ESTRO 2020 Abstract book

S807 ESTRO 2020

PO-1427 Comparison of Flattening Filter (FF) and Flattening-Filter-Free (FFF) out of field surface doses M.T. García Hernández 1 , A. Vicedo González 2 , B. Sánchez Nieto 3 , M. Romero Expósito 4 , J. Roselló Ferrando 1 1 ERESA. Hospital General Universitario de Valencia, Servicio de Radiofísica, Valencia, Spain ; 2 ERESA. Hopsital General Universitario de Valencia, Servicio de Radiofísica, Valencia, Spain ; 3 Pontificia Universidad Católica de Chile- Santiago- Chile., Instituto de Física, Valencia, Spain ; 4 Universitat Autónoma de Barcelona- Barcelona- Spain, Departamento de física-, Barcelona, Spain Purpose or Objective The study of Secondary cancers, as a consequence of peripheral doses received by photon radiotherapy patients, is becoming a topic of growing interest due to the longer life expectancy accomplished nowadays. On the other hand, new developments such as flattening filter free (FFF) beams have the benefits of higher dose rates associated with shorter treatment times. Also, removal of the flattening filter reduces scatter and improves the efficiency of the treatment dose delivery. Therefore, it has the potential benefit of reducing out-of-field dose. Skin is an organ at risk of secondary cancer during radiotherapy, however there are only few articles that investigate surface peripheral dose. The objective of this work is to compare peripheral surface doses between FF and FFF beams for 10 MV and 6 MV under different Measurements were carried out in a Varian TrueBeam linac (Varian Medical Systems, PaloAlto, CA) using gafchromic EBT3 films calibrated using the procedure developed by Tamponi et al, to reduce energy dependence. Films of 2.5x3 cm2 were placed on the surface of an anthropomorphic phantom (Rando phantom Alderson Laboratories, Stamford) at distances from 2 to 50 cm to the field border for energies of 6 MV, 6 MV FFF, 10 MV and 10MV FFF. Field sizes of 3x3, 10x10 and 20x20 cm2, modifying source surface distance and distance between multileaf collimator and adjustable jaws were analysed (figure1a). Additionally, one film was introduced sandwiched at 8 cm from the border of a 10x10 cm2 field to measure peripheral dose from the surface up to 5.8 cm depth (figure 1b). In order to get a dose level covered by the dose range of the irradiated calibration curve, the fluency administered ranged from 6000 to 72000 MU. irradiation conditions. Material and Methods

PO-1426 A comparative study for Dose Length Product (DLP) methods of Cone Beam CT scans A. Abuhaimed 1 , C. J. Martin 2 1 King Abdulaziz City for Science and Technology, The National Center for Applied Physics, Riyadh, Saudi Arabia ; 2 University of Glasgow, Department of Clinical Physics and Bioengineering, Glasgow, United Kingdom Purpose or Objective Dose length product (DLP) plays a vital role in CT dosimetry in estimation of patient doses. With the evolution of CT technology, use of wide beams for cone beam CT (CBCT) scans has many advantages and has become common practice. However, the volume CT dose index (CTDI vol ), upon which DLP is based, cannot be assessed directly for CBCT scans because of the beam geometry. The purpose of this study is compare between two methods to estimate DLP values for CBCT scans. measured in the standard CTDI phantoms with a narrow beam width of 20 mm by width of the cone beam of interest . The second method (DLP CBCT ) uses modified weighted CTDI 100 , which is recommended by the International Electrotechnical Commission (IEC) for CBCT scans, and is multiplied by width of the beam. A validated Monte Carlo model of a Varian kV on board imagining (OBI) system mounted on a Truebeam linac was used. Simulations were run to assess DLP CT and DLP CBCT for head and body scan protocols using 120 kV for scan beam widths ranging from 20 to 320 mm. Results Comparisons between the DLP CT and DLP CBCT for the head and body protocols are shown in figure 1. For both protocols, differences began to appear with increases in the beam width of interest. The differences resulted from the correction factors used for each beam width in the modified CTDI 100 , which equate to unity for the narrow beams. Comparisons show that most differences were within ±1%, with larger differences occurring for the body protocol. Material and Methods The first method (DLP CT by multiplying CTDI vol ) is based on determination of DLP

Results Generally, surface stray dose is lower for FFF beams with differences ranging from 4% to 57% for 6 MV and from 4% to 71 % for 10 MV, increasing with distance to the field border. When moving the collimator 3 cm away from the MLC edge (figure 2), there is an increase in the surface out-of-field dose (ranging from 37 % to 170 %) on average higher than that observed when comparing FF versus FFF. The most important decrease of surface peripheral dose is achieved when the treatment field size is reduced (with differences up to 200 %). We find peripheral surface dose higher than in depth dose by a factor of 2.5 and 2.8 for 6 MV and 10 MV FF respectively, at 8 cm from the field border. For FFF beams this ratio is reduced, being surface peripheral dose 1.8 and 1.7 times in depth dose for 6 MV and 10 MV FFF beams, respectively.

Conclusion Results of the comparisons indicate that use of the DLP CT concept to assess DLP of scans acquired with wide beam leads to over or underestimation by about 1%. However, application of the correction factor is recommended as it will vary from one system to another, and is affected by parameters of scans.