Geometry optimisation of graphite energy degrader for proton therapy. (August 2020)

• Record Type: Journal Article
• Authors: Oponowicz, E.; Owen, H.L.; Psoroulas, S.; Meer, D.
• Is Part Of: Physica medica. Volume 76(2020)
• Page Start: 227
• Page End: 235
• Publication Date: 2020-08
• Subjects: Energy degrader -- Monte-Carlo simulations -- Proton therapy -- Graphite; Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2020.06.023
• Languages: English
• ISSNs: 1120-1797

Abstract

Highlights: Alternative energy degrader geometries for proton therapy facility. Single block offers higher transmission than multiple wedges degrader. Emittance calculations require beam core extraction from non-Gaussian distribution. Abstract: Introduction : Cyclotron-based proton therapy facilities use an energy degrader of variable thickness to deliver beams of the different energies required by a patient treatment plan; scattering and straggling in the degrader give rise to an inherent emittance increase and subsequent particle loss in the downstream energy-selection system (ESS). Here we study alternative graphite degrader geometries and examine with Monte-Carlo simulations the induced emittance growth and consequent particle transmission. Methods : We examined the conventional multiple-wedge degrader used in the Paul Scherrer Institute PROSCAN proton therapy system, the equivalent parallel-sided degrader, and a single block degrader of equivalent thickness. G4Beamline Monte-Carlo tracking of protons was benchmarked against measurements of the existing degrader for proton energies from 75 to 230 MeV, and used to validate simulations of the alternative geometries. Results : Using a careful calculation of the beam emittance growth, we determined that a single-block degrader placed close to the collimators of the ESS is expected to deliver significantly larger transmission, up to 17% larger at 150 MeV. At the lowest deliverable of 75 MeV there is still a clear improvement in beam transmission. Conclusions : Whilst dose rates are not presently limited on the PROSCAN system at higher energies, a single-block degrader offers the ability to access either lower energies for treatment or a larger dose rate at 75 MeV in case transmission optimisation is desired. Single-block degraders should be considered for the delivery of low-energy protons from a cyclotron-based particle therapy system.