Chemotherapy-induced normalization of FDG uptake by colorectal liver metastases does not usually indicate complete pathologic response

Dramatic responses are being observed in colorectal cancer liver metastases treated with newer chemotherapeutic regimens. These have been associated with normalization of [¹⁸F]fluoro-2-deoxy-d-glucose (FDG) uptake (complete metabolic response) on follow-up Positron Emission Tomography with [ ¹⁸F]fluoro-2-deoxy-d-glucose (FDG-PET) scans in some patients. It is unclear how often complete metabolic response is indicative of complete tumor destruction. We analyzed a subset of patients who had neoadjuvant chemotherapy for hepatic metastases from colorectal adenocarcinoma. Inclusion criteria were: (1) FDG-avid hepatic lesions before initiation of chemotherapy; (2) complete metabolic response of the same lesions after chemotherapy; and (3) histopathologic examination of hepatic lesions. Complete pathologic response was defined as no histologically identifiable viable tumor. Fourteen patients fit the inclusion criteria. All had synchronous, hepatic-only colorectal metastases. On microscopic examination, complete pathologic response to the neoadjuvant regimen was found in only 5 of 34 lesions (15%) and in only 3 of the 14 patients (21%). Seven lesions had complete metabolic response and disappeared on computed tomography (CT); of these, six still contained viable tumor. We conclude that complete metabolic response on FDG-PET after neoadjuvant chemotherapy is an unreliable indicator of complete pathologic response. Therefore, currently, curative resection of liver metastases in these patients should not be deferred on the basis of FDG-PET findings.