3132 - Ultra-Sharp Upright Radiotherapy with Extended-Distance 2.5 MV Beams: An Alternative Mode of Use for FLASH-Capable Sources

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 8

POSTER

Presenter(s)

Lawrie Skinner, PhD - Department of Radiation Oncology, Stanford University, Stanford, CA

L. E. K. Ghomsi¹, R. M. Ashraf², V. J. Heng², and L. Skinner³; ¹Department of Radiation Oncology, Stanford University, Palo Alto, CA, ²Stanford University, Stanford, CA, ³Department of Radiation Oncology, Stanford University, Stanford, CA

Purpose/Objective(s): Standard 6 MV radiotherapy is fundamentally limited by secondary electron range, with minimum penumbra widths of 2–3 mm. These soft beam edges limit dosimetric conformity and organ-at-risk sparing. This study investigates an upright radiotherapy paradigm that uses filtered 2.5 MV beams to sharpen the beam edge, combined with extended distance to produce penetration comparable to conventional setups. The objective is to achieve at least a twofold improvement in beam edge sharpness while maintaining standard 6MV-like depth dose fall-off.

Materials/Methods:

A 2.5 MV flattening-filter-free (FFF) beam was delivered at a 4-meter source-to-phantom distance and hardened with a 6 mm lead filter (2.5 MV-ED). This beam was collimated with 8 cm thick tungsten-copper bars (W75-Cu25 alloy) placed 0.5 m upstream from the phantom surface. Lateral profiles and percent depth doses (PDD) were measured in a solid-water phantom using radiochromic film and a parallel-plate ion chamber. These measurements were used to benchmark Monte Carlo simulations. The validated 2.5 MV-ED model was then used to simulate multi-beam plans. Small targets and spatially fractionated (lattice) radiotherapy plans were calculated. Comparisons were made against equivalent 6 MV-FFF coplanar deliveries at 1 m from the source.

Results:

The single 28×28 mm² field with a 2.5 MV-ED beam produced a measured 80–20% penumbra width of 1.0 ± 0.1 mm, versus 2.4 ± 0.1 mm for the standard 6 MV-FFF beam. The PDD at 10 cm was 52% for 2.5 MV-ED and 56% for 6 MV-FFF (for the 28 x 28 mm² field size), while the surface dose was 22% for 2.5 MV-ED compared with 38% for 6 MV-FFF. Multi-beam Monte Carlo simulations using the 2.5 MV-ED beam showed significantly sharper composite dose fall-off in all cardinal directions than coplanar 6 MV-FFF plans. In a lattice example with 5 mm diameter high-dose spheres, the 2.5 MV-ED approach achieved a peak-to-valley dose ratio of 4.5–5.2, compared with 2.8–2.9 for a 6 MV-FFF plan on a clinical system.

Conclusion:

Low-energy, extended-distance photon beams can provide sharper penumbra and lower surface dose while maintaining depth-dose penetration comparable to standard 6 MV setups. Simulations and measurements indicate that these geometries with upright positioning can deliver ultra-sharp dose distributions, offering improved treatment conformity and potential reductions in toxicity. This approach which requires 20-40 times more photon fluence than standard 6MV radiotherapy setups, provides an alternative mode of use for FLASH-capable photon machines potentially unlocking advanced radiotherapy with higher spatial fidelity.