Feasibility of Single-Energy Proton Pencil Beam Scanning Using a Programmable Energy Modulation Filter
Abstract
Purpose
Proton pencil beam scanning (PBS) offers excellent dose conformity but remains limited by delivery inefficiency associated with energy layer switching time. A programmable energy modulation filter (PEMF) was developed to enable single-energy proton PBS delivery. This study evaluates the feasibility of PEMF-based single-energy proton therapy.
Methods
The PEMF consists of multiple macrospots, each formed by an array of modulator rods with discrete heights, followed downstream by a fixed 5-cm-thick PMMA slab. Each macrospot has a diameter of 1.6 cm. Protons traversing different rod thicknesses are energy-modulated but are regarded as originating from a common nominal spot position. Dose distributions were constructed from the lateral profiles and depth–dose characteristics of individual rods. Spot size evolution of protons traversing the PEMF in water was estimated using the Fermi–Eyges formalism and independently verified using Monte Carlo simulations (RayStation 11B, RaySearch Laboratories, Sweden). Feasibility criteria included ripple-free lateral dose profiles and proton spot full width at half maximum (FWHM) exceeding the macrospot diameter.
Results
Analytic evaluation showed that negligible ripple in the lateral profiles of individual rods is a necessary condistion fpr generating a uniform SOBP. For incident proton energies ranging from 100 to 240 MeV, spot sigma values ranged from 1.0 to 1.7 cm between the 50% dose depth and the Bragg peak depth. Analytic and Monte Carlo results agreed within 1 mm for all energies and depths, with FWHM values consistently exceeding 1.6 cm. Lateral profile ripple from individual modulated energies remained below 3%, confirming that a uniform SOBP can be achieved. Film dosimetry comparison of 5-cm-wide SOBP plans demonstrated good agreement between PEMF-based single-energy and conventional multi-energy PBS deliveries.
Conclusion
This study demonstrates that single-energy proton treatments are feasible using the proposed PEMF. The PEMF approach provides a potential pathway toward simpler, faster, and more cost-effective proton therapy delivery systems.