SCC has been reported to have a higher SUV of the primary tumor compared to adenocarcinoma [20, 25], and a higher SUV of the primary tumor is correlated with a higher likelihood of occult nodal metastasis [35–37], which might be one of the reasons why the CA9-positive SCC cases in the present study had more false-negative outcomes. This evidence concerns the gene CA9 and adenocarcinoma.