THEME: Task-Hierarchical Evidence Completion for In- complete Multimodal Brain Tumor Segmentation

An unavailable MRI sequence removes information needed for brain tumor segmentation, but its effect differs across tumor regions. We propose THEME, which predicts a separate learned summary for each missing sequence and each of three region tasks. Modality-specific encoders extract spatial features and task summaries; a completion module estimates the summaries of missing sequences from those observed. One router fuses only acquired spatial features, and a second combines acquired and predicted summaries for each task before decoder modulation. We evaluate every one of the 15 non-empty subsets of four MRI sequences. On BraTS 2018, THEME obtains 88.44/81.16/64.32% Dice for whole tumor/tumor core/enhancing tumor, improving on the strongest baseline by 1.50/0.35/1.87 points. The corresponding BraTS 2020 scores are 89.65/83.04/68.16%, with gains of 1.49/0.36/1.87 points. Removing direct summary supervision reduces enhancing-tumor Dice by 5.78 points. These experiments show the benefit of the complete architecture under simulated missing-sequence evaluation.