Comparison of CT number calibration techniques for CBCT-based dose calculation

Area of Science:

• Medical Physics
• Radiotherapy Physics
• Imaging Science

Background:

• Cone beam CT (CBCT) is increasingly used for image-guided radiotherapy (IGRT).
• Accurate dose calculation in CBCT requires precise CT number calibration.
• Various CT number calibration techniques exist, necessitating validation for dose accuracy.

Purpose of the Study:

• To compare and validate different computed tomography (CT) number calibration techniques.
• To assess their impact on cone beam CT (CBCT) dose calculation accuracy.
• To evaluate these techniques across pelvic, lung, and head and neck (H&N) treatment sites.

Main Methods:

• Assessed CBCT dose calculation accuracy using physics-based scatter correction (CBCTr) and density override methods (W, WB, WL).
• Evaluated CBCT density assignment using a commercial treatment planning system (RSauto) with six density levels.
• Compared CBCT dose distributions against planning CT ground truth using dose-difference maps and dose-volume statistics.

Main Results:

• All CT number calibration methods yielded <1.5% average dose-volume deviation for pelvic sites.
• RSauto showed higher errors in pelvic cases with significant adipose tissue.
• Head and neck (H&N) cases had <1.0% average dose-volume difference; CBCTr and RSauto performed best (0.5-0.6%).
• WL and RSauto methods produced lung dose distributions most similar to ground truth.

Conclusions:

• The RSauto density override approach is a viable option for CT number adjustments across multiple anatomical sites.
• RSauto methods were validated for dose calculations consistent with diagnostic CT.
• This consistency was demonstrated for lung, H&N, and pelvic RT cases (without excess adipose tissue).