ESTRO 37 Abstract book

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ESTRO 37

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. Films were scanned with an EPSON 1000XL flatbed scanner and converted to dose. Profiles on the horizontal axis of each film (IEC 1217 x- and y-axes for films perpendicular and parallel to the B 0 - field respectively), and the mean dose in a 1 cm 2 area centered on the axis intersecting the film (y- and x-axes for films perpendicular and parallel to the B 0 - field respectively) were measured. Doses were converted to percentages of the maximum dose (D max ) at isocenter from a 10x10 cm 2 field at 10 cm depth. Purpose or Objective The B 0

2.8±0.2%. However, doses to all films covered by at least 3 mm of solid water (active layers at least 3.9 mm deep) were consistent to within 0.4% of D max . Dose distributions to surface films perpendicular to the B 0 - field were non-uniform; doses in the region parallel to the photon beam were 2.5 to 3 times higher than doses outside this region. Profiles across films perpendicular to the B 0 - field are shown in Fig. 2. Note that profiles at depths of 3.9 mm and 5.2 mm were very similar to the profile recorded at depth 6.5 mm. Films perpendicular to the B 0 - field covered by 1 mm and 2 mm of solid water (active layers 1.4 mm and 2.7 mm deep respectively) also showed higher doses inside than outside the region parallel to the photon beam. Doses to films parallel to the B 0 - field were not obviously higher within the region parallel to the photon beam.

Fig. 2: Profiles along horizontal x-axis on films positioned as shown in Fig. 1 (i.e. perpendicular to B 0 -field). Conclusion Contaminant electrons deposit dose to surfaces perpendicular to the magnetic field outside the primary beam of the MRI-linac. However, doses to films at least 2.7 mm deep are consistent along the four axes, suggesting that the contaminant electrons are of low energy and are absorbed within a few millimeters of the phantom surface. PV-0142 Time-resolved commissioning of HDR brachytherapy applicators G.P. Fonseca 1 , M.R. Van den Bosch 1 , R. Voncken 1 , M. Podesta 1 , F. Verhaegen 1 1 Department of Radiation Oncology MAASTRO- GROW- School for Oncology and Developmental Biology- Maastricht University Medical Center, Physics, Maastricht, The Netherlands Purpose or Objective There are several reports in the literature of offsets and variations between HDR 192 Ir brachytherapy applicators of the same model. Therefore, accurate applicator commissioning is essential to ensure the correct treatment delivery. Dwell position verification is routinely performed using radiochromic films whose time integrated response has several limitations. We developed and validated a novel time-resolved measurement method using an Imaging Panel (IP) to acquire an image of the applicator and accurately measure dwell times and positions (patent pending). Material and Methods A ring applicator (never used with patients) for 192 Ir High Dose Rate (HDR) gynecological treatments was tested as an example case due to variations of few millimeters reported in the literature. A custom holder with an imaging channel (Figure 1a) was placed on top of an IP that registered the source movement (9 frames per second). The HDR 192 Ir source first goes into the imaging

Fig 1. Measurement schema for films oriented perpendicular to B 0 -field. Results The mean dose in 1 cm 2 on surface films perpendicular to the B 0 - field were 5.4±0.2% of D max , whereas the mean dose on surface films parallel to the B 0 - field was