PET-CT in melanoma staging
FDG-PET/CT; 18F-fluorodeoxyglucose PET
FDG-PET/CT is a problem-solving rather than first-line imaging modality in melanoma. Its strengths are detection of distant nodal, soft-tissue and visceral metastatic disease and clarification of equivocal findings on standard CT. Its weaknesses are limited sensitivity for small CNS metastases (where MRI is preferred) and a high false-positive rate from inflammation. UK practice generally reserves PET-CT for stage IIB melanoma and above, suspected metastatic recurrence, and pre-operative confirmation of resectability for advanced disease.
When to use PET-CT
- Stage IIB melanoma upwards — NICE NG14 (recommendation 1.7) endorses contrast-enhanced CT as the baseline staging modality; PET-CT is a problem-solving adjunct used selectively (e.g. to clarify equivocal CT findings, or where adjuvant systemic therapy is being considered).
- Suspected nodal or distant recurrence to clarify equivocal CT findings.
- Pre-operative confirmation of resectability for advanced or recurrent disease before metastasectomy or complex regional surgery.
- Response assessment in selected patients on systemic therapy when CT findings are ambiguous.
- Investigation of melanoma of unknown primary.
When NOT to use PET-CT
- Stage 0, IA, IB — pre-test probability of distant disease too low; imaging not indicated.
- Stage IIA — generally not indicated (low yield); local protocols vary.
- Detection of small (< 5 mm) CNS metastases — use MRI brain instead.
- Routine surveillance of resected stage III in patients already on protocol CT — adds limited information.
- Indolent very-low-grade lesions where management is not influenced.
Limitations
- Brain — PET-CT has poor sensitivity for small CNS metastases due to high background cortical FDG uptake; MRI brain is the imaging of choice for intracranial disease.
- Inflammation — reactive lymph nodes, post-surgical inflammation, post-RT change can be FDG-avid; correlate with clinical and prior imaging.
- Mucosal melanoma — sometimes low FDG uptake; supplement with dedicated regional imaging.
- Resolution — sub-centimetre lesions may be missed.
- Radiation dose — ~ 7–10 mSv per scan; weigh against benefit.
Interpretation pearls
- SUVmax — semi-quantitative; trend over scans more informative than single value.
- Brown adipose tissue uptake (neck, supraclavicular) — common in cold environments and slim patients; do not over-call as nodal disease.
- Reactive in-transit FDG avidity in the limb after recent surgery — wait 6 weeks post-op for clean baseline.
- Read PET-CT alongside CT and any MRI — multimodality discussion at MDT.
References
- Schroer-Gunther MA et al. F-18-FDG PET or PET/CT for the detection of recurrent melanoma — meta-analysis. Eur J Nucl Med Mol Imaging; 2012.
- NICE NG14. Melanoma: assessment and management. London: NICE; 2015 (last updated 27 July 2022), recommendation 1.7.
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