SU-E-T-530: Relative Electron Density Phantom Comparison.

Area of Science:

• Medical Physics
• Radiotherapy
• Image Analysis

Background:

• Modern radiotherapy treatment planning systems rely on accurate conversion of Hounsfield Units (HU) to relative electron density (RED).
• This conversion is critical for heterogeneity corrections during dose calculations, directly impacting treatment accuracy.
• Different CT phantom models can yield varying HU to RED curves, necessitating an evaluation of their impact.

Purpose of the Study:

• To assess the influence of different CT phantom models on the generation of HU to RED curves.
• To compare the clinical significance of HU to RED curves derived from distinct phantom models in treatment planning.

Main Methods:

• Two phantoms, GAMMEX 467 and CATPHAN 500, were imaged across multiple CT scanners.
• HU to RED curves were generated and implemented in the Eclipse 8.10 treatment planning system.
• Dose calculations were performed on a heterogeneity phantom and compared to measurements; 3D and IMRT plans were analyzed for clinical differences.

Main Results:

• HU to RED curves showed reasonable consistency across scanners but differed between phantom models, especially for high-density materials.
• Differences in higher HU values were attributed to the effective atomic number influencing photoelectric effect and Compton scatter.
• The GAMMEX phantom's HU to RED curve resulted in better agreement between Eclipse AAA calculations and measured doses.

Conclusions:

• The CT phantom model has a greater influence on HU to RED curves than the CT scanner itself.
• The GAMMEX phantom provided more accurate dose distribution calculations compared to the CATPHAN.
• While minor differences were observed in patient plans, the choice of phantom model is crucial for accurate radiotherapy dose calculations.