LooBox: Loose-box-supervised 3D Tumor Segmentation with Self-correcting Bidirectional Learning

Deep learning-based tumor segmentation methods typically require precise pixel-level annotations, which are costly in clinical practice. While bounding box supervision offers a more efficient alternative, existing approaches assume unrealistically tight box annotations, leading to performance degradation when applied to the loose boxes commonly produced by medical annotators. To address this challenge, we propose LooBox, a novel 3D segmentation framework that utilizes loose box annotations through a self-correction and bidirectional rectification paradigm. For the self-correction part, we propose a noise cleaner that comprehensively utilizes deterministic outer box information by integrating three complementary perspectives for predictive self-rectification: entropy mapping, gradient monitoring, and foreground-background affinity measurement. For the bidirectional rectification part, we introduce an augmentation-driven comprehensive consistency constraint strategy. Specifically, the framework incorporates: an asymmetric co-teaching architecture comprising a basic UNet and an enhanced UNet variant with a noise adapter, and an augmentation-driven consistency mechanism that computes pairwise loss between self-corrected predictions after each training iteration to ensure robust tumor feature extraction. Comprehensive evaluations on LIDC-IDRI, MSD-Lung, and MSD-Pancreas datasets demonstrate that LooBox achieves superior segmentation accuracy compared to state-of-the-art box-supervised methods.