# Comparison of protons and very high-energy electrons transmission pencil-beam-scanning for FLASH radiotherapy

## TL;DR

This study compares proton and electron beam therapies for FLASH radiotherapy, finding that protons offer higher dose rates but electrons can be viable with higher frequencies.

## Contribution

A new model-independent FLASH index was introduced to quantify treatment plan FLASH potential.

## Key findings

- Proton plans achieved higher FLASH indices due to larger ultra-high-dose rate coverage.
- Electron beams required ≥500 Hz, ideally 1000 Hz, to match proton dose rates.
- VHEE treatment plans showed comparable quality to proton plans.

## Abstract

•Transmission pencil beam scanning with protons and electrons achieved similar quality.•A new model-independent FLASH index quantified the treatment plan FLASH potential.•Proton plans reached higher FLASH indices due to larger ultra-high-dose rate coverage.•Electron beams needed ≥500 Hz, ideally 1000 Hz, to reach proton dose rates.

Transmission pencil beam scanning with protons and electrons achieved similar quality.

A new model-independent FLASH index quantified the treatment plan FLASH potential.

Proton plans reached higher FLASH indices due to larger ultra-high-dose rate coverage.

Electron beams needed ≥500 Hz, ideally 1000 Hz, to reach proton dose rates.

Very High-Energy Electron (VHEE) radiotherapy stands as a promising alternative to proton therapy in view of the FLASH effect, which allows for differential sparing of healthy tissues under ultra-high dose rate irradiation. This study compared the quality of transmission Pencil-Beam-Scanning proton and VHEE treatment plans, with emphasis on dose rate quantification relevant to the assessment of the FLASH effect.

Proton (250 MeV) and VHEE (200 MeV and 150 MeV) treatment plans were designed for four patient cases. Plans were optimized using an in-house developed spot weight and position optimization algorithm. Objectives were set through scorecards based on RTOG protocols. Dose rate estimations were based on beam parameters of the ProBeam system for protons and of a realistic VHEE system. For each structure, a new, model-free FLASH index was calculated.

VHEE treatment plans achieved a quality comparable to that of protons. While the conformity and homogeneity were similar, FLASH indices tended to be higher for protons due to higher dose rates covering more healthy tissues. A pulse repetition frequency of 500 Hz was found necessary to attain FLASH-compatible dose rates (≥40 Gy/s).

The importance of treatment parameters (such as pulse repetition frequency) for VHEEs to reach ultra-high dose rates was assessed. With their high plan quality, VHEEs could constitute a viable alternative to proton transmission plans and a promising modality for FLASH therapy. The proposed FLASH index offers a robust tool to compare the FLASH potential across treatment modalities.