Density calibrated cone beam CT as a tool for adaptive radiotherapy

Area of Science:

• Medical Physics
• Radiotherapy
• Image-guided radiation therapy

Background:

• Radiotherapy plan adaptation relies on visual inspection of anatomical changes on daily cone-beam CT (CBCT) images.
• Direct dose calculations on CBCTs can improve adaptation by identifying dosimetric changes.
• This study evaluates the accuracy of dose calculations performed directly on CBCT images.

Purpose of the Study:

• To investigate the accuracy of dose calculations on cone-beam CT (CBCT) images.
• To determine if CBCT dose calculations can reliably guide radiotherapy plan adaptation.
• To assess the clinical viability of dose calculations on CBCTs.

Main Methods:

• Calibration curves were generated for CBCT imagers across nine accelerators.
• CBCT scans of phantoms with varying densities were acquired in Head-Neck and Pelvis modes.
• Dose distributions on CBCTs were calculated and compared to planning CT (pCT) data, with uncertainty quantified by dosimetric differences. Patient data (Head-Neck, Lung, Pelvis) were analyzed using standard and low-HU corrected calibration curves.

Main Results:

• Phantom data showed minimal dose differences between pCT and CBCT (mean deviations < ±0.6%) for all dose-volume histogram (DVH) parameters.
• Low-HU corrected calibration curves yielded the best performance for patient data.
• Mean deviations for target coverage and mean dose were within 0.1%±0.6% and 0.2%±0.7%, respectively, across all patient groups.

Conclusions:

• Dose calculation on CBCT images achieves clinical acceptability with approximately 1% accuracy for target coverage and mean dose.
• CBCT mode-specific calibration curves are transferable across identical imaging devices and patient populations.
• This validates the use of direct CBCT dose calculations for radiotherapy adaptation.