ESTRO 38 Abstract book

S907 ESTRO 38

EP-1687 Variation of size-specific dose estimates (SSDE) of cone-beam CT (CBCT) scans with the scan length 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 Length of a cone-beam CT (CBCT) scan is defined mainly by the clinical need. Recently, size-specific dose estimates (SSDE) has been recommended to estimate the imaging dose received by a specific patient from a given scan. SSDE is based on determination of size of the area of interest for the patient, i.e. region being scanned. Since length of this area is varied by the scan length, this investigation aimed to study impact of the scan length on SSDE values for CBCT scans. Material and Methods Monte Carlo simulations with user codes, BEAMnrc and Cavity, that were based on EGSnrc system, were employed to model a Varian on-board imager (OBI) kV system integrated into a linear accelerator (Truebeam), and to assess doses in water phantoms of diameters ranging from 10 – 40 cm. This allowed derivation of size conversion factors required to assess SSDE. The factors were derived in water phantoms to be linked to the patient size, which is reported in terms of a water equivalent diameter (D w ). SSDE of CBCT scans were assessed by applying the conversion factors to the dose index CTDI IEC , which is recommend by IEC as a dose index for CBCT scans. A MATLAB-based code developed in house to calculate D w of the region of interest was utilized. The ICRP adult male and female reference phantoms were used in this study to represent average sizes for adult patients. Impact of the scan length was investigated in the trunk, which was divided to three regions, thorax, abdomen, and pelvis. Eleven scan lengths ranging from 80 – 280 mm, with increments of 20 mm, were used for each region, and centers of each scan wat set at middle of these regions. The protocol used in the clinic to scan the body was applied, and the beam widths in the z-axis for CBCT scans were set the scan lengths studied. Results Minimum, maximum, and mean values of D w for the region of interest were assessed for each scan. CTDI IEC of the body scan was evaluated to be 2.46 mGy/100 mAs, and this value was converted to SSDE using the different values of D w as shown in Figure 1. Use of the mean and maximum values of D w to assess SSDE indicated less dependency on the scan length, where SSDE values were only varied by up to ±5% over all the scan lengths studied. However, SSDE values based on the minimum values of D w were affected by the scan length, particularly for the thorax region, where extension of the scan length from middle of the region to the upper part included the shoulder that had a higher attenuation area. The lower variations in SSDE values based on the mean D w values were found for the abdomen region, all being within ±1%, as the attenuation values for the tissues in this region are comparable.

Figure 1: Influence of the scan length on SSDE values based on minimum, maximum, and mean values of D w for the regions of interest. Conclusion Estimation of SSDE values based on the mean D w for the regions of interest showed to give a good indication for the patient dose as the scan length had a minimal influence on SSDE values as well as it took into account the minimum and maximum values of D w . EP-1688 Monte Carlo simulations to quantify out-of- field doses due to the electron streaming effect V.N. Malkov 1 , S.L. Hackett 1 , J.W.H. Wolthaus 1 , B.W. Raaymakers 1 , B. Van Asselen 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands Purpose or Objective In the context of magnetic resonance guided radiation therapy (MRgRT), the influence of the magnetic field on surface doses has been an ongoing area of research. The electron return effect (ERE) can contribute to a significant increase in surface doses at boundaries (Raaijmakers et al.). In addition to the ERE, in orthogonal magnetic field orientations, electrons are swept out of the irradiated air volume. This leads to a reduction of in-field entry surface doses, but contaminant streaming electrons can contribute up to about 5.4% to out of field doses (Hackett et al., 2018). Further, Park et al. (2018) studied treatment accelerated partial breast plans with either a 0 or 0.35 T perpendicular magnetic field and found that the presence of the magnetic field induced out-of-field doses as large as 15% of the prescription dose to extend in the air along the magnetic field. In this work, the out-of-field surface doses contribution due to backscattered or ejected electrons, focused by the magnetic field, is studied. This electron streaming effect (ESE) can contribute to substantial out-of-field doses and requires detail study. Material and Methods The EGSnrc Monte Carlo package is used to simulate a water phantom with an incident 10x10 cm 2 7MV FFF beam. As shown in Figure 1, the phantom entry or exit surface is inclined with respect to the magnetic field, and an out-of- field water panel is positioned 10cm away from, and centered, on the isocenter. The surface dose profiles in the water panel are calculated with either a 0, 0.35, or 1.5 T magnetic field and inclines of 10, 30, or 45 o .