Consistency Check of Deformable Image Registration-Based Dose Summation for Off-Line Adaptive Re-Planning

Abstract

<h3>Purpose/Objective(s)</h3> Uncertainty of deformable image registration (DIR) based dose accumulation is a concern due to the uncertainty with the DIR, in particular for a large deformation. In this work, we introduce a practical workflow of using a DVH overlay technique to check consistency in DIR-based dose accumulation during off-line adaptive replanning for large deformations observed during radiation therapy for abdominal tumors. <h3>Materials/Methods</h3> Required inputs to the DVH overlay workflow are deformably registered primary (original plan) and secondary (re-plan) images, primary regions-of-interest (ROIs), and a secondary dose distribution. Primary ROIs are forward warped to the secondary image, secondary dose is inversely warped to the primary image, and the ROI DVHs for each image are overlaid. Differences between the DVHs at a given dose point (e.g., V_45Gy) quantify the dose uncertainty due to inverse consistency error (ICE) inherent in the DIR. We applied the workflow to a pancreas case treated using modulated arc therapy (mARC) on a Linac equipped with integrated kVCT-on-rails IGRT. For the original plan, 65.25-Gy and 50.75-Gy PTVs were concurrently treated over 29 fractions. After fraction 21, the patient was observed to have lost sufficient weight to justify an adaptive mARC plan based on an updated kVCT image. A composite dose distribution was compiled on the original plan CT by summing 21 fractions of the original planned dose and 8 fractions of a dose distribution warped from the re-plan CT to the original plan CT. Warping was performed using a hybrid DIR algorithm combining contour-based and image-based registration. For each organ-at-risk (OAR) DVH dose point, the workflow-derived ICE (scaled to 8 fractions) was additively applied as an uncertainty to the summation DVH. <h3>Results</h3> For the re-planned pancreas case, ICE based on the DVH overlay workflow was greatest for the small bowel V_45Gy (approximately 5%) and for the stomach V_45Gy and V_56Gy (approximately 2% for both). ICE was also approximately 4% at V_15Gy for the duodenum and colon. For all other ROIs, ICE was 1% or less from the DVH overlay, indicating a favorable DIR for those regions. When applying the DVH overlay-based ICE the resulting accumulated dose remained compliant with the original plan objectives for all OARs except for the stomach V_56Gy, which equaled 7.9 cm³ with ICE included (plan objective < 5 cm³). <h3>Conclusion</h3> A DVH overlay technique was used to estimate ICE attributable to the uncertainty in DIR-based dose accumulation. The method was applied to a pancreas case for which dose accumulation was performed due to off-line adaptive re-planning for a large anatomic change during the treatment course. In instances where the ICE is over a pre-defined threshold or the summation dose exceeds plan objectives if ICE is included, it is prudent to review the DIR dose transfer and accumulation before using it to make clinical decisions.