Ultra-Sharp Upright Photon Radiotherapy via Low Energy Extended Distance: An Alternative to FLASH for high flux Sources

TL;DR

This study demonstrates that low-energy, extended-distance photon beams in upright radiotherapy can produce sharper dose penumbra and lower surface doses compared to standard high-energy beams, enabling more precise and conformal treatments.

Contribution

The paper introduces a novel approach using 2.5 MV low-energy beams at extended distances to achieve ultra-sharp dose distributions, surpassing the limitations of conventional 6 MV radiotherapy.

Findings

80%-20% penumbra of 1.0 mm with 2.5 MV-ED beam

Sharper dose fall-off in simulations with conical geometry

Higher peak-to-valley dose ratio in lattice plans

Abstract

Standard 6 megavolt (MV) radiotherapy is limited by source size and secondary electron range to minimum radiological penumbra widths of ~2-3 mm. This study investigates sharper beams via upright radiotherapy with lower energies and extended source-to-patient distances. A 2.5 MV beam from a clinical linac was delivered at a 4 m source-to-phantom distance (2.5 MV-ED). Lateral profiles and percent depth doses were measured in a solid water phantom with radiochromic film and an ion chamber. These single beam measurements were used to benchmark TOPAS Monte Carlo simulations. The validated 2.5 MV-ED model was then used to simulate upright deliveries with a conical beam geometry. These simulations were compared against equivalent plans generated for standard 6 MV-FFF coplanar deliveries at 1 m from the source. The 2.5 MV-ED single 28x28 mm^2 beam produced a measured 80%-20% penumbra of 1.0 +-0.1 mm, compared to 2.4 mm Jaw-defined penumbra for a standard 6MV-FFF beam. The doses at 10 cm depth were 52% vs. 56%, and the surface doses were 22% vs. 38% for the 2.5MV-ED and standard 6 MV-FFF respectively. Conical geometry Monte-Carlo simulations using the 2.5 MV-ED beams demonstrated significantly sharper composite dose fall-off in all cardinal directions compared to coplanar 6MV-FFF plans. For a spatially fractionated lattice example plan with 5 mm diameter high dose spheres, the 2.5 MV-ED conical approach achieved a peak-to-valley dose ratio of 4.5-5.2, compared to the 2.6-2.9 achievable with a standard 6 MV-FFF clinical system. Low-energy, extended-distance photon beams can provide sharper penumbra and lower surface dose, while maintaining comparable depth-dose penetration as standard 6 MV setups. Combined with upright patient setups, ultra-sharp dose distributions with enhanced treatment conformity, reduced toxicity, and higher-fidelity dose modulation are possible.