# Impact of discrepancies between CT numbers of brain-tissue-equivalent density plug and actual brain tissue on dose calculation accuracy

## TL;DR

This study shows that differences in CT numbers and composition between brain-tissue-equivalent plugs and real brain tissue have minimal impact on radiation therapy dose calculations.

## Contribution

The study quantitatively evaluates the impact of CT number and elemental composition discrepancies on dose calculation accuracy in radiation therapy.

## Key findings

- Significant differences in elemental composition were found, especially in carbon and oxygen content.
- The effective atomic numbers and CT numbers of the plug and brain tissue showed notable discrepancies.
- Dose calculation deviations were minimal, typically within ±0.2% using different algorithms.

## Abstract

This study quantitatively evaluated the impact of differences in computed tomography (CT) numbers and elemental compositions between commercially available brain-tissue-equivalent density plugs (BDPs) and actual brain tissue on dose calculations in a radiation therapy treatment planning system (RTPS). The mass density and elemental composition of BDP were analyzed using elemental analysis and X-ray fluorescence spectroscopy. The CT numbers of the BDP and actual brain tissue were measured and compared, with effective atomic numbers (EANs) calculated based on compositional analysis and the International Commission on Radiological Protection Publication 110 data for brain tissues. The theoretical CT numbers were derived using the stoichiometric CT number calibration (SCC) method. The dose calculations were performed using the modified CT number-to-relative electron density (RED) and mass density (MD) conversion tables in Eclipse v16.1, employing AAA and Acuros XB algorithms, employing the physical material table in AcurosXB_13.5. The dose metrics D2%, D50%, and D98% were evaluated. Significant differences in elemental composition were found, particularly in carbon (73.26% in BDP vs. 14.3% in brain tissue) and oxygen (12.52% in BDP vs. 71.3% in brain tissue). The EANs were 6.6 for BDP and 7.4 for brain tissue. The mean CT numbers were 23.30 HU for the BDP and 37.30 HU for brain tissue, a 14 HU discrepancy. Nevertheless, dose calculation deviations were minimal, typically within ± 0.2%, with a maximum discrepancy of 0.6% for D98%. Although CT numbers and elemental compositions exhibited notable differences, their impact on dose calculations in the evaluated RTPS algorithms was negligible.