Ultra-Sharp Upright Radiotherapy: An Alternate Modality for Flash-Capable Photon Sources Utilizing Lower Energy, Large-Source, Extended Distance Setups.
Abstract
Purpose
Secondary electron range constrains standard 6MV radiotherapy to 80%-20% dose penumbra of ~2.4–3 mm, limiting conformity and organ sparing. Photon FLASH likely necessitates even larger penumbra, which is potentially problematic as the dose rates can be non-FLASH there. Instead, we investigate what can be achieved by combining powerful sources with extreme collimation, to achieve sharper dose distributions and enhanced depth-dose penetration at standard dose rates. Advanced SRS, SBRT, and spatially fractionated radiotherapy (SFRT) are possible.
Methods
Extended source-to-patient distances are combined with lower energies, and conical arcs. Specifically, percent depth doses (PDDs) from a 2.5 MV beam (Varian TrueBeam) at a 4 m Source-to-Axis Distance (SAD), with an additional lead filter, and external collimator, were measured in a 30x30x20 cm2 solid water phantom using radiochromic film (Gafchromic EBT3) and an Exradin A10 parallel-plate ion chamber. These extended distance beams (2.5MV-ED) were then benchmarked TOPAS Monte Carlo simulations. Monte Carlo calculations (with larger source sizes to regain dose rate) were made for upright patient conical arc deliveries with a 15° tilt between the beam and patient rotation axes. Simulations of a small cylindrical target and an SFRT pattern were investigated and compared with standard-geometry 6MV-FFF coplanar TrueBeam deliveries using small jaw-defined fields, and HD-MLC VMAT.
Results
The measured 2.5MV-ED beam achieved an 80–20% penumbra of 1.0 mm, compared with 2.4 mm for a standard 6 MV-FFF beam , while the extended-distance and filtration enabled comparable PDD10 52% vs 56% (for a 28mm2 field). In the SFRT model, the 2.5MV-ED conical approach achieved sharper dose fall-off in all cardinal directions with peak-to-valley dose ratios of 4.5–5.2, exceeding 2.8–2.9 of the 6 MV-FFF reference.
Conclusion
Low-energy, extended-distance photon beams delivered via upright conical arcs can overcome conventional linac penumbra limits while maintaining depth-dose fall-off. This opens up potential for higher-fidelity radiotherapy.