CT number calibration audit in photon radiation therapy

Area of Science:

• Medical Physics
• Radiotherapy
• Imaging Science

Background:

• Inaccurate computed tomography (CT) number calibration curves can compromise radiation dose calculation accuracy in treatment planning systems (TPSs).
• The consistency of CT number calibration curves with human tissues is crucial but has not been thoroughly assessed across various phantoms and TPSs.

Purpose of the Study:

• To investigate the accuracy of CT number calibration curves used for mass density (ρ) and relative electron density (ρ_e) determination in radiotherapy.

Main Methods:

• An audit phantom was scanned at 24 Japanese photon therapy institutes using their clinical CT protocols.
• Institute-specific CT calibration curves were compared against theoretical curves derived using a stoichiometric method.
• Differences in ρ and ρ_e were calculated for lung, soft tissues, and bone, categorized by phantom vendor (Gammex, CIRS) and TPS, including a separate analysis for Tomotherapy TPSs.

Main Results:

• Variations in CT number calibration curves were observed, with mean differences in mass density (Δρ_m) and relative electron density (Δρ_e,m) varying by phantom type and tissue simulated.
• Tomotherapy TPSs showed larger deviations for soft tissues compared to other TPSs.
• Specific tissue-equivalent materials (TEMs), like the Gammex brain phantom, exhibited slightly inferior tissue equivalency.

Conclusions:

• Latent deviations exist between human tissue properties and the tissue-equivalent materials used in CT calibration phantoms across different institutes.
• These findings highlight the need for standardized CT number calibration procedures to ensure accurate dose calculations in radiotherapy.