Validation of a Monte Carlo Framework for Out-of-Field Dose Calculations in Proton Therapy

Frontiers in Oncology

Guillaume Boissonnat¹  Miguel Rodriguez³  Beate Timmermann⁴  Isabelle Thierry-Chef⁷  Christian Bäumer⁸  Johannes Esser⁹  Lorenzo Brualla^1,10  Nico Verbeek^1,10  Florian Stuckmann^1,11  Stephan Radonic^1,12  Uwe Schneider^1,12  Fabiano Vasi^1,12  Jérémie Dabin^1,14  Olivier Van Hoey^1,14  Racell Nabha^1,14  Marijke De Saint-Hubert^1,14  Jörg Wulff^1,15 

[1] 0CEA, Université Paris-Saclay, Palaiseau, France;1Hospital Paitilla, Panama City, Panama;2Instituto de Investigaciones Cientificas y de Alta Tecnología INDICASAT-AIP, Panama City, Panama;3Department of Particle Therapy, University Hospital Essen, Essen, Germany;4Radiation Programme, Barcelona Institute for Global Health (ISGlobal), Barcelona, Spain;5University Pompeu Fabra, Barcelona, Spain;6CIBER Epidemiología y Salud Pública, Madrid, Spain;Department of Physics, TU Dortmund University, Dortmund, Germany;Faculty of Mathematics and Science Institute of Physics and Medical Physics. Heinrich-Heine University, Düsseldorf, Germany;Faculty of Medicine, University of Duisburg-Essen, Essen, Germany;Klinikum Fulda GAG, Universitätsmedizin Marburg, Fulda, Zurich, Germany;Physik Institut, Universität Zürich, Zürich, Switzerland;Radiation Oncology and Imaging, German Cancer Consortium DKTK, Heidelberg, Germany;Research in Dosimetric Applications, Belgian Nuclear Research Center (SCK CEN), Mol, Belgium;West German Cancer Center (WTZ), Essen, Germany;West German Proton Therapy Centre Essen WPE, Essen, Germany;

关键词: proton therapy;    anthropomorphic pediatric phantom;    Monte Carlo simulation;    out-of-field dosimetry;    TLD;    bubble detector;   

DOI  :  10.3389/fonc.2022.882489

来源: DOAJ

【 摘 要 】

Proton therapy enables to deliver highly conformed dose distributions owing to the characteristic Bragg peak and the finite range of protons. However, during proton therapy, secondary neutrons are created, which can travel long distances and deposit dose in out-of-field volumes. This out-of-field absorbed dose needs to be considered for radiation-induced secondary cancers, which are particularly relevant in the case of pediatric treatments. Unfortunately, no method exists in clinics for the computation of the out-of-field dose distributions in proton therapy. To help overcome this limitation, a computational tool has been developed based on the Monte Carlo code TOPAS. The purpose of this work is to evaluate the accuracy of this tool in comparison to experimental data obtained from an anthropomorphic phantom irradiation. An anthropomorphic phantom of a 5-year-old child (ATOM, CIRS) was irradiated for a brain tumor treatment in an IBA Proteus Plus facility using a pencil beam dedicated nozzle. The treatment consisted of three pencil beam scanning fields employing a lucite range shifter. Proton energies ranged from 100 to 165 MeV. A median dose of 50.4 Gy(RBE) with 1.8 Gy(RBE) per fraction was prescribed to the initial planning target volume (PTV), which was located in the cerebellum. Thermoluminescent detectors (TLDs), namely, Li-7-enriched LiF : Mg, Ti (MTS-7) type, were used to detect gamma radiation, which is produced by nuclear reactions, and secondary as well as recoil protons created out-of-field by secondary neutrons. Li-6-enriched LiF : Mg,Cu,P (MCP-6) was combined with Li-7-enriched MCP-7 to measure thermal neutrons. TLDs were calibrated in Co-60 and reported on absorbed dose in water per target dose (μGy/Gy) as well as thermal neutron dose equivalent per target dose (μSv/Gy). Additionally, bubble detectors for personal neutron dosimetry (BD-PND) were used for measuring neutrons (>50 keV), which were calibrated in a Cf-252 neutron beam to report on neutron dose equivalent dose data. The Monte Carlo code TOPAS (version 3.6) was run using a phase-space file containing 1010 histories reaching an average standard statistical uncertainty of less than 0.2% (coverage factor k = 1) on all voxels scoring more than 50% of the maximum dose. The primary beam was modeled following a Fermi–Eyges description of the spot envelope fitted to measurements. For the Monte Carlo simulation, the chemical composition of the tissues represented in ATOM was employed. The dose was tallied as dose-to-water, and data were normalized to the target dose (physical dose) to report on absorbed doses per target dose (mSv/Gy) or neutron dose equivalent per target dose (μSv/Gy), while also an estimate of the total organ dose was provided for a target dose of 50.4 Gy(RBE). Out-of-field doses showed absorbed doses that were 5 to 6 orders of magnitude lower than the target dose. The discrepancy between TLD data and the corresponding scored values in the Monte Carlo calculations involving proton and gamma contributions was on average 18%. The comparison between the neutron equivalent doses between the Monte Carlo simulation and the measured neutron doses was on average 8%. Organ dose calculations revealed the highest dose for the thyroid, which was 120 mSv, while other organ doses ranged from 18 mSv in the lungs to 0.6 mSv in the testes. The proposed computational method for routine calculation of the out-of-the-field dose in proton therapy produces results that are compatible with the experimental data and allow to calculate out-of-field organ doses during proton therapy.

【 授权许可】

Unknown