ESTRO 37 Abstract book

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ESTRO 37

to produce a static MLC defined aperture of 1x10 cm 2 . Gantry rotation was in both directions. The detector was fixed to the accessory tray and equipped with a synchronized inclinometer attached to the gantry head to provide real-time dosimetric and angle measurements at each linac pulse. The radiation sensor is a monolithic silicon transmission detector (DUO) with two orthogonal linear arrays of 256 diodes, each diode has a size of 0.04 x 0.8 mm 2 and a pitch of 0.2 mm The sensor is calibrated in terms of counts/cGy with respect to an ionization chamber in reference conditions (for 6MV, ion chamber placed at 10 cm depth in water, source-to-surface distance of 100cm and field size of 10x10cm 2 ). The synchronicity test is required to assess the effect of inertia on the delivered dose and gantry angles during continuous acceleration and deceleration of the gantry and simultaneous large variations of dose rate. VMAT plans were designed according to the recommendations of the Code of Practice in section 2.5.2.4 of the report. Measurements obtained with the detector DUO are compared to dynalog data. Results

EP-1777 Dosimetric characterisation and clinical commissioning of a high-field inline MRI-Linac U. Jelen 1 , J. Begg 1,2,3 , G. Liney 1,2,3,4 , B. Dong 1 , K. Zhang 1 , B. Whelan 5 , L. Holloway 1,2,3,4,5 , P. Keall 5 1 Ingham Institute for Applied Medical Research, Department of Medical Physics, Liverpool, Australia 2 Liverpool and Macarthur Cancer Therapy Centre, Department of Medical Physics, Liverpool, Australia 3 University of New South Wales, South Western Sydney Clinical School, Sydney, Australia 4 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 5 University of Sydney, Radiation Physics Lab, Sydney, Australia Purpose or Objective The pursuit of real-time image-guided radiotherapy with optimal soft-tissue contrast has prompted the advent of hybrid devices coupling MRI scanners with radiotherapy treatment units, usually linear accelerators (linacs). One challenge in developing and operating such systems is the effect of the magnetic field on radiation beam generation and dose deposition. A prototype system, constructed under the Australian MRI-Linac program, explores the configuration in which the linac is oriented parallel to the magnetic field (inline configuration), which has the potential to minimise or even exploit some of these effects. The purpose of this work was to dosimetrically characterise and commission this system for an upcoming application in patient treatment. Material and Methods The system consists of (1) a dedicated 1.0 T split-bore magnet (Agilent) integrated with the Avanto control system (Siemens Healthcare), Tesla gradient coils, Magentica radiofrequency coils and (2) a portable 6 MV FFF linear accelerator (Linatron-MP, Varex) combined with a 120-leaf MLC (Millennium, Varian). To minimise the beam output loss and the deflection of electrons in the linac, a twofold strategy was employed, guided by investigative measurements of the beam properties: a precise physical alignment of the radiation head to the magnetic field was complemented by an optimised design of the magnetic shielding of the target. Subsequently, mechanical parameters and dosimetric data characterising the radiation beam were acquired in line with the relevant national and international standards and recommendations (i.a. IEC, AAPM). Results Alignment of radiation beam and imaging isocentre within 2 mm was obtained through iterative adjustments, using a dedicated phantom (Leeds Test Objects), ball-bearing crosshair phantoms and a stand-alone EPID panel (Perkin Elmer). Reproducibility, proportionality and calibration of the dose monitoring system fulfilled the IEC criteria. Absolute dose, TPR 20,10 and output factor measurements were performed in a 1D water tank using a Farmer-type ionisation chamber (Scanditronix Wellhofer). Absorbed dose depth and lateral profiles were acquired using EBT3 (Ashland) films placed in solid water. In the depth dose profiles, focusing of the contamination electrons was visible. Penumbra of the lateral profiles did not show asymmetry inherent to perpendicular configurations. The profile symmetry was measured using the Starcheck MR maxi ionisation chamber array (PTW) and remained under 103% for the entire range of field sizes. Conclusion A comprehensive set of commissioning measurements has been conducted to characterise the unique high-field split-bore inline MRI-linac. The key parameters were within tolerances specified by the applicable standards and will form a baseline for future clinical quality assurance protocols.

The dose rate measured with the transmission system was within 1% in comparison to dynalog, whereas the deviation in gantry speed in the constant speed section was within 3%. In the synchronicity test, the width of the spokes measured by the system is 2˚ broader than the nominal gap of 2˚. Dynalog data also reported a gap broadening of approximately 1.5˚. The dose measured with the proposed technique is in agreement with the expected and dynalog values at angles 0˚, 40˚ and -40˚ (320˚) however, with larger angles, the measured dose decreases by approximately 8%. This decrease is caused by the sagging of MLC leaves due to gravity.

Conclusion A new, fast and reliable system is proposed to perform routine linac QA for VMAT as recommended by the NCS Code of Practice Report 24. The system is based on a high spatial resolution radiation hard transmission detector DUO combined with an inclinometer and placed on the accessory tray of the linac. Such system allows accurate reconstruction of the gantry speed and dose rate measurements independently and in real-time and will be extended to verify MLC leaf speed eliminating the need for film or multiple detectors to complete the QA checks. In addition to 6 MV FF VMAT modalities, results of tests performed on Truebeam FFF will be presented.