Dual-Energy Computed Tomography Proton-Dose Calculation with Scripting and Modified Hounsfield Units

Area of Science:

• Medical Physics
• Radiotherapy
• Image Processing

Background:

• Accurate proton stopping-power ratio (SPR) calculation is crucial for precise dose delivery in proton therapy.
• Traditional single-energy computed tomography (SECT) has limitations in accurately determining SPRs, impacting treatment planning.
• Dual-energy computed tomography (DECT) offers potential for improved material characterization and SPR calculation.

Purpose of the Study:

• To implement and validate dual-energy computed tomography (DECT) for calculating proton stopping-power ratios (SPRs) within a commercial treatment planning system.
• To establish a workflow for the safe deployment of DECT in clinical practice, using SECT as a verification measure.
• To assess the impact of DECT-based SPR calculations on treatment planning, including range uncertainty margins and organ-at-risk doses.

Main Methods:

• DECT images were acquired at 80 kVp and 140 kVp, processed to derive electron density and effective atomic number.
• Validation involved comparing DECT-derived SPRs against reference values from tissue-equivalent phantoms using a commercial treatment planning system's API.
• Clinical workflows included optimizing plans with SECT then recalculating with DECT SPR, and optimizing directly with DECT SPR for reduced margins.

Main Results:

• DECT demonstrated lower root mean square errors in SPR compared to SECT for various tissue surrogates (e.g., 1.08% vs. 2.29% for Gammex phantom).
• DECT-based optimization for head and neck plans with 2% range uncertainty achieved reduced organ-at-risk doses compared to SECT plans with 3.5% margins.
• DECT successfully identified and corrected erroneous SPR values in a liver case involving lipiodol within an SECT plan.

Conclusions:

• Dual-energy computed tomography (DECT) can be safely and effectively implemented for proton dose calculation in commercial treatment planning systems.
• The use of DECT allows for a reduction in range uncertainty margins to 2% in certain treatment sites, such as head and neck.
• DECT-based SPR calculations enhance the accuracy of proton therapy planning, potentially leading to improved clinical outcomes.