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From active shape model to active optical flow model: a shape-based approach to predicting voxel-level dose distributions in spine SBRT

Prediction of achievable dose distribution in spine stereotactic body radiation therapy (SBRT) can help in designing high-quality treatment plans to maximally protect spinal cords and to effectively control tumours. Dose distributions at spinal cords are primarily affected by the shapes of adjacent planning target volume (PTV) contours. In this work, we estimate such contour effects and predict dose distributions by exploring active optical flow model (AOFM) and active shape model (ASM). We first collect a sequence of dose sub-images and PTV contours near spinal cords from fifteen SBRT plans in the training dataset. The data collection is then classified into five groups according to the PTV locations in relation to spinal cords. In each group, we randomly choose a dose sub-image as the reference and register all other sub-images to the reference using an optical flow method. AOFM is then constructed by importing optical flow vectors and dose values into the principal component analysis (PCA). Similarly, we build ASM by using PCA on PTV contour points. The correlation between ASM and AOFM is estimated via a stepwise multiple regression model. When predicting dose distribution of a new case, the group is first determined based on the PTV contour. The prediction model of the selected group is used to estimate dose distributions by mapping the PTV contours from the ASM space to the AOFM space. This method was validated on fifteen SBRT plans in the testing dataset. Analysis of dose-volume histograms revealed that the important D2%, D5%, D10% and D0.1cc dosimetric parameters of spinal cords between the prediction and the clinical plans were 11.7  ±  1.7 Gy versus 11.8  ±  1.7 Gy (p = 0.95), 10.9  ±  1.7 Gy versus 11.1  ±  1.9 Gy (p = 0.8295), 10.2  ±  1.6 Gy versus 10.1  ±  1.7 (p = 0.9036) and 11.2  ±  2.0 Gy versus 11.1  ±  2.2 Gy (p = 0.5208), respectively. Here, the ‘cord’ is the spinal cord proper (not the thecal sac) extended 5 mm inferior and superior to the involved vertebral bodies, and the ‘PTV’ is the involved segment of the vertebral body expanded uniformly by 2 mm but excluding the spinal cord volume expanded by 2 mm (Ref. RTOG 0631). These results suggested that the AOFM-based approach is a promising tool for predicting accurate spinal cord dose in clinical practice. In this work, we demonstrated the feasibility of using AOFM and ASM models derived from previously treated patients to estimate the achievable dose distributions for new patients.