Abstract Book

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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.

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 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

Conclusion The Elekta MRI-Linac beam meets international specifications. Limitations were verified. Special features such as the PDD ERE, surface dose, asymmetric profiles, penumbra, and cryostat scatter and attenuation have been characterized. The system is suitable for clinical use. PV-0141 Contaminant electrons increase doses to surfaces outside the photon beam of the MRI-linac S. Hackett 1 , B. Van Asselen 1 , J. Wolthaus 1 , H. Bluemink 1 , J. Kok 1 , J. Lagendijk 1 , B. Raaymakers 1 1 UMC Utrecht, Department of Radiotherapy, Utrecht, The Netherlands - field of an MRI-linac influences the trajectories of contaminant electrons generated in air by the photon beam. In a transverse B 0 - field these electrons are swept out of the beam and deposit dose to surfaces perpendicular to the B 0 - field. This study aims to characterize the dose from these contaminant electrons. Material and Methods Doses were measured with EBT3 model Gafchromic film (Ashland ISP Advanced Materials, NJ, USA) on an MRI- linac, which has a 7 MV beam oriented perpendicular to a 1.5T B 0 - field. A stack of six films, each separated by a slab of solid water 1 mm thick, were fixed to the surface of a solid water phantom. The phantom surface was 5 cm from the geometric edge of a 10x10 cm 2 field (see Fig. 1). Films were exposed with 2000 MU. The process was repeated with films parallel to each side of the radiation beam. Purpose or Objective The B 0