Quantifying intra-fractional prostate motion trajectory for establishing a new gating strategy: a preliminary study

Background: Existing gated delivery methods adopting predefined intervention thresholds, and better delivery accuracy comes at the expense of lower therapeutic efficiency. So we present a new gating strategy based on trajectory quantification and pattern discrimination. Material and methods: 1265 real-time ultrasound monitoring trajectories of 61 prostate cancer patients were retrospectively analyzed. The trajectories included four typical patterns: stable target at baseline, transient excursion, continuous drift, and persistent excursion. Trajectories were randomly sampled to train a discriminant model that utilizes a sliding window approach with a 30-s window. The discriminant accuracy and receiver operating characteristic (ROC) curve were analyzed to assess the model's performance. Results: The discriminant model to identify intra-fractional motion patterns was successfully trained. The model had statistical significance (p < 0.001) and correctly classified 94.0% of trajectories in the time window. The area under the ROC curve, sensitivity, and specificity were all greater than 0.80 for different motion patterns. Conclusion: As an essential component of the new gating strategy, trajectory quantification makes it possible to automatically discriminate intra-fractional motion patterns; pattern discrimination further allows active guidance for subsequent interventions. This preliminary work has shown promise in delivering radiotherapy accurately and cost-effectively.