Predicting dose accumulation reliability at the planning stage, with an application to adaptive proton therapy

Abstract Objective. Online adaptive proton therapy could benefit from reoptimization that considers the total dose delivered in previous fractions. However, the accumulated dose is uncertain because of deformable image registration (DIR) uncertainties. This work aims to evaluate the accuracy of a tool predicting the dose accumulation reliability of a treatment plan, allowing consideration of this reliability during treatment planning. Approach. A previously developed deep-learning-based DIR uncertainty model was extended to calculate the expected DIR uncertainty only from the planning computed tomography (CT) and the expected dose accumulation uncertainty by including the planned dose distribution. For 5 lung cancer patients, the expected dose accumulation uncertainty was compared to the uncertainty of the accumulated dose of 9 repeated CTs. The model was then applied to several alternative treatment plans for each patient to evaluate its potential for plan selection. Results. The average accumulated dose uncertainty was close to the expected dose uncertainty for a large range of expected uncertainties. For high expected uncertainties, the model slightly overestimated the uncertainty. For individual voxels, errors up to 5% of the prescribed dose were common, mainly due to the daily dose distribution deviating from the plan and not because of inaccuracies in the expected DIR uncertainty. Despite the voxel-wise inaccuracies, the method proved suitable to select and compare treatment plans with respect to their accumulation reliability. Significance. Using our tool to select reliably accumulatable treatment plans can facilitate the use of accumulated doses during online reoptimization.