Is Dose Deformation-Invariance Hypothesis Verified in Prostate IGRT?

Area of Science:

• Radiation Oncology
• Medical Physics
• Image-Guided Therapy

Background:

• Image-guided radiation therapy (IGRT) relies on accurate dose delivery.
• Prostate cancer treatment often uses intensity-modulated radiation therapy (IMRT).
• Cone-beam CT (CBCT) enables image guidance but can introduce dose uncertainties.

Purpose of the Study:

• To evaluate dose uncertainties in prostate IGRT arising from the dose deformation-invariance hypothesis.
• To determine if rigidly propagated planned dose distributions accurately estimate fraction dose distributions.

Main Methods:

• Twenty prostate cancer patients undergoing IMRT received weekly CT scans.
• Dose distributions were obtained by recalculating the original plan and by rigidly propagating the planned dose.
• Deformable image registration estimated cumulative doses in organs at risk.
• Per-voxel and dose-volume histogram (DVH) metrics compared recalculated and propagated doses.

Main Results:

• Fraction dose differences were generally <1 Gy for rectum and bladder, but maximum differences reached 8.0 Gy.
• DVH differences for fraction and cumulative doses were within acceptable ranges for most organs at risk.
• Dose differences correlated with rectal gas volume and bladder contour variations.

Conclusions:

• The dose deformation-invariance hypothesis is largely supported in prostate IGRT.
• Exceptions include significant changes in rectal gas volume or bladder external contour.
• Rigid propagation is a reliable method for estimating dose distributions in most clinical scenarios.