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原文摘要

Recent research on medical MLLMs has shifted its focus from image-level understanding to fine-grained, pixel-level comprehension. Although segmentation serves as the foundation for pixel-level understanding, existing approaches face two major challenges. First, they introduce implicit segmentation tokens and require joint fine-tuning of the MLLM and external pixel decoders, increasing the risk of catastrophic forgetting and limiting out-of-domain generalization. Second, most methods rely on single-pass reasoning and lack the ability to iteratively refine segmentation results. To overcome these limitations, we propose IBISAgent--a novel agentic MLLM that reformulates segmentation as a vision-centric, multi-step decision-making process. IBISAgent enables MLLMs to generate interleaved reasoning and text-based click actions, invoke segmentation tools, and produce high-quality masks without architectural modifications. We also design a two-stage training framework consisting of cold-start SFT and agentic RL with tailored, fine-grained rewards. Through iterative multi-turn visual reasoning, IBISAgent naturally facilitates mask refinement and enhances robustness in complex medical referring and reasoning segmentation tasks. Extensive experiments demonstrate that IBISAgent consistently outperforms both closed-source and open-source SOTA methods.