Improving Proton Dose Delivery for Swelling HN Targets Via Range-Based Patient Setup
Abstract
Purpose
Proton head-and-neck (HN) patients often experience target volume changes. Our investigation of daily CBCTs showed the changes from swelling to shrinkage happening usually within one week. Offline adaptation takes about one week to create a replan and thus very possibly the replan for the swelling target is used in the already shrinking target. We propose range-based registration instead of replanning to improve dose delivery to swelling targets.
Methods
5 proton HN patients replanned due to target swelling were studied. A range-based registration algorithm was developed to register the QACTs to the planning CT. The algorithm rigidly searched the registration parameters (shift only in this work) to minimize the deviation of water equivalent thickness (WET) of the target and meanwhile balance the WETs between their corresponding D95 and V98. The WET deviations corresponding to D95=95 and V98=90 were determined by analyzing the robustness of the treatment plan. With the extra shifts obtained from range-based registration, standard QACT-plan evaluation procedures were performed to deform the target contours from the planning CT to QACTs and then to calculate the dose from the treatment plan to the QACTs in the treatment planning system.
Results
For the 5 investigated QACTs, the mean D95 increased from 90.9% to 97.1% of the prescribed dose. Extra lateral shifts were from 1 to 4 mm determined by our range-based registration. One of them had D95=91.2% in a QACT registered to the planning CT by image-based setup but increased to 98.7% in the QACT with extra shifts (-2, 0, 1) mm in the lateral, AP and SI directions.
Conclusion
Image-guided patient setups for all cases used in this study were acceptable, but delivered doses were not acceptable unless extra shifts were applied. Range-based registration significantly improved proton dose delivery to the intended targets.