Oncologic Imaging with PET/CT and SPECT/CT
FDG biology and response criteria
F-18 FDG enters cells via GLUT transporters, is phosphorylated by hexokinase to FDG-6-phosphate, and is trapped because the phosphate cannot be further metabolized and dephosphorylation is slow. Uptake therefore reflects transporter density and hexokinase activity, not simply glucose consumption. Fasting 4–6 h, glucose under about 150–200 mg/dL, and a 60-minute uptake period standardize the study; recent insulin, carbohydrate loading, or exercise shunts tracer into muscle and fat.
Quantification matters more than any single SUV cutoff. Weight-normalized SUV is confounded by adiposity, so lean-body-mass SUV (SUL) is preferred in PERCIST. PERCIST uses SULpeak in a fixed 1-cm³ sphere, with a 30% reduction plus a 0.8-unit absolute fall defining complete metabolic response. Lymphoma uses the Deauville 5-point scale referenced to mediastinal blood pool and liver: score 4 is uptake above liver, score 5 markedly above liver (roughly twice).
Know the tracer palette: Ga-68 DOTATATE for somatostatin receptor imaging, Ga-68 or F-18 PSMA ligands for prostate cancer, I-123/I-131 MIBG for pheochromocytoma and neuroblastoma, F-18 NaF for bone, and I-131 for differentiated thyroid cancer.
Traps: brown fat (supraclavicular, paraspinal), thymic rebound in children, marrow activation from G-CSF, post-radiation inflammation, and insulin-driven muscle uptake are false positives; small mucinous or necrotic metastases can be false negatives. Never compare SUVs across scanners or uptake times.
Worked example: baseline SULpeak 8.0 falls to 5.0, a 37.5% decrease that exceeds the 30% PERCIST threshold and counts as complete metabolic response if all target lesions behave similarly.