Scanning or Scattering? Evaluating Very High Energy Electron Delivery Strategies for Ultra-High Dose-Rate Radiotherapy
Abstract
Purpose
To evaluate and compare double-scattered 3D conformal (DS 3D-CRT) and pencil beam scanning (PBS) for ultra-high dose rate (UHDR) very high energy electron (VHEE) radiotherapy.
Methods
A DS 200 MeV VHEE beamline with a copper scatterer, aluminum flattening filter, and tungsten collimation was optimized for a flat field at 10cm depth. The DS beam was recorded as a phase-space and imported into a RayStation research build with VHEE dose calculations benchmarked against Monte Carlo for both DS and PBS configurations. Treatment plans for a 66cm³ lung tumor used 37 beam angles with 5mm pencil beams at 5mm spacing for PBS, while DS beams with static MLCs enabled UHDR 3D-CRT. Plans were evaluated for PTV coverage, OAR sparing, and conformity. Instantaneous dose rates, local dose rates, and treatment times for each plan were calculated using experimentally acquired beam parameters from the CERN linear electron accelerator for research (CLEAR).
Results
The optimized DS beamline presented profiles with 1.30% flatness and 9.5mm penumbra. RayStation dose calculations agreed with Monte Carlo within 5.3% for PDDs, 3.1% for PB profiles, and 18% for DS profiles. For the lung cancer patient, the VHEE PBS plan reduced the mean dose to the heart and right lung by 13% and 28%, respectively, compared to the clinical VMAT plan. In contrast, the DS 3D-CRT plan demonstrated higher OAR doses by up to 168% in the spinal cord. The conformity index (CI₁₀₀) was 0.97 for VMAT, 1.02 for PBS and 2.90 for DS 3D-CRT. Instantaneous dose rates were 1.45×10⁸ Gy/s for PBS and 8.66×10⁵ Gy/s for DS 3D-CRT, with treatment times of 18.44s and 4.86s, respectively.
Conclusion
This work demonstrates that PBS offers reduced normal tissue dose and higher instantaneous dose rates compared to DS 3D-CRT, establishing PBS as the preferred delivery method for clinical VHEE UHDR radiotherapy.