SU-E-J-22: Alternative Method for Evaluating the Need for Replanning in Patient's with Significant Anatomical Changes




The use of IGRT is highlighting the issue of anatomical patient changes during treatment due to weight loss and/or tumor shrinkage. A means of quickly reviewing the need to replan is important to maximize efficiency and review the necessity of modifying treatment planning. The goal of this project is to use an alternative method of evaluation for replanning that does not involve extra patient scanning or specific deformable registration software.


Five patients that had undergone daily CBCT imaging during treatment and were noted to have considerable changes in anatomy due to weight loss or tumor shrinkage were evaluated with two methods. First, CBCT external volumes were subtracted from original planning CT to obtain a “change-in-volume” contour. The density of this contour was overridden to that of air and original plan recalculated. Second, the original plan was recalculated on a new CT scan taken the same day as the CBCT for comparison. The contours remained unchanged from the original planning scan.


So far 1 patient has been evaluated using both methods. The DVH comparison shows similar results between each method for the target tissues (CTV and PTV) as well as critical structures (right parotid and spinal cord). Using the CBCT method mean doses for each structure examined were within 3.2% of those calculated with a new CT scan. The remaining four patients will be evaluated similarly after a new CT scan is obtained.


By using the patient's daily CBCT scan from setup prior to treatment, a quick method for evaluating anatomical changes and the need to replan a patient during the course of treatment was demonstrated. This method prevents the need to rescan the patient initially and allows a quick visualization of possible dose differences caused by a significant change in body habitus.