ESTRO 38 Abstract book

S905 ESTRO 38

Figure 2: Relative readings [%] measured at a horizontal radial distance of 1.5 m from the isocentre at 15° increments. Results at 2 m from isocentre follow the same trend as the results at 1.5 m. Relative measurements in full scatter conditions are currently under investigation. Conclusion The relative radiation measurements in a horizontal plane throughout the bunker were plotted as a function of room angle for various distances from the Halcyon isocentre. Measurement results reported above fall within quoted reference values. These suggest that a primary barrier may not be required when designing a bunker for the Halcyon as the primary transmission through the primary beam stopper is negligible. This agrees with the Varian Designers’ Desk Reference, Halcyon Accelerator Edition (2017) which states that a primary shielding wall is not required for the bunker. EP-1685 Impact of acquisition mode of CBCT scans on size-specific dose estimates (SSDE) conversion factors A. Abuhaimed 1 , C. Martin 2 , O. Demirkaya 3 1 King Abdulaziz City for Science and Technology, The National Centre for Applied Physics, Riyadh, Saudi Arabia ; 2 University of Glasgow, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom ; 3 King Faisal Specialist Hospital & Research Centre, Department of Biomedical Physics, Riyadh, Saudi Arabia Purpose or Objective This study aimed to investigate influence of an acquisition mode, namely the partial scan, which is used for cone- beam CT (CBCT) scans to image patients partially over 200°, on size conversion factors of size-specific dose estimates (SSDE), which were derived with the full 360° scan mode. Material and Methods The SSDE factors are used to account for patient size by converting a dose index that is measured in phantoms of reference sizes known as CTDI head and body phantoms. For a CBCT scan of nominal beam width > 40 mm, one of the dose indices, recommended by IEC, is called CTDI IEC . It is based on dose measurements at central and peripheral positions of the CTDI phantoms with a reference beam width of ≤ 40 mm, and free-in-air dose measurements for the beam of interest and the reference beam. SSDE factors for a Varian kV imaging system, namely on-board imaging (OBI), were derived to convert CTDI IEC values using Monte Carlo simulations with BEAMnrc and Cavity user codes under different conditions. SSDE factors were assessed in water phantoms of diameters ranging from 10 to 40 cm. The diameters range was selected to match the patient size that is given as a water equivalent diameter. Two clinical scan protocols, head and body, similar to those used in the clinic were employed. Four tube potentials of 80 – 140 kV and the full and partial scan modes were used to derive SSDE factors.

Figure 1: Illustration defining the room angles used for measurements. Central axis (CAX) measurements in solid water, at depth of dose maximum, 100 cm SSD, for a 10 cm x 10 cm field, corrected for daily output variation, were used as a reference to give the relative reading [%]. Transmission through the beam stopper was measured for gantry 90° and 270° with the chamber at 1.5 m from the isocentre. Head leakage measurements were made at 1 m behind the target, with closed MLC, collimator angle 0°, 1000 MU/reading, at room angles 90° and 270°. Results Initial horizontal plane measurement results at 1.5 m from the isocentre are shown in Figure 2. The transmission through the beam stopper at a distance of 1.5 m from the isocentre was 0.003%. The inverse square law was used to calculate transmission at 1 m from the isocentre (0.007%). This agrees with the Varian Designers’ Desk Reference, Halcyon Accelerator Edition (2017) which states that the transmission 1 m from the isocentre is < 0.1%. The highest measurements were 0.28% and 0.27% at room angles 90° and 270° respectively, at 1.5 m from the isocentre. The average result of the head leakage measurements was 0.05%. Head leakage is required to be less than 0.1% as per IEC standard 60601-2-1. Gantry angles 0° and 180° provided similar results for the left and right (Figure 1) of the Halcyon, while Gantry angles 90° and 270° provided mirrored results.