Effects of varying lateral spot size and beam energy spread in proton therapy: a phantom and patient case study

Why clinicians should know about it

• Picked for Medical Physics (paper of the day, 14 August 2026): Spot size and energy spread effects on proton plan quality

Abstract

Objective. The dielectric wall accelerator (DWA) is proposed as a low-cost, compact system for proton therapy. It offers additional degrees of freedom over conventional accelerators for controlling beam parameters. This study seeks to determine clinically desirable proton beam characteristics by evaluating their implications for treatment plan quality; particularly, the influence of variations in lateral spot size and beam energy spread were investigated.Approach.A linear optics model of the DWA, developed in TRANSOPTR, was used to define realistic beam parameters at the accelerator exit, from which an analytical method selected those yielding the desired spot sizes and energy spreads at isocentre. These parameters were subsequently passed through a generic nozzle model in TOPAS. Lateral spot size was varied from 1 to 10 mm. Beam energy spread was varied from 0.00005% and 15%. Robustly optimized treatment plans were generated in RayStation for a homogeneous water phantom and a sample pediatric central nervous system cancer case. Results were validated against conventional proton beam data.Main results.In the water phantom study, lower energy spreads decreased the normalized surface dose and increased the depth of maximum dose. In both phantom and patient cases, reducing the lateral spot size yielded greater OAR sparing while maintaining comparable target coverage, which is in agreement with the literature. A similar trend of improved OAR sparing was observed with reduced beam energy spread for all cases; albeit, the effect was less pronounced. For realistic optimization of the DWA system, the beam parameters identified as most favourable were a lateral spot size of approximately 1 mm and an energy spread on the order of 0.05% to 0.5% for 20 to 226 MeV.Significance.These results should prove useful to both hardware and numerical efforts, and form the basis for future studies of the clinical characteristics of prospective DWA machines.

Abstract as published, via PubMed.