Correlation of In Vivo and In Vitro Measures of Carbonic Anhydrase IX Antigen Expression in Renal Masses Using Antibody 124I-cG250

The goal of this study was to determine whether there is a potential correlation between quantification of radiolabeled macromolecular uptake in tumors determined in vivo using PET/CT and in vitro using autoradiography and gamma-counting of tumor tissue. Methods: Twenty-six patients with renal masses scheduled for surgical resection received I-124-labeled antibody cG250. Tumor specimens obtained from resection were studied. Fifteen of these patients had clear cell cancer demonstrated by positive findings on PET/CT images and histopathology. Radioactivity in tumors was measured on PET/CT images and expressed as percentage injected dose per gram. These values were then normalized to measurements of known serum radioactivity from a venous blood sample obtained at the time of PET/CT. Comparable measurements were obtained in vitro using gamma-well counting and digital autoradiography of tumor tissue. Results: There was a significant correlation between tumor radioactivity estimated in vivo and in vitro (Spearman correlation coefficient comparing normalized PET measurements with well counting of 0.84, P < 0.000001, and with autoradiography of 0.88, P < 0.000001). PET/CT measurements of tumor uptake were lower than measurements obtained with either of the in vitro methods, and digital autoradiography resulted in the highest measurements. Conclusion: PET/CT can be reliably used to quantify radiolabeled macromolecular uptake in vivo, suggesting important implications for quantitative pharmacokinetic estimates of macromolecular biodistribution.