ESTRO 2021 Abstract Book

S747

ESTRO 2021

Conclusion Direct application of the van Herk margin as SRS in the Dutch robustness evaluation protocol leads to overly conservative PT treatment plans. We recommend deriving margin recipes for the use in robust optimization and evaluation in PT.

PD-0905 Pencil Beam Proton Flash Therapy, the challenge of scanning. A. Pin 1 , L. Hotoiu 1 , S. Deffet 2 , E. Sterpin 2,3 , R. Labarbe 1

1 Ion Beam Applications, Research, Louvain-la-Neuve, Belgium; 2 Université Catholique de Louvain, Imagerie Médicale, Radiothérapie et Oncologie, Louvain-la-Neuve, Belgium; 3 KU Leuven, Laboratorium Experimentele Radiotherapie, Leuven, Belgium Purpose or Objective FLASH proton therapy requires irradiation at high dose rate for fast treatment dose delivery (above 40Gy/sec). An important challenge consists into using existing treatment planning systems to create a treatment plan deliverable in FLASH conditions. The FLASH plan will be more dependent on the proton delivery patterns than a conventional IMPT plan. In a proton treatment delivered with pencil beam scanning (PBS), it is important to optimize the order of delivery of each spot. This work presents an impact study of the spot sequence delivery over the dose and dose rate in the targeted field. Materials and Methods Assuming a proton PBS delivery we consider a local definition of the dose rate. The dose rate is computed per voxel, as the ratio of dose received by the voxel and the time required to deliver the PBS spot contributing to the dose of the voxel. The dose rate within a RT-structure can then be characterized by the average (or the median) of the dose rate of all the voxels in that structure. The dose received by one voxel, as considered for the dose rate computation, is presented in figure 1. Changing the scanning pattern will change the dose delivery pattern of each voxel.