Modality Bottleneck

The modality bottleneck and the capacity bottleneck in the MIL projection layer are largely orthogonal problems, and resolving one does not diminish the benefit of resolving the other. Existing projection-layer redesigns improve exploitation of the morphological feature space but cannot introduce molecular axes that the foundation model never observed during pretraining. Most clinically relevant pathology endpoints are governed by molecular tissue state rather than purely morphological appearance under H&E stain. Frozen pathology foundation models organize their feature space along morphological axes because they are pretrained almost exclusively on H&E images. Histologically similar tissue regions can correspond to entirely distinct molecular programs, a situation morphology-only MIL cannot resolve regardless of encoder or aggregator power.