ESTRO 37 Abstract book

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ESTRO 37

Purpose or Objective The Elekta MRI-linac delivered its first clinical treatments in May 2017. As a prerequisite, it was necessary to characterize the 7 MV flattening-filter-free (FFF) radiation beam with surface-axis distance 143.5 cm, in its clinical configuration within the 1.5 T magnetic field. The aims of this work were to (i) identify limitations and assess the beam against international standards, and (ii) measure beam data suitable for modeling in the Monaco treatment planning system (TPS). Material and Methods Following acceptance testing, beam characterization data was acquired with Semiflex3D (PTW 31021), microDiamond (PTW 60019), and Farmer-type (PTW 30013 and IBA FC65- G) detectors in an Elekta 3D scanning water phantom and a PTW 1D water phantom. EBT3 Gafchromic film and ion chamber measurements in a buildup cap were also used. Special consideration was given to scan offsets, detector effective points of measurement and avoiding air gaps. Results Expected limitations have been verified. Co-planar, step-and-shoot IMRT using leaves of width 7.2 mm, up to a maximum field size of 57 cm x 22 cm is achievable, and the system satisfies the relevant beam requirements of IEC60976. Beam data was acquired for field sizes between 1x1 and 57x22 cm 2 . New techniques were developed to measure PDD curves including the electron return effect (ERE) at beam exit, which exhibits an electron-type practical range of 12 ± 1 mm (Figure 1a). The Lorentz force acting on the secondary charged particles increases the entrance skin dose (film measurement ~ 36% of D max ) and creates an asymmetry in the crossline profiles with an average shift of +2.4 mm (Figure 1b). For a 10x10 cm 2 beam, scatter from the cryostat contributes 1% of the dose at isocentre. This affects the relative output factors (s cp ), collimator (and cryostat) scatter factors (s c ) and beam profiles, both in-field and out-of-field. The average 20%-80% penumbral width of 7.0 mm (measured with PTW semiflex3D) is the same as that of the Elekta Agility linac MLC. Cryostat transmission as a function of gantry angle was quantified so that it could be incorporated into the TPS calculations.

surface of the plug. RADPOS position tracker recorded the phantom motion with time steps of 100 ms. 4D CT scans of the moving phantom were ac quired using a Big Bore helical CT scanner. Static 3×3 cm 2 square and VMAT plans were created on the end-of-inhale CT scans of the phantom in Monaco V.5.11.01 to deliver 100 cGy to the center of the tumour. A previously validated BEAMnrc model of our 6MV Elekta Agility linac was used for all simulations 2 . DOSXYZnrc and 4DdefDOSXYZnrc 3 user codes were used, respectively, for stationary and moving anatomy dose simulations with 8×10 7 histories to achieve a statistical uncertainty of 0.7% on a dose grid resolution of 2.0×2.0×2.0 mm 3 . Data from delivery log files were extracted to generate input files for simulations. For 4D simulations, deformation vectors were obtained by deformably registering 4DCT scans of the end-of-exhale to end-of-inhale states using Velocity AI 3.2.0. Deformation vectors, along with the phantom motion trace measured with RADPOS, were used to model the phantom motion. It was assured that the exact same motion as in irradiations was used in simulations by synchronizing the start of the phantom motion with the linac beam-on time. Results Motion reproducibility of the phantom was found to be 1.2 mm as measured with RADPOS detectors. Dose values from MC simulations and measurements at the center of the tumor and top surface of the plug from were found to be within 2% and 2σ of experimental uncertainties (3.5%), respectively, for all deliveries. On the stationary phantom all data points from MC simulations passed a 3%/2 mm gamma analysis. On the moving phantom, passing rates were better than 98%.

Conclusion Our 4D Monte Carlo simulation using the 4DdefDOSXYZnrc code accurately calculates dose delivered to a deforming anatomy. The ongoing future work is on replacing the phantom's DC motor with a programmable one and establishing the accuracy of our method using patient respiratory motion patterns. 1 Cherpak A et al. Med Phys. 2011;38(1):179-187. 2 Vujicic M et al. Med Phys. 2015;42:3480. 3 Gholampourkashi S et al Med Phys. 2017;44(1):299-310. PV-0140 Beam characterization of the Elekta MRI-linac for the first clinical trial S. Woodings 1 , J.J. Bluemink 1 , J.H.W. De Vries 1 , Y. Niatsetski 2 , B. Van Veelen 2 , J. Schillings 2 , J.G.M. Kok 1 , J.W.H. Wolthaus 1 , S.L. Hackett 1 , B. Van Asselen 1 , H.M. Van Zijp 1 , S. Pencea 2 , D.A. Roberts 2 , J.J.W. Lagendijk 1 , B.W. Raaymakers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands 2 Elekta Ltd., Linac House, Crawley, United Kingdom