A Scissor-Beam Planning Framework for Cylindrical Spatial Dose Modulation In Proton Therapy
Abstract
Purpose
Spatially fractionated proton therapy enables spatial dose modulation that may improve normal tissue tolerance; however, generating controlled peak–valley dose patterns while maintaining adequate target coverage remains challenging. In particular, achieving clinically relevant spatial modulation geometries without compromising uniform target dose is nontrivial. This work proposes a planning-based scissor-beam framework to generate cylindrical spatial dose modulation in proton therapy while preserving uniform target dose.
Methods
A scissor-beam (SB) planning approach was developed by splitting a field into primary and complementary angled beams with a small relative rotation. Beam spots were selectively assigned to the two beams to produce peak–valley dose modulation in normal tissue while maintaining uniform dose at depth. By alternating spot assignment along both x- and z-directions in the beam’s eye view and adapting the rotation angle to tumor depth, a two-dimensional cylindrical spatial dose pattern was generated. Building upon the base scissor-beam framework, an additional optimization layer incorporating L1/L2 norm–based constraints was introduced to enhance peak-to-valley dose ratio (PVDR) while maintaining overall plan quality.
Results
The proposed method was evaluated in different clinical cases and compared with conventional intensity-modulated proton therapy (IMPT). Scissor-beam plans generated distinct cylindrical peak dose patterns in normal tissue while maintaining uniform target coverage using single or dual fields. Incorporating the optimization layer consistently increased PVDR relative to the base scissor-beam approach, with typical improvements on the order of 20–30%, without degrading target coverage or organ-at-risk sparing. Similar trends were observed across all anatomical sites.
Conclusion
This work demonstrates that cylindrical spatial dose modulation can be achieved in proton therapy through a planning-driven scissor-beam framework while preserving uniform target dose. By incorporating optimization-driven enhancement of peak-to-valley dose ratio, the approach provides a flexible means of integrating spatial fractionation concepts into conventional proton treatment workflows without altering delivery hardware.