Room: Exhibit Hall | Forum 2
Purpose: To evaluate daily changes in the dose distribution to determine the necessity of plan adaptation using deformable image registration (DIR) to cone beam CT imaging for head and neck cancer patients without dose re-computation, allowing for a fast online assessment.
Methods: This study involved 18 head-and-neck cancer patients treated with CBCT image guidance who had their treatment plan modified on day X based on a new CT simulation (ReCT). Anatomical changes were analyzed using DIR between two pairs of images i) planning CT (PCT) to day X CBCT and ii) PCT to ReCT. Dose effects were determined through applying either the planned dose distribution or the recalculated dose from the ReCT. Changes in the dose to ipsilateral and contralateral parotid glands, as a result of anatomy changes, were evaluated through voxel-by-voxel dose changes. The necessity of plan adaptation was assessed by the dose to 95% of the planning target volume (D95) and mean dose to the parotids.
Results: Compared to the gold standard, the best results were produced using the planning CT to day X CBCT using the ReCT dose distribution with a relative voxel wise dose error of 7% and 8% for the ipsilateral and contralateral parotid respectively. The simplest method using the DIR from the PCT to day X CBCT with the planned dose yielded a voxel-wise dose error of 13% and 14% for the ipsilateral and contralateral parotid, respectively. The treatment plans for all 18 patients were adapted clinically but only 6 needed an adaptation and using the DIR from PCT to day X CBCT with the planned dose would have avoided 8 unnecessary adaptations.
Conclusion: Using the DIR between the planning CT and daily CBCT can successfully flag cases for plan adaptation without requiring a dose recalculation.