ESTRO 37 Abstract book

S922

ESTRO 37

1 University of wollongong, School of Physics, Wollongong, Australia 2 Research Center- King Faisal Specialist Hospital and Research Center, Biomedical Physics, Riyadh, Saudi Arabia 3 Sir Charles Gairdner Hospital, Radiation Oncology, Perth, Australia 4 Ministry of Health, Department of Radiology, Riyadh, Saudi Arabia 5 University of Western Australia, School of Physics and Astrophysics, Perth, Australia Purpose or Objective Cyberknife Robotic SRS allows conformal dose distributions and adopts hypo-fractionation regimes which require accurate Quality Assurance (QA) to avoid plan or operator mistakes. Effective QA of small field, multidirectional and non-isocentric photon beams requires radiation dosimeters that have small sensitive volume, angular independence and operating in real time. The CMRP has developed an innovative angular independent silicon diode (edgeless technology) for dosimetry in megavoltage radiotherapy. The aim of this work is to evaluate the implementation the edgeless diodes in Cyberknife for machine and patient QA. Material and Methods The edgeless diodes are manufactured using n-type silicon substrates with 0.5 mm and 0.1 mm thicknesses. By using lateral ion implantation techniques, a 3D cathode junction is fabricated on 0.5×0.5x0.5 mm 3 and 1×1x0.5 mm 3 silicon chip with an anode implanted on the top side. The chip is packaged using the patented technology 'drop-in” which consists of a Kapton tail 0.5 mm thick and 3 mm wide. The kapton tail provides the electrical connections of the silicon sensor to the electrometer. The characterization of the edgeless diodes has been performed in two different Cyberknife machines (G4 and M6) with fixed cones and Iris collimators. The probes were tested in terms of basic quality assurance parameters such as tissue-phantom ratio (TPR), output factor, off axis ratio (OAR) and beam profiling. In addition, measurement has been performed to verify the accuracy of the diodes to measure the dose delivered in a point using three patient-specific plans. The results of the edgeless diode have been compared to PTW 60018, PTW 60016, SN edge, Pin-point ionization chamber, Gafchromic EBT3 film and treatment planning system (TPS). Results The TPR measurement performed by edgeless diode shows an agreement within 2.2% with data obtained by PTW 60018 and PTW 60016 diodes (Fig.1a). Output factor for both fixed cones and Iris collimators agrees within 2.6% with that measured by PTW 60018 and SN edge diodes which have been corrected according with Francescon et al. (Fig.1b). The beam profiles with both fixed and Iris collimators match PTW 60018 and SN edge diodes with a measured FWHM within 2.57% and penumbra widths within 0.21 mm (Fig.1c). The patient- specific QA measurements of the edgeless samples demonstrate an agreement within 4.72% and 3.11% with TPS and EBT3 film, respectively (Tab.1).

EP-1724 Off-axis ratio measurement of stereotactic beams. Does detector orientation matter? D. Radomiak 1 , S. Adamczyk 1,2 1 Greater Poland Cancer Centre, Medical Physics, Poznan, Poland 2 IntraOp Medical Corporation, Clinical Science and Technology, Poznań, Poland Purpose or Objective The aim of this study was to compare the influence of changing the detector’s orientation and the direction of scanning stereotactic beams on measured penumbra width and symmetry measure with different detectors. Material and Methods Off-axis ratio was measured using: 2 cylindrical chambers: PTW Semiflex 31010, PTW PinPoint 31016, 2 silicon diodes: PTW Diode E 60017, PTW Diode P 60016 and plane-parallel chamber: PTW Markus 23343. A 6 MV flattening filter free beam with a high dose-rate of 1000 MU min -1 were generated by CyberKnife® System. Beams were collimated by Fix Collimator. The source-surface distance was set to 78.5 cm and the effective point of measurement was set at 1.5 cm depth from the surface of the phantom. Off-axis ratio using cylindrical ionization chambers and silicon diodes were measured in a water phathom in 3 different settings - parallel and perpendicular orientation of the detector to the water surface, in the parallel orientation of the detector off- axis ratios were measured in two directions - transversal (in the detector’s long axis direction) and sagittal (in the detector’s short axis direction). Due to technical aspects, off-axis ratio measured using Markus chamber was performed only in perpendicular orientation. In this study penumbra width was defined as a lateral distance between the 80%-20% isodoses lines. For symmetry analysis Area Ratio formula was used, where integral over the left part and right part of the profile are compared. In this study, fields from 5mm to 15mm diameter were analyzed. Results As it was predicted, large differences in penumbra width and symmetry were observed for parallel and transversal settings, especially for ion chambers where the shape and volume of active part of detector does matter. For field size 20mm diameter, orientation (between transversal and perpendicular) of the detector may provide large difference in penumbra width, for Semiflex 0,45mm and 0,3mm for PinPoint. Large differences in symmetry values are observed for 3 settings for Diode P, for transversal setting 2,5% and for perpendicular setting 0,26% were measured. This large difference is caused by shielded plate. Markus chamber due to its symmetrical build measures symmetry 0,05%, 0,02% respectively for 5mm and 7.5mm field sizes. Penumbra width measured by Markus for every analisied field size are higher than for any analisied detector, for the smallest field size 3,39mm for Markus and 1,9mm for Diode E respectively. Conclusion It has been shown that penumbra width and beam symmetry values measured with cylindrical, plane- parallel and silicion diode vary in the results. The detector’s orientation as well as the direction of scanning influences penumbra width and symmetry value when measured cylindrical chambers and silicon diodes. Symmetry and penumbra width is independent of direction of scan in Markus chamber. Setting a detector perpendicularly to the water surface is recommended for the measurement of off-axis ratio. EP-1725 Quality assurance of Robotic SRS (Cyberknife) by an innovative angular independent silicon detector S. Alhujaili 1 , G. Biasi 1 , F. Alzorkany 2 , G. Grogan 3 , M.A. Al Kafi 2 , J. Lane 3 , B. Hug 3 , A.H. Aldosari 4 , S. Alshaikh 4 , P.R. Farzad5 5 , M.A. Ebert 3 , B. Moftah 2 , A.B. Rosenfeld 1 , M. Petasecca 1