ESTRO 2023 - Abstract Book

S1724

Digital Posters

ESTRO 2023

Conclusion A novel and CPU-efficient approach obtaining MC based microdosimetric characterization of proton beams was developed and applied. The results illustrate the dependence of lineal energy on site size and may be related to the known variation of relative biological effectiveness in proton therapy. The developed tools could be used to support in vivo and in vitro radiobiological experiments, enable comparison of experimental measurements from different detectors and to generate data for nano and microdosimetry-based treatment planning in proton therapy.

PO-1961 Linear Energy Transfer Distributions for Pencil Beam Scanning of Pediatric Brain Tumors

P. Laegdsmand 1 , A. Vestergaard 1 , L. Muren 1 , Y. Lassen-Ramshad 1 , L. Toussaint 1

1 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark

Purpose or Objective The relative biological effectiveness (RBE) uncertainties associated with the variation in linear energy transfer (LET) of proton beams is a challenge for treatment planning of pediatric central nervous system (CNS) tumors near the brainstem. However, the LET distributions of pencil beam scanning (PBS) plans still need to be investigated for these patients. Therefore, our study aimed to survey the distribution of LET and RBE in the brainstem for patients treated at our institution. Materials and Methods A cohort of 15 patients was selected from 47 pediatric CNS patients (aged less than 18 years (figure 1a)) treated with PBS. None had experienced brainstem toxicity. The selection criteria were: 1) Focal brain irradiation only, 2) Maximum dose to the brainstem at least 50 GyRBE. All plans used a minimum of 3 fields spaced by at least 30 degrees to mitigate high LET. The prescribed dose was 50.4 GyRBE (6 plans) or 54 GyRBE (9 plans) in 1.8 GyRBE fractions. Some plans included a boost of 5.4 GyRBE to a smaller target volume not considered in this study. The brainstem core was defined as the brainstem subtracted a 3 mm margin in the transversal directions. The dose-averaged LET (LETd), including only protons, was calculated for all plans by Monte Carlo simulations in Tool for Particle Simulation (TOPAS) v.3.5. Variable RBE was calculated