To accommodate the inter- and intra-fractional motion of internal organs in prostate cancer treatment, a large
margin (5mm-25mm) has often to be considered during radiation therapy planning. Normally, the inter-fractional
motion is more substantial than the intra-fractional counterpart. Therefore, the study of inter-fractional motion
pattern is of special interest for adaptive radiation therapy. Existing methods on organ motion analysis mainly
focus on the deviation of an organ's shape from its mean shape. The deviation information is helpful in choosing a
statistically proper margin, but is of limited use for plan adaptation. In this paper, we propose a new deformation
analysis method that can be directly used for plan adaptation. First, deformation estimation is accomplished by
a fast deformable registration method, which utilizes a contour based multi-grid strategy to register treatment
cone-beam CT (CBCT) images with planning CT images. Second, dominant deformation modes are extracted
by a novel deformation analysis approach. To be specific, a cooperative principal component analysis (PCA)
method is developed to analyze the deformation field in a coarse-to-fine strategy. The deformation modes are
initialized by applying PCA on the organs as a whole and refined by analyzing the individual organs subsequently.
The experimental results show that the organ motion can be well characterized by a few dominant deformation
modes. Based on the dominant modes, a corresponding set of dominant modal plans could be generated for
further optimization. Ultimately, an adaptive plan for each treatment can be obtained on-line while the margin
can be effectively reduced to minimize the unnecessary radiation dosage.
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