P32. Impact of CT acquisition parameters on the electronic density curve

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Introduction The electronic density curve is the basis of dosimetric calculation. This affects the dose calculated in planning. We purpose to study the variations of the CT Number generated by different acquisition parameters. Materials and Methods We used Lightspeed RT16 (GE) at 120 kV. The first point was to check the performance of the scanner through the water phantom (diameter 32 cm) provided by the manufacturer. Repeatability, reproductibility, uniformity, noise and water CT number were explored. In a second step we evaluated the CT number variations due to different acquisition parameters using phantom CIRS 062 M with various electron densities. The 2 geometry (16 cm and 32 cm diameter) were tested. Following parameters were studied for FOV (Field of View) Large and Small: - Differences between two identical density insert; - Insert position; - Acquisition type: axial or helix; - Tube rotation time: 0.8–4 s; - Thickness: 16 × 0.625 mm; 16 × 1.25 mm; 8 × 2.5 mm; 4 × 3.75 mm; - Intensity: 50–440 mA. Results CT Number, uniformity and noise does not exceed french tolerances. Repeatability: (Standard deviation) σ Large 0.3 HU and σ Small 0.2 HU ; Reproductibility: σ Large 1.6 HU ; Table influence: (Maximal CT number difference) Δ MAX _ Large = 3.5 HU and Δ MAX _ Small = 1.8 HU ; Differences between two identical densities insert: Δ MAX _ Large = 4.8 (for − 750) HU and Δ MAX _ Small = 8.7 (for 1050) HU; Insert Position: Δ MAX _ Large = 76.6 HU and σ Large = 21.3 HU ; Δ MAX _ Small = 27.1 HU and σ Small = 8.9 HU (RED (Relative Electronic Density) insert = 1.7); Acquisition type: more noise for Helix (16.5 HU vs 11.6 HU); Intensity: Noise decrease as intensity increase; Δ MAX _ Large = (6.8 ± 1.5) HU and Δ MAX _ Small = (11.5 ± 3.9) HU; Tube rotation time: Δ MAX _ Large = 2.1 HU and Δ MAX _ Small = 1.0 HU ; Thickness: Δ MAX _ Large = 27.6 HU and Δ MAX _ Small = 6.5 HU ; 32 cm diameter phantom FOV Large vs FOV Small: Δ MAX _ Small = 12.7 HU (RED insert = 1.7); 16 cm diameter phantom FOV Small vs FOV Large: Δ MAX _ Small = 62.5 HU (RED insert = 1.7); 32 cm diameter phantom FOV Large vs 16 cm diameter phantom FOV Small: Δ MAX _ Small = 254.7 HU (RED insert = 1.7). Conclusion The reference acquisition for calibration of the electron density curve seems hard stage in particular due to inserts position and FOV. We noticed for RED higher than 1.1, important variations appeared for CT number. A study about dosimetric impact was started.