# Assessment of the backscattered skin dose from the beam stopper on Halcyon and Ethos radiotherapy systems

## TL;DR

This study used simulations to determine how much radiation from a beam stopper affects skin dose during radiotherapy, finding it contributes less than 2%.

## Contribution

The study is the first to assess backscatter from the beam stopper in Halcyon and Ethos systems using Monte Carlo simulations.

## Key findings

- The maximum backscatter contribution to skin dose was 2% under worst-case conditions.
- Backscatter contribution remains minimal even with large field sizes and close proximity to the bore.
- Spectral analysis of backscattered photons was conducted for one phantom setup.

## Abstract

The backscatter contribution from the beam stopper to the exit‐side skin dose on the Halcyon and Ethos radiotherapy systems has not been studied, and this contribution cannot be separated by experimental methods.

This study aimed to assess how the beam backscatter from the beam stopper contributes to the exit‐side skin dose.

The beam stopper and a simplified model of the bore and MV imager were implemented in a validated Monte Carlo model of Halcyon and Ethos radiotherapy systems. To analyze the worst‐case scenarios of the backscatter, the largest field size was used, and the dose deposition in a water phantom was calculated with four different phantom setups, varying the distance from the bore and the beam stopper. The backscatter contribution to the dose was isolated, and the spectral properties of the backscattered photons were analyzed for one phantom setup.

The depth dose curves and the separated backscatter contribution were presented, with a maximum backscatter contribution of 2% of the local dose at the exit‐side skin dose observed when the phantom was closest to the bore and the beam stopper.

The backscatter contribution to the skin dose was found to be minimal, even with the largest field size and the phantom positioned extremely close to the bore.