SU‐E‐J‐27: Appropriateness Criteria for Deformable Image Registration and Dose Propagation

Purpose: Several commercial software packages have been recently released that allow the user to apply deformable registration algorithms (DRA) for image fusion and dose propagation. Although the idea of anatomically tracking the daily patient dose in the context of adaptive radiotherapy or merely adding the dose from prior treatment to the current one is very intuitive, the accuracy and applicability of such algorithms needs to be investigated as it remains somewhat subjective. In our study, we used true anatomical data where we introduced changes in the density, volume and location of segmented structures to test the DRA for its sensitivity and accuracy. Methods: The CT scan of a prostate patient was selected for this study. The CT images were first segmented to define structure such as the PTV, bladder, rectum, intestines and pelvic bone anatomy. To perform our study, we introduced anatomical changes in the reference patient image set in three different ways: (i) we kept the segmented volumes constant and changed the density of rectum and bladder in increments of 5% (ii) we changed the volume of rectum and bladder in increments of 5% and (iii) we kept the segmented volumes constant but changed their location by moving their COM in increments of 3mm. Using the Velocity software, we evaluated the accuracy of the DRA for each incremental change in all three scenarios. Results: The DRA performs reasonably well when the differential density difference against the background is more than 5%. For the volume change study, the DRA results became unreliable for relative volume changes greater than 10%. Finally for the location study, the DRA performance was acceptable for shifts below 9mm. Conclusion: Site specific and patient specific QA for DRA is an important step to evaluate such algorithms prior to their use for dose propagation.