ESTRO 2021 Abstract Book

S497

ESTRO 2021

Conclusion Satisfactory target coverage was achieved in all photon plans and in most proton plans for all cases. The dose to the whole brain was largely reduced in the supratentorial case when using protons, but this was less pronounced in the infratentorial cases. Overall, the brainstem was better spared in the proton plans but the range across metrics was large. Although there was a common treatment planning guideline, doses to OARs varied between the centers independent of the modality. OC-0633 Cardiac substructure avoidance in lung cancer using photon vs proton radiotherapy: A planning study K. Banfill 1 , J. Wood 2 , F. Charlwood 2 , D. Lines 2 , V. Rompokos 3 , C. Hiley 4 , M. van Herk 1 , C. Faivre-Finn 1 , M. Schmitt 5 , A. Salem 1 , A. McWilliam 1 1 University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 2 The Christie NHS Foundation Trust, Radiotherapy Physics, Manchester, United Kingdom; 3 University College London Hospitals NHS Foundation Trust, Radiotherapy Physics, London, United Kingdom; 4 University College London, Cancer Research UK Lung Cancer Centre of Excellence, London, United Kingdom; 5 Manchester University Foundation Trust, Cardiovascular Division, Manchester, United Kingdom Purpose or Objective Higher radiotherapy dose to selected cardiac structures is associated with cardiac events and death in patients with lung cancer. Studies reveal a dose response relationship for structures at the base of the heart including the right atrium and ascending aorta, with a significant dose threshold of 23Gy (McWilliam et al, 2020). Our objective was to define a cardiac avoidance area and investigate the ability of optimised photon and proton plans to spare these structures compared to standard photon radiotherapy. Materials and Methods A cardiac avoidance area (CAA) was defined, based on previous studies, comprising the superior vena cava, right atrium, aortic root, and proximal coronary arteries (figure 1). 12 patients with stage 3 NSCLC treated at 2 UK centres who had a 4D planning CT and ITV within 5cm of the heart were selected. Plans were generated in Raystation v7R to achieve a dose of 66Gy in 33fractions using standard dose parameters. Cases were re-planned using an additional dose constraint of D1cc<23Gy to the CAA for optimised photon plans. For pencil beam scanning proton (PBSP) plans Varian Eclipse (v13.7) was used for replanning. Photon plans used VMAT with two 360° arcs. PBSP plans used single field optimisation with 3 or 4 fields. PBSP plans were robustly optimised for ITV as target using 5mm setup error and 5% range uncertainty. Plans were compared using analysis of variance and pairwise comparisons.