SU-E-T-520: Deriving Energy Spectrum From Depth Dose Measurements for X-Rays of CT Scanner

Abstract

Purpose: The goal of this study is to derive x-ray energy spectrum from the measured depth dose data for any CT scanner without a priori knowledge of x-ray tube configurations. The obtained energy spectrum, together with other beam characteristics, can be used as an effective way to estimate the patient-specific organ doses from CT scans clinically. Methods: EGSnrc Monte Carlo code was used to generate a series of percent depth dose (PDD) curves in water for mono-energetic x-rays with energy ranging from 11 keV to 150 keV. Specifically, a fan beam was simulated with a fan angle of 55 degrees and 2 cm thickness along superior-inferior direction. Based on BEAMnrc Monte Carlo characterization of x-rays from CT scanners, an empirical function with three variables was proposed to represent the energy spectrum of those beams. An iterative multivariate search algorithm has been developed to derive the energy spectrum based on the correlation coefficient between the measured PDD and the fitted PDD. The developed algorithm has been tested with a BEAMnrc simulated x-ray spectrum serving as the ‘original’ spectrum and the subsequent dose simulation serving as the ‘measured’ PDD. Results: Except for the characteristic energy peaks, the derived energy spectrum reproduced the ‘original’ spectrum very well with a smooth energy dependency, peaking around 39 keV. The maximum error between the measured PDD and the fitted PDD was less than 1%, indicating the validity of our proposed method for spectrum derivation. Conclusion: According to this study, it is possible to derive the energy spectrum from the measured PDD data for the x-rays of a CT scanner, without a priori knowledge of x-ray tube configurations. Future work will seek to demonstrate its clinical efficacy by establishing a patient-specific dosimetry system for any CT scanners, particularly for organ dose estimation on real patients.