Dynamic iterative coarse-to-fine prompt learning based on SAM for precise esophageal cancer gross target volume segmentation.

Accurate segmentation of the gross tumor volume from computed tomography images is a core step in the development of precise radiotherapy planning for esophageal cancer, which directly affects treatment efficacy and normal tissue protection. Recently, foundation models represented by the segment anything model (SAM) have been increasingly applied in medical image segmentation, and several studies have extended them to tumor target segmentation with promising progress. However, existing SAM-based segmentation methods rely on manual prompts, leading to significant limitations in esophageal cancer. Manual localization of tumor prompt points is difficult, and inappropriate prompts easily cause segmentation errors, increasing the risk of damage to organs-at-risk during radiotherapy. To address this, this study proposes a segmentation method based on SAM that streamlines the inference process by using a coarse-to-fine prompt generation strategy. Inspired by the stepwise refinement of clinical target volume delineation, the core design lies in a coarse-to-fine prompt generation strategy. Specifically, coarse segmentation results generated by nnUNet are first converted into initial prompts to provide global anatomical priors for SAM. Furthermore, a dynamic iterative prompt update mechanism is introduced. During inference, the iterative correction prompts are derived solely from the model's own prediction uncertainty, forming a closed-loop refinement that gradually improves segmentation accuracy. Experimental results based on multi-center datasets show that this method, which iteratively updates prompts based on its own uncertainty, achieves better segmentation performance than existing single-pass methods on both internal and external validation sets. The framework is highly consistent with the logic of clinical target volume delineation and can provide a reliable efficient scheme for the formulation of precise radiotherapy plans for esophageal cancer.