Validation of a deformable image registration technique for cone beam CT-based dose verification

Area of Science:

• Medical Physics
• Radiotherapy
• Image Guidance

Background:

• Adaptive radiotherapy requires accurate dose monitoring and adaptation to patient changes.
• Image guidance is crucial for modern radiation therapy techniques like intensity modulated radiotherapy (IMRT).

Purpose of the Study:

• To validate a method for evaluating delivered IMRT dose using multimodal deformable image registration (DIR) for prostate treatments.
• To assess the suitability of DIR for dose calculation based on planning CT and daily CBCT data.

Main Methods:

• A pelvic phantom was scanned using CT and CBCT, then digitally deformed.
• Deformable image registration (DIR) using B-spline algorithm in Plastimatch was performed.
• Geometric and dosimetric evaluations, including dose-volume histograms and gamma analysis, were conducted.

Main Results:

• CBCT calibration reduced mean HU differences by up to 60% and vector field length differences from 10.1 to 2.5 mm.
• Mean dose differences were 0.2% (calibrated CBCT) and 3.9% (uncalibrated CBCT) compared to the primary deformed CT.
• Gamma analysis pass rates were 100% with calibrated CBCT and 86% without, indicating high accuracy.

Conclusions:

• Deformable registration using planning CT and daily CBCT is feasible for evaluating IMRT dose.
• The method provides accurate dose distribution, minimizing uncertainties from CBCT artifacts.
• This is a key step towards image-guided adaptive radiotherapy for prostate cancer.