Room: AAPM ePoster Library
Purpose: the workflow built into Velocity to generate adaptive CT images (aCT) from cone beam computed tomography (CBCT) for proton therapy. The goal of the workflow is to determine if the changes observed during the workflow could trigger action earlier due to any anatomical changes and lead to a verification CT (CTver) for an adaptive plan.
Methods: is a retrospective study on patients that underwent pencil beam scanning proton therapy where anatomical changes during treatment were observed during a CTver leading to an adaptive plan. The aCT was generated using Velocity 4.1 which considers the anatomical deformation for HU re-assignment and the online matching to account for setup errors based on the CBCT and the planning CT (CTP). Velocity is used to accumulate dose in each aCT generated. Target and the critical structures are used to evaluate the deformation using indices such as deformation volume change (?VDef), Jacobian determinants, and subtractive dose volume histogram. After evaluating the deformation, the dose from the generated aCTs is averaged and accumulated to the CTP and the original dose is subtracted away for comparison.
Results: dosimetric results from the accumulated aCTs agree with the CTver and corresponding verification plan. Three criteria used for correlating adaptive plan action: ?VDef greater than 5%, large spread in Jacobian determinant, and skewed subtractive dose volume histogram. In patients with anatomical changes we have observed accumulated dose differences from the aCTs to the target volume of more than 10% compared to the prescription dose prior to the patient receiving a CTver.
Conclusion: This workflow detects anatomical changes in target and organs-at-risk for proton therapy ahead of a scheduled CTver. It provides an indicator for proton adaptive planning and can be used to provide a safer and more effective adaptive treatment.