PET-CT Scan

What It Shows

• Metabolically active tumours — primary and metastatic sites simultaneously
• Lymph node involvement and distant metastases in a single whole-body scan
• Treatment response: whether tumours are responding to chemotherapy or targeted therapy (metabolic response precedes anatomical shrinkage)
• Cancer recurrence after treatment — especially useful when tumour markers are rising but CT is equivocal
• Differentiation of viable tumour from scar tissue / fibrosis post-treatment
• Brain glucose metabolism (FDG brain PET for neurological cancers)
• Cardiac and infection imaging (non-cancer applications with the same platform)

How It Works

The patient fasts for 4–6 hours. A small amount of FDG (fluorodeoxyglucose — radioactive glucose analogue) is injected intravenously. The patient rests quietly for 60 minutes while FDG distributes throughout the body. Areas of high metabolic activity (cancer cells, inflamed tissue, brain) absorb more FDG. Positrons emitted from FDG annihilate with electrons, producing pairs of gamma rays detected by the PET scanner ring. The CT scan is acquired simultaneously, providing anatomical reference. A nuclear medicine physician fuses and interprets both datasets together.

• Initial staging — to determine cancer extent before treatment planning
• Mid-treatment response assessment — is chemotherapy working?
• End-of-treatment response — confirming remission or residual disease
• Surveillance — detecting recurrence in asymptomatic patients
• When CT alone is equivocal and tissue biopsy is not feasible
• Pre-surgical planning to avoid unnecessary surgery in metastatic disease

PET-CT is guideline-standard for staging and response assessment in lymphoma, lung, colorectal, head and neck, and many other cancers. In Hodgkin lymphoma, interim PET-CT response (Deauville score) directly guides de-escalation or intensification of chemotherapy — one of the most practice-changing imaging advances in oncology. The IELSG-32 trial demonstrated PET-guided treatment in primary CNS lymphoma. Multiple meta-analyses confirm PET-CT superiority over CT alone for nodal staging in NSCLC (sensitivity 79% vs 60%). Novel tracers beyond FDG — PSMA for prostate cancer, DOTATATE for neuroendocrine tumours — extend PET-CT utility significantly.

Preparation & What to Expect

Fast for 4–6 hours (water allowed). Avoid strenuous exercise for 24 hours before (muscles take up FDG). Blood glucose must be <11 mmol/L (200 mg/dL) — diabetics need special preparation. Wear comfortable loose clothing. Arrive 30 minutes early for cannula and paperwork. Entire appointment: 2.5–3.5 hours.

• FDG uptake is non-specific — inflammation, infection, and post-surgical changes can mimic cancer
• Brain always shows high FDG uptake — primary brain tumours often require MRI instead
• Slow-growing or low-grade tumours (e.g. prostate adenocarcinoma, some indolent lymphomas) may be FDG-negative
• Tumours smaller than 5–8 mm may be below resolution threshold
• Hyperglycaemia reduces FDG uptake in tumours — poorly controlled diabetes impairs scan quality

• Moderate radiation dose (~14 mSv) — avoid in pregnancy; breastfeeding should be paused for 12–24 hours post-scan
• FDG is radioactive — patient should limit close contact with pregnant women and children for several hours after scan
• Requires a cannula (IV line) — communicate needle phobia to team in advance
• Results must be interpreted by a specialist nuclear medicine physician alongside clinical context