ESTRO 2022 - Abstract Book

S1418

Abstract book

ESTRO 2022

Conclusion The greater magnitude and variation in similarity metrics in children with brain tumours suggests auto-segmentation tools perform worse than in healthy children. Contour differences remained within 4mm in children with brain tumours. FreeSurfer and CAT were the most consistent and showed the fewest failures, and therefore show promise for use in paediatric radiotherapy research. Further work validating against clinical contours is needed.

PO-1627 Peripheral dose sparing of volume-of-interest CBCT using a novel 2.5 MV low-Z beam

J. Borsavage 1 , A. Cherpak 2 , J. Robar 2

1 Dalhousie University, Department of Physics and Atmospheric Science, Halifax, Canada; 2 Dalhousie University, Department of Radiation Oncology, Halifax, Canada Purpose or Objective Megavoltage imaging offers the advantageous perspective from the treatment beam's-eye-view (BEV), however, due to the high energy photons in an MV spectrum, images acquired with this modality suffer from limited image quality per unit imaging dose. A commercial 2.5 MV beam at our centre was recently modified to replace the copper target with a low-Z sintered diamond alternative. This 2.5 MV beam contains a significant population of diagnostic-energy photons, yielding improved planar CNR versus dose characteristics compared to the commercial imaging beam. To further reduce the dose to healthy tissues in 2.5 MV low-Z CBCT, the MLC was used to collimate the beam to the relevant anatomy, through a volume-of-interest (VOI) CBCT approach in this study. Materials and Methods A sintered diamond target was integrated into the TrueBeam (Varian Medical Systems, Palo Alto, CA) target arm replacing the commercial copper imaging target. Using this experimental beam, the sinuses of an anthropomorphic phantom was imaged to investigate the dose sparing capacity of the VOI technique. VOI regions were defined in the treatment planning system as concentric volumes where the smallest VOI conformed to the sinuses, and additional VOIs were created by adding 0.5, 1, and 1.5 cm margins (Figure 1). MLCs were fit to the BEV of each volume throughout a full 360 degree arc. The corresponding MLC sequences were used to construct XML scripts for acquisitions using Developer Mode. For comparison, an additional full field-of-view (FOV) CBCT was also acquired. During each CBCT acquisition, calibrated EBT3 radiochromic film was placed between phantom slices along the transverse plane to measure axial dose distributions.

Results