Effect of Varying Lateral Spot Size and Beam Energy Spread on Proton Therapy Treatment Plans
Abstract
Purpose
Proton therapy (PT) has demonstrated superior healthy tissue sparing compared to photon radiotherapy for various sites; however, PT systems are expensive, which limits global availability. The dielectric wall accelerator (DWA) is proposed as a low-cost, compact PT system that provides unique control over spot size and energy spread. In this work, we investigate the effect of varying lateral spot size and beam energy spread on PT treatment plans.
Methods
A linear beam optics model of the DWA was developed using TRANSOPTR. Proton bunches (20–230 MeV) were simulated through a DWA beamline using analytically derived parameters to modulate spot sizes and energy spreads at isocenter. To evaluate the impact of lateral spot size and beam energy spread variations on plan quality, treatment plans were generated and compared across both homogeneous water phantoms and five pediatric CNS patients. For comparison, a conventional proton beam using IBA beam data was used as reference.
Results
In both water phantom and patient cases, reducing the lateral spot size yielded greater OAR sparing while maintaining comparable target coverage (e.g., a 2-mm spot size reduction reduced critical OAR mean dose by up to 50%). A similar trend of improved OAR sparing was observed with reduced beam energy spread; however, the effect was less pronounced, due to changes in energy spread predominantly influencing the lower-dose tail rather than the Bragg peak. Lower energy spreads also decreased the normalized surface dose and increased the depth of maximum dose.
Conclusion
Decreasing lateral spot size and beam energy spread resulted in greater OAR sparing while maintaining target coverage in water phantom and pediatric CNS cases. 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 of ~0.05% for 20 to 230 MeV.