12.4 Lymphoscintigraphy & Sentinel Lymph Node Mapping

12.4 Lymphoscintigraphy & Sentinel Lymph Node Mapping

Principle and Purpose

Lymphoscintigraphy images the lymphatic system by injecting a particulate or receptor-binding radiocolloid that drains through lymphatic channels and accumulates in regional lymph nodes. Its principal oncologic application is sentinel lymph node (SLN) mapping: identifying the first lymph node (or nodes) that receives lymphatic drainage directly from a primary tumor. Because the sentinel node is the first stop for metastatic spread, its biopsy status predicts the status of the entire nodal basin. A negative sentinel node spares the patient a full, morbid lymph-node dissection.

The blueprint groups lymphoscintigraphy into three sub-areas — breast, skin lesion, and lymphangiography — reflecting its dual role in sentinel-node oncology and in lymphedema evaluation.

Radiopharmaceuticals

Two Tc-99m agents dominate clinical practice.

Lymphatic capillaries are permeable to particles smaller than roughly 50 nm, so very large colloid particles remain at the injection site and drain poorly. Filtered sulfur colloid (0.22-µm filter) is preferred to reduce particle size. Because colloids settle by gravity, the syringe must be gently rotated immediately before injection to resuspend particles.

Tc-99m tilmanocept is a small synthetic receptor-binding molecule that targets mannose receptors (CD206) on reticuloendothelial cells within lymph nodes. Its small size enables fast clearance from the injection site, and its specific receptor binding produces durable uptake in first-echelon nodes — permitting two-day mapping (injection the day before surgery). The standard tilmanocept dose is 0.5 mCi, administered at least 15 minutes before intraoperative mapping and not delayed more than about 15 hours. Sulfur colloid is often injected the day before or morning of surgery at activities around 0.3–1.0 mCi depending on protocol.

Injection and Imaging Technique

1. Injection site depends on the tumor: intradermal or subcutaneous injection in the periareolar region or around the known breast tumor; intradermal around a primary melanoma or skin lesion; interdigital web-space injection for lower-extremity lymphedema.
2. Use a fine needle (25–26 gauge) and a shielded syringe. After injection, apply gentle pressure and dab the site to prevent skin contamination, which can mimic a lymph node.
3. Massage the injection site to promote lymphatic uptake.
4. Acquire dynamic imaging immediately after injection to visualize lymphatic channels and transit, followed by static images to identify and localize sentinel nodes; mark the node on the overlying skin for the surgeon.
5. Some centers forego preoperative imaging and rely solely on intraoperative probe detection.

Intraoperative Detection

In the operating room the surgeon uses a handheld gamma-detecting probe (neoprobe) to locate the hot sentinel node transcutaneously and then in the surgical field. SLN mapping is routinely combined with vital blue dye (isosulfan or methylene blue) injected at the same site; the node is confirmed sentinel when both radioactive and blue, though either signal alone may suffice. The node is excised and sent to pathology; if negative, the nodal dissection is omitted.

The 10% Rule (Melanoma)

For melanoma and other cutaneous lesions, a node is considered a sentinel node if its in-vivo or ex-vivo count is ≥10% of the hottest node's count (the "10% rule"). This accounts for the multiple-channel drainage common in melanoma, where several true sentinel nodes may exist.

Lymphangiography for Lymphedema

For suspected lymphedema (primary, as in congenital lymphedema, or secondary, after surgery/radiation or in tropical filariasis), interdigital web-space injection of filtered Tc-99m colloid images the superficial lymphatic channels of the limb. Normal studies show symmetric, linear tracer transit to regional nodes; lymphedema shows dermal backflow, delayed transit, or absent nodal visualization. Lymphoscintigraphy cannot visualize the deep lymphatic system well, an inherent limitation. Findings guide lymphatic-venous anastomosis or other surgical planning.

Radiation Safety and Special Populations

Sentinel-node lymphoscintigraphy delivers a low effective dose — typically less than many routine nuclear studies — because the activity is small and largely confined to the injection site and a few nodes. Nonetheless:

• Use standard ALARA precautions and syringe shielding during injection.
• Pregnancy is a relative consideration; sentinel-node biopsy has been performed in pregnant patients with acceptable fetal dose, but the decision involves the surgeon, nuclear physician, and obstetrician.
• Document injection site, radiopharmaceutical, activity, and time so the surgeon can correlate probe counts with imaging.

flowchart TD
    A[Inject Tc-99m colloid intradermal/subcutaneous around lesion] --> B[Massage + dynamic then static imaging]
    B --> C[Mark sentinel node on skin]
    C --> D[Surgery: gamma probe + blue dye]
    D --> E{Melanoma?}
    E -->|Yes| F[Apply 10% rule: node counts >=10% of hottest are sentinel]
    E -->|No| G[First-echelon hot/blue node = sentinel]
    F --> H[Excise sentinel node(s) for pathology]
    G --> H