SU-E-J-174: ITV Variations as a Function of CT Geometry and Scan Time : A Simulation Study Using Patient Data

Area of Science:

• Medical Physics
• Radiotherapy
• Imaging Technology

Background:

• The internal target volume (ITV) is crucial for accounting for tumor motion and shape uncertainties in radiotherapy planning.
• Four-dimensional computed tomography (4DCT) is the standard imaging modality for ITV definition.
• Optimizing CT acquisition parameters is essential for accurate ITV delineation.

Purpose of the Study:

• To investigate the influence of CT scanner geometry and scan duration on internal target volume (ITV) definition.
• To utilize gold fiducial markers in lung tumors to represent tumor motion for simulation.
• To assess the impact of varying scan durations and CT geometries on ITV size.

Main Methods:

• Geometric simulations were performed in MATLAB, excluding image reconstruction variations.
• Simulated CT scanner geometries included a 4-slice scanner (10 mm axial field of view) and an 'ideal' scanner (100 mm axial field of view).
• Scan durations were varied from 6 to 30 seconds per bed position, with a 25 mm diameter spherical tumor model.

Main Results:

• Mean ITV consistently increased with increasing scan duration across all patients.
• The maximum observed increase in mean ITV was 25% with extended scan duration.
• Scan duration was identified as the primary factor influencing ITV size, surpassing scanner geometry effects.

Conclusions:

• Modified CT protocols, such as increasing scan duration, can potentially reduce ITV uncertainties.
• Decreasing scanner mA while increasing scan duration offers a strategy to manage radiation dose.
• Scan duration is a more significant determinant of ITV than CT scanner geometry, guiding protocol optimization.