Room: Exhibit Hall | Forum 5
Purpose: Advanced adaptive radiotherapy strategies rely on deformable image registration (DIR) for contour propagation or dose mapping. The goal of this study is to compare two approaches to register the planning CT to a follow-up image: (i) using direct DIR between the two images as typically done or (ii) using DIR between each pair of sequential longitudinal images acquired during the treatment and calculating the composite deformation vector field (DVF).
Methods: Five patients treated for oropharyngeal cancer with radiotherapy were retrospectively analyzed. For each patient and week n, direct DIR Tn was performed between the weekly and planning CT using the intensity-based DIR method available in RayStation v7. Incremental DIR K(n) was also performed between each weekly CT and corresponding previous week CT. For each weekly CT, the DVF T(n) was compared to the DVF Q(n) obtained by combination of the previous week-to-week DVF (Q(n) = K(n) âˆ˜ Q(n-1)). Vector norm differences between the two DVFs were calculated. The planned dose distribution was propagated forward to each weekly CT by application of the corresponding inverted DVFs and absolute dose differences were calculated.
Results: The direct or composite DVFs yielded to visually similar deformed CTs and propagated contours with Dice scores ranges of 0.91-0.98 for the parotid glands and mandible. The DVF differences increased as the treatment advanced over time. The 95áµ—Ê° percentile (p95) ranges of the DVF differences in the 75% idosose region for the 5 patients were 0.8-1.8 mm at week 2 and 1.7-4.1 mm at week 5. The corresponding dose difference p95 ranges were 38-144 and 125-257 cGy.
Conclusion: Performing DIR only between sequential images during radiotherapy is feasible but this study showed that it could lead to substantially different dose mapping results compared to the traditional approach, justifying the need for further accuracy evaluation studies.
Funding Support, Disclosures, and Conflict of Interest: Funding from RaySearch Laboratories
Not Applicable / None Entered.