alrowaiili 1 , M. Carolan 2 , M. Petasecca 1 , M. Lerch 1 , A. Rosenfeld 1 1 University of Wollongong, Center for Medical Radiation Physics, Wollongong, Australia 2 Wollongong Hospital, Illawara Cancer Care Centre, Wollongong, Australia Purpose or Objective We evaluate the impact of an air gap on the MP512 irradiation response at depth in a phantom and optimize this gap for accurate small field dosimetry in clinical photon and electron beams. Material and Methods MP512 is a 2 dimensional silicon monolithic detector manufactured on a p-type substrate. The array consists of 512 pixels with detector size 0.5x0.5 mm 2 and pixel pitch 2 mm. The overall area of the active part of the detector is 52x52 mm 2 . The output factor (OF) and the percentage depth dose (PDD) were measured with MP512 in 6MV and 10MV photon beams. The OF was measured at a depth of 10 cm in a solid water phantom for square field sizes ranging from 0.5 to 10 cm 2 . The PDD was measured for field sizes 2x2cm 2 , 5x5cm 2 and 10x10cm 2 by scanning the MP512 from the depth of 0.5 cm to 10 cm. Both the OF and PDD were measured at all field sizes with an air gap immediately above the detector of 0.5, 1.0, 1.2, 2.0 and 2.6 mm respectively. The PDD for 6, 12 and 20 MeV electron beams with a standard applicator providing 10x10 cm 2 field size, were measured using an air gap of 0.5mm and 2.6mm. Results The OF measured by the MP512 reduces with increasing air gap above the detector. The impact of the air gap is