Geometry optimisation of graphite energy degrader for proton therapy

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.

Technical challenges for FLASH proton therapy

“…4. Improved degrader geometries may help somewhat [60,61], but it also suggests the need for dedicated lower-energy intense cyclotrons that avoid the need for some of the degrader material. Should a single cyclotron be used to cover all the clinical energy ranges, and assuming the existing transmission loss of 99% at low energies, currents approaching 100 µA will need to be extracted to meet the requirements for the 1 litre volume set out in Section 2.…”

Technological Developments and Future Perspectives in Particle Therapy: A Topical Review

“…Some research is also ongoing to find an optimal shape. Typically they are wedge-shaped, but parallel-sided degraders and single block degraders have also been recently proposed [55]. Dipole magnets and collimators located downstream the degrader can be used to limit the energy spread in the beam and to define the emit- tance and transverse beam distribution.…”

Improving delivery efficiency using spots and energy layers reduction algorithms based on a large momentum acceptance beamline

“…But, it increases the transverse beam emittance and momentum spread, which eventually leads to beam loss 29,30 . To minimize these side effects, several groups have optimized the geometry structure and used materials with low atomic numbers and high density 31,32 . Moreover, Maradia et al.…”

“…29,30 To minimize these side effects, several groups have optimized the geometry structure and used materials with low atomic numbers and high density. 31,32 Moreover, Maradia et al developed a beamline that transports a larger emittance beam (100 π⋅mm⋅mrad) using asymmetric collimators. 33 The beamline resulted in the overall transmission increased by 40%, especially for the 70 MeV beam, where the transmission was improved from 0.13% to 0.72%.…”