ESTRO 2020 Abstract book

S950 ESTRO 2020

PO-1639 Surface Image Guided Radiotherapy for breast treatments on Halcyon F. Crop 1 , P. Comte 1 , J. Laffarguette 1 , R. Cayez 1 , D. Pasquier 2 , X. Mirabel 3 , T. Lacornerie 1 1 Centre Oscar Lambret, Medical Physics, Lille, France ; 2 Centre Oscar Lambret & CRIStAL, Academic department of radiation oncology & CRIStAL UMR CNRS 1189, Lille, France ; 3 Centre Oscar Lambret, Academic department of radiation oncology, Lille, France Purpose or Objective The combination of Surface Image Guided Radiotherapy (SIGRT) on halcyon is new. A conventional three camera Catalyst setup around the patient is not possible as the Halcyon system is a closed gantry system. Therefore, three cameras are positioned in front of the gantry. The central camera is closer and oriented towards the real isocenter to enable real-time tracking and breath hold techniques inside the gantry. We installed Catalyst with a slightly different geometry on two halcyon systems (H1 and H2) in order to test the compromise between patient position precision and motion tracking inside the gantry. A Halcyon system requires a daily Quality Control (QC) with a dedicated phantom (MPC). We evaluate if the daily SIGRT QC can be combined with the daily machine QC. Material and Methods We evaluate the precision of the SIGRT breast patient positioning by analyzing the results of Cone Beam CT (CBCT) to planning CT-matching of 1721 sessions. We evaluate the variance differences between H1 and H2 setups with the modified Brown-Forsythe-Levine test (BFL). The precision of the MPC phantom as daily QC device for Catalyst was evaluated by manual displacements in all three and combined directions. Results The H1 Catalyst setup performs slightly better than the H2 setup for positioning in the IEC Y (feet-head) direction (BFL test p<0.001) and in the IEC X (left-right) direction (p=0.02). Most likely this is due to the lateral camera angles and the closer central camera. Mean absolute displacements after CBCT were (X,Y,Z, in mm) (2 ; 2.2 ; 1.8) with standard deviations (2.7, 2.9, 2.4): figures 1 and 2. These results are close to the inter-user fusion variability. There was a remaining table flex of 1.9 mm on H1 and 3.6 mm on H2 for breast patients in the ant-post direction (p < 0.001). Applying the Van Herk principle, a PTV margin of (4.3 ; 4.8 ; 4.7) mm suffices for treatment planning on the H1 setup. In practice, it is recommended to not apply this PTV margin for IMRT and apply a robust optimization scheme for respiration and position uncertainty. Manual displacement tests of the MPC phantom result in a mean error vector of 0.3 mm (largest 0.6 mm). Displacements in the Y direction (Head-Feet) have the largest uncertainty associated in the feet direction: mean error of 0.7 mm on H2 (other directions < 0.5 mm).

Figure 1 : Distribution of manual CBCT corrections after Catalyst setup. Z was corrected for table sag.

Figure 2: CBCT displacements after Catalyst positioning for Halcyon breast treatments (X: red, dots, Y: green, dashed, Z: blue, dashdot (corrected for table sag)). The black line corresponds with the inter-user MVCT fusion variability taken from Crop et al 2016. Conclusion The adapted 3-camera setup for breast patient positioning by Catalyst on Halcyon is precise and CBCT use could be reduced. The use of the MPC halcyon phantom (with the addition of a white scotch on the handle) is precise for daily Catalyst QA. There remains a small table flex for halcyon, which can be compensated by SIGRT. PO-1640 Dosimetric impact of rectum and bladder anatomy during hypofractionated prostate radiation therapy M. Roch 1 , P. Castro 1 , C. Anson 1 , D. Hernandez 1 , F. Garcia de Vicente 2 , A. Zapatero 3 , A. Viñals 1 , R. Fayos-Sola 1 , L. Perez 1 1 Hospital Universitario La Princesa, Radiophysics / Radiation Oncology, Madrid, Spain ; 2 Hospital Universitario Ramón y Cajal, Radiophysics, Madrid, Spain ; 3 Hospital Universitario La Princesa, Radiation Oncology, Madrid, Spain