ESTRO 2020 Abstract book

S1029 ESTRO 2020

PO-1756 Phantom investigation of the geometric accuracy of 4D-CBCT V. Giacometti 1 , C.E. Agnew 2 , A. Ali 3 , A.R. Hounsell 2 , C.K. McGarry 2 1 Centre for Cell Biology and Cancer Research- Queen's University Belfast, School of Medicine- Dentistry And Biomedical Sciences, Belfast, United Kingdom ; 2 Northern Ireland Cancer Centre, Radiotherapy Physics, Belfast- Northern Ireland, United Kingdom ; 3 Queen's University Belfast, Physics, Belfast, United Kingdom Purpose or Objective The aim of this study was to investigate the geometrical accuracy of four-dimensional cone beam computed tomography (4D-CBCT) when tracking a target in motion at different motion amplitudes. Material and Methods A 27 cm 3 cubic insert of the QUASAR TM respiratory motion phantom (Modus Medical Devices Inc., London, ON) was sinusoidally moved using the QUASAR programmable respiratory motion software v3.0. A clinically relevant motion period of 4 s and amplitudes of 20 mm, 25 mm and 30 mm were selected. 4D-CBCT images were acquired using a thorax acquisition protocol (125kV. 672 mAs) on two Varian TrueBeam v2.7 linacs (Varian Medical Systems, Palo Alto, CA) with advanced imaging and motion management. The reconstructed images (slice thickness 0.15 cm) were sorted into 10 phase bins and imported into Varian Eclipse (v15.6) for contouring and measuring the size of the cubic insert. A threshold of 220 - 500 HU was applied to appropriately isolate and contour the cube volume against the background. The superior - inferior cube length was measured using the ruler function. The motion velocity ( v ) in each phase bin ( p ) for a set motion amplitude ( A ) and motion period ( T=4s ) was calculated as described in [1]. The impact of motion velocity in generating geometrical artefacts was investigated by plotting length deviations as a function of the motion velocity. Results Length deviations of up to 3.5 mm were measured. The increase in length with increasing velocity was consistent across the two linear accelerators used to generate the 4D- CBCT images (linear accelerator 1: slope = 0.0055, intercept = 3.0825; linear accelerator 2: slope = 0.0042, intercept = 3.0617). The volume variation was found to be between -3.0% and +15.0%, with consistency observed across the linear accelerators across all velocities.

Conclusion This phantom study demonstrated that the latest 4D-CBCT technology results in significantly smaller length deviations compared to 4D-CT [1]. Further studies will be conducted to establish the margins to be adopted for contouring a target in motion from 4D-CBCT images, taking into account phase dependant acquisition uncertainties determined from this study. References [1] O'Connell et al , Optimizing geometric accuracy of four- dimensional CT scans acquired using the wall- and couch- mounted Varian® Real-time Position Management™ camera systems, Br J Radiol. 2015; 88(1046). PO-1757 Peer Review in Stereotactic Body Radiotherapy: The Impact of Case Volume M. Huo 1 , L. Morley 1 , L. Dawson 1 , J. Bissonnette 2 , J. Helou 1 , M. Giuliani 1 , A. Berlin 1 , D. Shultz 1 , A. Hosni 1 , A. Shessel 3 , A. Barry 1 1 Princess Margaret Cancer Centre, Radiation Oncology, Toronto, Canada ; 2 Princess Margaret Cancer Centre, Radiation Physics, Toronto, Canada ; 3 Princess Margaret Cancer Centre, Radiation Therapy, Toronto, Canada Purpose or Objective Peer review is an essential component of the radiotherapy (RT) paradigm, optimizing treatment quality and patient safety. A growing body of literature has demonstrated significant rates of suggested plan changes resulting from peer review, in the order of 10%. Furthermore, peer review is well indicated for stereotactic body radiotherapy (SBRT) due to a high risk of morbidity if performed sub-optimally. There remains a paucity of data on the impact of peer We conducted a retrospective review of all SBRT cases that underwent peer review from July 2015 to June 2018, under an institutional ethics waiver. At our institution, peer review occurs via weekly chart rounds grouped according to primary cancer subsite. We compiled learning moments (LM), defined as suggested changes or an educational discussion beyond the routine management of each case, and critical errors recorded during peer review. We also review rounds on SBRT. Material and Methods Poster: Physics track: Implementation of new technology, techniques, clinical protocols or trials (including QA & audit)