CAIX PET (89Zr-girentuximab)

CAIX PET uses 89Zr-labelled girentuximab, an antibody against carbonic anhydrase IX, a HIF target expressed in more than 95% of clear-cell renal cell carcinomas because of VHL loss. Imaging 5 days after injection shows whether an indeterminate renal mass is clear-cell cancer without a biopsy. In the ZIRCON phase 3 (2023) it achieved sensitivity 86% and specificity 87%, giving histology-level specificity and whole-body staging in one scan. Telix's TLX250-CDx received an FDA complete response letter in 2025 over manufacturing, and resubmission is planned, so it is not yet approved; the five-day imaging delay and the 89Zr dose are practical limitations. The same antibody labelled with 177Lu is being tested therapeutically in the STARLITE trials, making CAIX a theranostic pair. It tells cancer from a benign kidney lump by lighting up a protein almost unique to clear-cell kidney cancer.