Finally, single-dose [<sup>177</sup>Lu]Lu-girentuximab treatment induced cytotoxicity in PDOs in vitro and strongly reduced tumor burden in 2 independent xenografted mouse models, with no signs of toxicity.<h4>Conclusion</h4>Our results demonstrate that CA-IX is a relevant target for a theragnostic strategy in mCRC, and provide the first demonstration in clinically-relevant models of metastasis that radiolabeled girentuximab can be used as a scouting agent to stratify and monitor mCRC patients and as a therapeutic alternative for patients with CA-IX-expressing tumors. Here, CA9 is linked to neoplasm.