TH-C-BRD-07: Minimizing Dose Uncertainty for Spot Scanning Beam Proton Therapy of Moving Tumor with Optimization of Delivery Sequence

Abstract

Purpose: Intensity modulated proton therapy (IMPT) has been shown to be able to reduce dose to normal tissue compared to intensity modulated photon radio-therapy (IMRT), and has been implemented for selected lung cancer patients. However, respiratory motion-induced dose uncertainty remain one of the major concerns for the radiotherapy of lung cancer, and the utility of IMPT for lung patients was limited because of the proton dose uncertainty induced by motion. Strategies such as repainting and tumor tracking have been proposed and studied but repainting could result in unacceptable long delivery time and tracking is not yet clinically available. We propose a novel delivery strategy for spot scanning proton beam therapy. Method: The effective number of delivery (END) for each spot position in a treatment plan was calculated based on the parameters of the delivery system, including time required for each spot, spot size and energy. The dose uncertainty was then calculated with an analytical formula. The spot delivery sequence was optimized to maximize END and minimize the dose uncertainty. 2D Measurements with a detector array on a 1D moving platform were performed to validate the calculated results. Results: 143 2D measurements on a moving platform were performed for different delivery sequences of a single layer uniform pattern. The measured dose uncertainty is a strong function of the delivery sequence, the worst delivery sequence results in dose error up to 70% while the optimized delivery sequence results in dose error of <5%. END vs. measured dose uncertainty follows the analytical formula. Conclusion: With optimized delivery sequence, it is feasible to minimize the dose uncertainty due to motion in spot scanning proton therapy.