ESTRO 2021 Abstract Book

S1630

ESTRO 2021

Conclusion The developed autoIMRT technique was proven to be advantageous for target coverage and homogeneity and more robust if compared with the standard 3DCRT technique. The use of automated planning reduces the planning workload while allowing improvements in plan quality. The technique entered in our clinical practice and is now used for the majority of our patients.

PO-1910 Proton therapy in breast cancer: how do different beam arrangements affect linear energy transfer? S. Settatree 1,2 , A. Bertolet 3 , A. Carabe 4 , D. Lines 5 , N. Harrold 5 , E. Harris 2 , A.M. Kirby 6,2 , S. Gulliford 7,8

1 Royal Marsden Hospital, Clinical Oncology, Breast, London, United Kingdom; 2 Institute of Cancer Research, Radiotherapy and Imaging, Sutton, United Kingdom; 3 Massachusetts General Hospital and Harvard Medical School, Radiation Oncology, Boston, USA; 4 Hampton University Proton Therapy Institute, Physics and Dosimetry, Hampton, VA, USA; 5 The Christie NHS Foundation Trust, Medical Physics, Manchester, United Kingdom; 6 Royal Marsden Hospital, Clinical Oncology - Breast, London, United Kingdom; 7 University College London, Medical Physics, London, United Kingdom; 8 University College London Hospitals NHS Foundation Trust, Radiotherapy, London, United Kingdom Purpose or Objective It is considered that protons have a variable relative biological effectiveness (RBE) which is related to linear energy transfer (LET). LET is variable along the proton track with highest values distally. For a breast and internal mammary nodal proton beam therapy (PBT) plan, this could affect the dose to organs at risk (OARs) such as heart, left anterior descending coronary artery (LAD) and ribs. This study compares dose-averaged LET (LET d ) distributions in OARs from different beam arrangements with the aim of identifying which could mitigate the potential clinical effects of high LET d . Materials and Methods Five left-sided breast patients comprising a spectrum of chest wall shapes were identified. Three patients had varying degrees of pectus excavatum (Haller Index 2.7-4.9). Breast/ reconstructed chest wall, axilla levels 1-4 and internal mammary clinical target volumes were contoured according to ESTRO consensus [1]. Four pencil beam scanning PBT plans were prepared for each patient using the beam arrangements described in Table 1. The prescription was 40.05 Gy in 15 fractions. All plans were optimised to achieve 95% dose coverage of 95% of all target volumes and satisfied uncertainty parameters of 5mm (set up) and +/-3.5% (range).

Treatment plans were calculated using an analytical pencil beam algorithm (Varian Eclipse). LET d

was calculated using a

bespoke script developed at the University of Pennsylvania [2,3].