Abstract Book

S953

ESTRO 37

Results

calculation and the RTP beam data measured using CC13 ion chamber were within an average of 2%. The treatment plan for the clinical validation using gamma index was analyzed by 3D-CRT and VMAT using solid water phantom and arbitrary patient data. The gamma evaluation results were calculated as 99.1% and 93.0% in 3D-CRT and VMAT plan using solid water phantom at 3% and 3mm, respectively. In the case of 3D-CRT and VMAT using CT data of the patient, 95.9% and 93.2% were The results of our study show that the most accurate dose calculation system using Monte Carlo QA system can be constructed and applied to clinical practice. The proposed QA system will be used for verification of clinical TPS such as SRS planning using small field, dose distribution for VMAT and secondary cancer risk which is difficult to verify dose calculation in TPS. EP-1776 Verification of the NCS Code of Practice Report 24 for VMAT QA using a high-resolution detector. F. Matar 1 , D. Wilkinson 2 , J. Davis 1 , T. Causer 2 , I. Fuduli 1 , A. Ceylan 2 , M. Carolan 2 , P. Metcalfe 1 , M. Petasecca 1 , A.B. Rosenfeld 1 1 University of Wollongong, Centre For Medical Radiation Physics, Wollongong NSW, Australia 2 Wollongong Hospital, Illawarra Cancer Care Centre, Wollongong, Australia Purpose or Objective This work aims to design and optimize a high spatial resolution silicon detector placed in the accessory tray of the linac to verify gantry angle and rotation speed, dose rate accuracy and synchronicity as suggested by the recommendations of the Code of Practice for VMAT QA published by the NCS Report 24. Material and Methods The VMAT CAP plan (Customer Acceptance Procedure) is designed to evaluate the synchronisation between gantry speed and dose rate during arc delivery. 10 sectors, each with a different MU weightings are generated for a Varian 21EX Clinac operating at 6 MV. The plan was customized 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. calculated. Conclusion

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. 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