Technical Note: The impact of deformable image registration methods on dose warping

Area of Science:

• Medical Physics
• Radiotherapy
• Image-guided therapy

Background:

• Deformable image registration (DIR) is crucial for accurate dose warping in radiotherapy.
• Discrepancies exist between image-based DIR (IM-DIR) and biomechanical model-based DIR (BM-DIR), particularly for organs with homogeneous intensity.

Purpose of the Study:

• To investigate the clinical-relevant discrepancies in dose warping between IM-DIR and BM-DIR for intensity-homogeneous organs.
• To evaluate the impact of DIR method on dose warping uncertainty.

Main Methods:

• Ten patients (Head & Neck, Prostate) underwent IM-DIR using a research tool.
• BM-DIR was performed by interpolating IM-DIR deformable vector fields to mesh boundary conditions and simulating displacement with a FEM solver.
• Geometrical (Target Registration Discrepancy) and dose warping discrepancies were quantified.

Main Results:

• Parotid glands showed significant shrinkage (75.7% ± 9% volume), with 6.5% ± 4.7% volume having TRD > 1.5 mm.
• Normalized Mean Dose Difference (NMDD) between IM-DIR and BM-DIR was -0.8% ± 1.5% for parotids, with a 99.0% ± 1.4% 2mm/2% gamma passing rate.
• Bladder showed a significant NMDD of -9.9% ± 9.7% (BM-DIR dose higher), while rectum showed minor deviation (0.5% ± 1.1%).

Conclusions:

• The impact of DIR method on dose warping is patient and organ-specific.
• Intensity-homogeneous organs with significant deformation experience greater dose warping uncertainty.
• BM-DIR can be beneficial for evaluating DIR and dose-warping uncertainty in such cases.