Effect of tube voltage and current on CT number–relative electron density calibration and radiotherapy dose calculation accuracy

Computed tomography (CT) is essential for radiotherapy simulation and dose calculation, which rely on the relationship between CT number and relative electron density (RED). Tube voltage (kVp) and current (mAs) affect CT numbers and may impact Monitor Unit (MU) accuracy if calibration curves do not match scan parameters. To evaluate the effects of varying kVp and mAs on the CT number–RED relationship, image quality, and MU calculation accuracy, and to establish kVp-specific calibration curves for clinical use. A 64-slice CT simulator scanned an electron density phantom at kVp of 80, 100, 120, and 140 kVp with fixed 100 mAs. The phantom was then rescanned at each kVp setting with mAs values of 100, 200, 300, and 400. CT numbers from 13 inserts (RED range: 0.20–6.92) were averaged over three repeated scans to generate CT number–RED calibration curves, which were imported into the treatment planning system (TPS). MU deviations under matched and mismatched calibration conditions were evaluated using thorax phantom scans and 3D conformal radiotherapy (3D-CRT) plans. Image quality was assessed using the Catphan 604 phantom according to AAPM TG-66 guidelines. Varying kVp caused noticeable CT number changes, especially in high-density materials (up to ±806 HU). In contrast, mAs had smaller effects (≤60 HU at 80 kVp; <30 HU at ≥100 kVp). MU deviations from mismatched curves stayed within 0.72 %, below the ±2 % tolerance from IAEA TECDOC 1583. Image quality assessment showed consistent uniformity (±4 HU), noise (3.78–4.62 HU), spatial resolution of 6 lp/cm, and CNR >1.2 across all kVp settings, aligning with AAPM TG-66 criteria. kVp-specific calibration curves improve MU calculation accuracy. Adjusting kVp based on patient size helps optimize image quality and calibration stability. Both 100 and 140 kVp are viable alternatives to 120 kVp for CT simulation.