Effects of the difference in tube voltage of the CT scanner on dose calculation

TL;DR

This study investigates how different tube voltages in CT scans affect dose calculation accuracy in radiation therapy, highlighting the importance of using appropriate conversion curves for each voltage to ensure precise treatment planning.

Contribution

It demonstrates the impact of varying CT tube voltages on dose calculation errors and emphasizes the need for voltage-specific CT number to electron density conversion curves.

Findings

Maximum dose difference was 1.3% in PDD and 1.1% in Monte Carlo simulations.

Dose differences in clinical cases were within 1.2% for mean, max, and min doses.

Using the same conversion curve for different voltages can lead to dose calculation inaccuracies.

Abstract

Computed Tomography (CT) measures the attenuation coefficient of an object and converts the value assigned to each voxel into a CT number. In radiation therapy, CT number, which is directly proportional to the linear attenuation coefficient, is required to be converted to electron density for radiation dose calculation for cancer treatment. However, if various tube voltages were applied to take the patient CT image without applying the specific CT number to electron density conversion curve, the accuracy of dose calculation would be unassured. In this study, changes in CT numbers for different materials due to change in tube voltage were demonstrated and the dose calculation errors in percentage depth dose (PDD) and a clinical case were analyzed. The maximum dose difference in PDD from TPS dose calculation and Monte Carlo simulation were 1.3 % and 1.1 % respectively when applying the same CT number to electron density conversion curve to the 80 kVp and 140 kVp images. In the clinical case, the different CT number to electron density conversion curves from 80 kVp and 140 kVp were applied to the same image and the maximum differences in mean, maximum, and minimum doses were 1.1 %, 1.2 %, 1.0 % respectively at the central region of the phantom and 0.6 %, 0.9 %, 0.8 % respectively at the peripheral region of the phantom.