FDG measures glucose use, not cancer: brown fat, muscle, bowel, infection, granulomas, fractures, post-radiotherapy and post-surgical change, and marrow stimulation after G-CSF all take it up; low-grade and mucinous tumours may not.
Orient first
- Report SUVmax with the uptake time and glucose level; compare against liver or mediastinal blood pool.
- False negatives: small lesions (< ~8 mm), mucinous, lobular breast, low-grade lymphoma, well-differentiated HCC, prostate, renal clear cell.
- Timing: avoid PET within ~3 months of radiotherapy and ~2 weeks of chemotherapy where possible (verify local practice).
Acquire the study
- Fasting ≥ 4–6 h, glucose checked, FDG with ~60 min uptake in a warm quiet room; PET-CT vertex to thighs (or whole body for melanoma).
The manoeuvre
- Check the acquisition: uptake time, blood glucose, extravasation at the injection site.
- Physiological uptake review on the MIP and fused series: brown fat (supraclavicular, paravertebral), muscles, bowel, urinary tract.
- Each focus: SUVmax, CT correlate, pattern (nodular vs linear vs diffuse).
- Benign avid patterns: diffuse marrow after G-CSF, linear rib fracture, symmetric hilar nodes (granulomatous), post-operative wound.
- Response: Deauville (lymphoma) or PERCIST (solid tumours) where applicable.
What confirms it
- An avid focus with a compatible CT correlate and clinical context.
What licenses you to exclude it
- A negative PET does not exclude a small or low-avidity tumour.
The classic misread
- Calling brown fat or ovarian physiological uptake a metastasis.