Mini-GRID radiotherapy on the CLEAR very-high-energy electron beamline: collimator optimization, film dosimetry, and Monte Carlo simulations.

Spatially-fractionated radiotherapy (SFRT) delivered with a
very-high-energy electron (VHEE) beam and a mini-GRID collimator was investigated
to achieve synergistic normal tissue-sparing through spatial fractionation and the
FLASH effect.
Approach: A tungsten mini-GRID collimator for delivering VHEE SFRT was
optimized using Monte Carlo (MC) simulations. Peak-to-valley dose ratios (PVDRs),
depths of convergence (DoCs, PVDR≤1.1), and peak and valley doses in a water
phantom from a simulated 150 MeV VHEE source were evaluated. Collimator
thickness, hole width, and septal width were varied to determine an optimal value
for each parameter that maximized PVDR and DoC. The optimized collimator (20 mm thick rectangular prism with a 15 mm×15 mm face with a 7×7 array of 0.5
mm holes separated by 1.1 mm septa) was 3D-printed and used for VHEE irradiations
with the CERN Linear Electron Accelerator for Research (CLEAR) beam. Open beam
and mini-GRID irradiations were performed at 140, 175, and 200 MeV and dose was
recorded with radiochromic films in a water tank. PVDR, central-axis (CAX) and
valley dose rates and DoCs were evaluated.
Main results: Films demonstrated peak and valley dose rates on the order of 100s
of MGy/s, which could promote FLASH-sparing effects. Across the three energies,
PVDRs of 2-4 at 13 mm depth and DoCs between 39-47 mm were achieved. Open
beam and mini-GRID MC simulations were run to replicate the film results at 200
MeV. For the mini-GRID irradiations, the film CAX dose was on average 15% higher,
the film valley dose was 28% higher, and the film PVDR was 15% lower than calculated
by MC.
Significance: Ultimately, the PVDRs and DoCs were determined to be too low for
a significant potential for SFRT tissue-sparing effects to be present, particularly at depth. 
Further beam delivery optimization and investigations of new means of spatial
fractionation are warranted.&#xD.