138 Beyond Rigid Registration: The Role of Deformable Image Registration for Rescans and Replans in H&N Cancer Radiotherapy.

This study investigates whether spinal deviations exceeding 5mm from the planned position during Head & Neck cancer radiotherapy, detected using a rigid registration IGRT approach, warrant rescans or replans. Varian's Velocity™ software package was employed to perform Deformable Image Registration (DIR) on ten radical H&N cases which required re-scanning and re-planning when evaluated using a rigid registration approach. The deformation field was used to generate Synthetic CT's (sCT) using Velocity to evaluate the dosimetric impact of the spinal deviations in Eclipse™. Our findings revealed that 6 out of the 10 plans (60%) exhibiting spinal deviations >5mm and evaluated using a rigid registration approach did not necessitate a replan or rescan, maintaining a dosimetric impact of <2% and adhering to institutional Dose Volume Constraint (DVC) tolerances. Conversely, only 4 of the 10 cases required a rescan and replan when deformation was utilised for evaluation. Rigid registration cannot always account for the anatomical deviations in spinal positions noted during Head & Neck Radiothearpy. The substantial workflow implications of unnecessary rescans and replans accentuate the need for effective adaptive workflows like DIR in making informed decisions in Head & Neck radiotherapy. [ABSTRACT FROM AUTHOR]