# Evaluation of the dose reduction effect of crystalline lens exposure in cone‐beam computed tomography with bismuth eye shield for image‐guided radiation therapy: An anthropomorphic phantom study

## TL;DR

This study shows that using a bismuth eye shield during head CT scans can significantly reduce radiation exposure to the crystalline lens, especially when the scan center is near the eye.

## Contribution

The study demonstrates the effectiveness of a bismuth eye shield in reducing lens exposure during head CBCT scans in a phantom model.

## Key findings

- The highest lens exposure occurred when the head phantom was moved 5 cm in the −Z direction.
- Using an eye shield reduced lens exposure by 31-32% in the −Z direction.
- Exposure reduction rates were 10-15% in other movement directions.

## Abstract

This study aimed to evaluate the dose‐exposure reduction effect of a crystalline lens with a bismuth eye shield using cone‐beam computed tomography (CBCT) for head image‐guided radiation therapy. The ocular surface dose of the head phantom (THRA‐1) is defined as a crystalline lens exposure dose and is measured using a radiophotoluminescence dosimeter (RPLD, GD‐352 M) with and without an eye shield (CT eye shield) while moving the head phantom from the reference position that is set at the center of the head in either the X or Z direction from −5 to +5 cm. The exposure doses were measured thrice at each movement position. The crystalline lens exposure doses at the reference position were 0.896 ± 0.024 mGy and 0.892 ± 0.016 mGy for the right and left sides, respectively. The exposure doses at the position where the head phantom was moved 5 cm in the −Z direction from the reference position were 2.812 ± 0.053 mGy and 2.576 ± 0.038 mGy for the right and left sides, respectively, with the highest doses at all movement positions. The crystalline lens exposure doses were reduced to 1.909 ± 0.046 mGy and 1.768 ± 0.043 mGy for the right and left sides with an eye shield in this position, causing an exposure dose reduction rate of −32% and −31%, respectively. The crystalline lens exposure dose reduction rate was approximately 10%–15% in the movement directions, except for the −Z direction. Head CBCT with an eye shield effectively reduced the crystalline lens exposure dose when the CBCT isocenter was set close to the eye. Head CBCT using an eye shield is a useful method that reduces the crystalline lens exposure dose.