# On the spectral changes of magna‐field photon beams used in total body irradiation and impact on dosimetry

## TL;DR

This study uses simulations to analyze how photon beam energy changes in large radiation fields and how it affects dosimeter accuracy.

## Contribution

The paper introduces a Monte Carlo-based method to calculate energy spectra and correction factors for dosimeters in large radiation fields.

## Key findings

- Photon beam energy decreases with depth but increases in deep regions due to scattered photons.
- Dosimeter energy dependence was up to 4.6% for OSLDs and 2.0% for TLDs in large fields.
- Beam softening in large fields significantly impacts dosimetry for TLDs and OSLDs.

## Abstract

Accurate dosimetry for magna fields can be challenging when dosimeters exhibit energy dependence. The aim of this work was to use Monte Carlo (MC) simulations to calculate energy spectra of magna photon fields and use the spectral information to derive energy dependence correction factors for commonly used dosimeters.

Photon beams of 6 and 10 MV energies were simulated using the TOol for PArticle Simulation (TOPAS) MC code. Reference and magna field conditions were simulated with differing field sizes and source‐to‐surface distances (SSDs). Average photon energy and tissue phantom ratios (TPRs) were calculated as a function of depth in addition to photon energy spectra at various depths. Previously reported known energy responses of a Farmer ion chamber, LiF‐thermoluminescent dosimeters (TLDs), and alumina‐optically stimulated luminescent dosimeters (OSLDs) were extracted to calculate the energy dependence correction factor kNR in magna fields.

The average photon energy decreased with depth, followed by an increase in the deepest regions due to the build‐up of scattered photons. At the phantom midplane, the average photon energy changed from 1.3 to 0.6 MeV for the 6 MV beam and from 2.3 to 1.1 MeV for the 10 MV beam with the transition from reference to magna field conditions. The energy dependence was found to be negligible for the ion chamber, up to 2.0% for the TLDs, and up to 4.6% for the OSLDs.

Large spectral changes were reported between the reference conditions and the magna field conditions. The influence of beam softening in magna field was determined to be considerable for dosimetry using TLDs and OSLDs.