Sentinel Lymph Node Mapping
Lymphoscintigraphy identifies the sentinel lymph node — the first node draining a tumour — so it can be selectively biopsied. A negative sentinel node usually means the nodal basin is clear, sparing patients the morbidity of a full lymph-node dissection. It is standard of care in breast cancer and cutaneous melanoma, and is used in vulvar, penile, early oral-cavity and gynaecological (cervical, endometrial) cancers.
Tumour cells spread first to the sentinel node, so its status predicts the rest of the basin. Radiotracer injected around the tumour or lesion migrates through lymphatics; dynamic and static imaging (with SPECT/CT) map the node(s), and an intra-operative gamma probe ± blue dye guides excision.
When to use
- Nodal staging in early breast cancer and clinically node-negative cutaneous melanoma.
- Selected vulvar, penile, early oral-cavity (head-and-neck) and gynaecological (cervical, endometrial) cancers.
- Mapping ambiguous or multiple drainage basins before surgery.
Technique & protocol
- Superficial (intradermal, subdermal, periareolar/subareolar) or deep (peritumoral) injection in breast, or intradermal injection around the lesion or biopsy scar in melanoma, using ⁹⁹ᵐTc-nanocolloid, ⁹⁹ᵐTc-sulfur colloid or ⁹⁹ᵐTc-tilmanocept.
- Dynamic imaging (standard in melanoma, optional in breast) to follow lymphatic channels, then static images, with SPECT/CT for anatomical localisation.
- Intra-operative gamma probe ± blue dye to retrieve the node.
How to read it
- Identify the first node(s) on a direct lymphatic channel from the injection site.
- Map all draining basins, including unexpected/in-transit nodes.
- Distinguish true sentinel nodes from second-echelon nodes.
Pitfalls
- Non-visualisation (obesity, tumour-replaced nodes, prior surgery) — delayed imaging/repeat injection helps.
- Injection-site “shine-through” can obscure nearby nodes.
- Second-echelon nodes may be mistaken for sentinel nodes.
Evidence & guidelines
- The joint EANM/SNMMI breast guideline (2013) and the SNMMI-endorsed EANM melanoma guideline (2015) standardise technique; EANM also publishes guidance for gynaecological (2014) and oral-cavity (2019) cancers.
- Multisociety (SNMMI-led) Appropriate Use Criteria for lymphoscintigraphy in sentinel node mapping and lymphoedema/lipoedema (published 2022; summary J Nucl Med 2023).
- SPECT/CT improves node localisation and surgical planning.
In depth
- Radiocolloids are retained in sentinel nodes by macrophage phagocytosis and particle trapping; migration is inversely related to particle size, so small-to-medium colloids (about 5–200 nm) are used, while particles larger than 200 nm stay mainly at the injection site.
- Small particles (e.g. antimony trisulphide, 5–30 nm) drain quickly, so imaging is usually complete within 1–3 h but more second-echelon nodes may be seen; medium particles (50–200 nm) may need images at 4–6 h or the next day; particles above 200 nm move slowly and remain largely at the injection site.
- ⁹⁹ᵐTc-tilmanocept (Lymphoseek, about 7 nm, binds the CD206 mannose receptor) was FDA-approved in 2013 for breast cancer and melanoma and in 2014 for oral cavity squamous cell carcinoma; it clears rapidly from the injection site with little second-echelon accumulation.
- Melanoma: intradermal injection in ≥4 aliquots of 0.1–0.2 mL around the lesion/scar (kept within 1 cm), avoiding subcutaneous injection.
- Breast: superficial injections use 0.05–0.5 mL and peritumoral 0.5–1 mL; ~5–30 MBq for same-day and ~150 MBq for next-day surgery.
- Protocol: dynamic imaging at one frame per minute (128×128) for 10–20 min helps separate true sentinel nodes from second-echelon nodes, followed by early 5-min static views and delayed static views at 1–3 h.
- Blue dye (patent blue V or isosulfan blue; methylene blue is an alternative) is injected 10–20 min before surgery in breast cancer or at the start of the operation in melanoma; in melanoma, 86% of sentinel nodes are both radioactive and blue, 13% radioactive only and 1% blue only.
- SPECT/CT improves sentinel node detection and localisation, especially near the injection site; it should be performed in head and neck melanoma and is mandatory, where available, for oral cavity cancer.
- Combining radiotracer with blue dye improves detection over dye alone, and experienced multidisciplinary teams routinely achieve sentinel node identification rates above 95%.
Sources: EANM melanoma SLN guideline 2015 (PMID 26205952) · EANM/SNMMI breast SLN guideline 2013 (PMID 24085499) · FDA approval for oral cavity SCC, 13 June 2014 · EANM oral cavity SCC SLN guideline 2019 (PMID 30564849)
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