We report the results of serial 18F-FDG PET/CT investigations in a 49-year-old woman presenting with an advanced cecal high-grade neuroendocrine carcinoma harboring a somatic BRAFV600E mutation. Patient was refractory to standard chemotherapy regimen showing life-threatening hyperlactatemia. Early after the beginning of BRAF-MEK therapy (dabrafenib and trametinib), impressive improvement in PET/CT imaging was achieved. The pathological 18F-FDG uptake in cecal primary tumor as well as in nodal, hepatic, and bone metastases drastically decreased. Moreover, the reduction of total lesion glycolysis on PET/CT images was strictly related to extraordinary patient clinical response and lactic acid level normalization.