Flow-MIL: Constructing Highly-expressive Latent Feature Space For Whole Slide Image Classification Using Normalizing Flow

Whole Slide Image (WSI) classification has been widely used in pathological diagnosis and prognosis prediction, and it is commonly formulated as a weakly-supervised Multiple Instance Learning (MIL) problem because of the large size of WSIs and the difficulty of obtaining fine-grained annotations. In the MIL formulation, a WSI is treated as a bag and the patches cut from it are treated as its instances, and most existing methods first extract instance features and then aggregate them into bag feature using attention-based mechanism for bag-level prediction. These models are trained using only bag-level labels, so they often lack instance-level insights and lose detailed semantic information, which limits their bag-level classification performance and damages their ability to explore high-expressive information. In this paper, we propose Flow-MIL, which leverages normalizing flow-based Latent Semantic Embedding Space (LSES) to enhance feature representation. By mapping patches into the simple and highly-expressive latent space LSES, Flow-MIL achieves effective slide-level aggregation while preserving critical semantic information. We also introduce Gaussian Mixture Model-based Latent Semantic Prototypes (LSP) within the LSES to capture class-specific pathological distribution for each class and refine pseudo instance labels. Extensive experiments on three public WSI datasets show that Flow-MIL outperforms recent SOTA methods in both bag-level and instance-level classification and offers improved interpretability.