FDG PET/CT in Oncology¹⁸F-FDG

Quick Answer

FDG PET/CT is the core oncologic imaging study for staging, restaging, treatment-response assessment, and radiotherapy planning across most solid and hematologic malignancies. Interpretation rests on recognizing physiologic and inflammatory uptake, understanding what SUV does and does not mean (it depends on uptake time, blood glucose, reconstruction, and lesion size), and applying the right response framework — PERCIST (metabolic) or RECIST (anatomic), and disease-specific criteria such as Deauville/Lugano in lymphoma.

SUV and its dependencies

The standardized uptake value normalizes lesion activity to injected dose and body size (usually body weight, SUVbw; lean-body-mass SUL is used in PERCIST). It is a semiquantitative surrogate for metabolism, not an absolute biological constant, and is sensitive to:

• Uptake time — SUV rises with longer post-injection intervals; keep it consistent (~60 min).
• Blood glucose — hyperglycemia competes with FDG and lowers tumor SUV/sensitivity.
• Reconstruction and scanner — different algorithms/settings change SUV; compare only matched protocols.
• Partial-volume effect — small lesions (below ~2–3× resolution) have underestimated SUV.

SUV is comparable across serial scans only when technique is held constant — same scanner, protocol, uptake time, and reference region. Treat SUV as a semiquantitative surrogate, not an absolute number.

Response assessment: PERCIST vs RECIST

• RECIST measures anatomic tumor size on CT (unidimensional). It can misclassify metabolically responding tumors that have not yet shrunk, or scar that remains measurable.
• PERCIST measures the metabolic response using SUL peak of the most active lesion against a consistent reference (typically liver), classifying complete/partial metabolic response, stable, and progressive metabolic disease.
• Disease-specific criteria exist where they perform better — e.g. Deauville/Lugano for lymphoma.

Metabolic change often precedes anatomic change, which is why PET-based criteria detect response earlier.

PERCIST specifics (board favorite): measure SULpeak (lean-body-mass–normalized, in a 1.2 cm spherical VOI) of the single most active lesion; the reference is a 1.5–3 cm sphere in normal right-lobe liver (use blood pool if the liver is abnormal). Complete metabolic response = lesion activity falls to ≤ liver background; partial = ≥30% decline in SULpeak; progressive = ≥30% increase or a new FDG-avid lesion. Measurable target requires lesion SUL > 1.5× liver SUL + 2 SD.

High-yield board facts

Learn the normal distribution and the two classic pitfall lists cold — most oncology PET questions turn on distinguishing physiologic/benign uptake from tumor, or remembering which cancers are FDG-low.

Patient preparation (detail)

Preparation is what makes FDG-PET interpretable, and most avoidable pitfalls trace back to it:

• Fasting 4–6 hours; only water. Withhold IV dextrose and glucose-containing fluids.
• Blood glucose checked before injection — most protocols require it below ~150–200 mg/dL; hyperglycemia competes with FDG and lowers tumor uptake/sensitivity. Manage diabetics per protocol (timing of insulin/oral agents; avoid recent short-acting insulin driving FDG into muscle).
• Uptake time ~60 min (kept consistent across serial scans), rest in a warm, quiet room to minimize muscle and brown-fat uptake.
• Avoid strenuous activity for 24 h (muscle uptake); consider warming and, per local policy, measures to reduce brown fat.
• Note recent G-CSF, chemotherapy, radiation, or surgery — all alter uptake and interpretation timing.

Immunotherapy-era response & pitfalls

Checkpoint-inhibitor and targeted therapies complicate FDG interpretation. Pseudoprogression and immune-related adverse events (sarcoid-like mediastinal/hilar nodes, thyroiditis, hypophysitis, colitis, pneumonitis) produce FDG-avid findings that mimic or coexist with tumor progression. Immune-adapted response frameworks (e.g. iPERCIST / imPERCIST) address the fact that a transient uptake increase or new lesions may not equal true progression — confirmation on follow-up or biopsy is often needed. Recognizing the pattern of immune-related uptake prevents both false calls of progression and missed treatment toxicity.

High-Yield Pearls

• Report the uptake time, glucose, and injected activity — they are prerequisites for interpreting SUV.
• Know the normal FDG distribution (brain, myocardium, liver/spleen, bowel, urinary tract) cold; deviations drive both findings and pitfalls.
• Metabolic response (PERCIST) may lead anatomic response (RECIST) — choose the framework the question demands.

Common Pitfalls

• Brown fat, muscle, and bowel uptake mistaken for nodal/metastatic disease.
• Post-treatment inflammation — surgery, radiation, infection, G-CSF marrow stimulation — causing false positives; timing of imaging matters.
• FDG-low tumors (some low-grade, mucinous, neuroendocrine) under-called.
• Comparing SUV across non-matched scans/scanners as though equivalent.

Related Pages

• Tracer: FDG; physics: SUV harmonization & EARL; reporting: Oncology response criteria (RECIST/PERCIST); pitfalls: Incidental findings on FDG-PET.

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