Sentinel Lymph Node (SLN)

Definition

"The first node to receive drainage from the tumor site is termed the sentinel node. This node is the node most likely to contain metastases, if metastases to that regional lymph node basin are present." - Schwartz's Principles of Surgery, 11th Ed.

History

• Sabiston Textbook of Surgery

Concept & Rationale

• Lymphatic drainage from a tumor follows a predictable (though not always anatomically intuitive) pathway.
• The SLN is the gateway: if cancer cells have spread via lymphatics, they are captured here first.
• If the SLN is negative on histopathology, the remaining regional nodes are almost certainly negative - avoiding full node dissection.
• If the SLN is positive, the surgeon proceeds to formal regional lymph node dissection (LND).
• This avoids morbidity (lymphedema, nerve injury, seroma) in patients whose nodes are truly negative.

Tracer Agents Used

• The combination of radioactive colloid + blue dye is more accurate than either alone.
• For melanoma: tracer is injected intradermally around the lesion/biopsy scar (raising a wheal). Subcutaneous injection is a common error and will fail to identify an SLN.
• For breast cancer: tracer is injected peritumorally, subareolarly, or subdermally near the primary tumor.

Preoperative Lymphoscintigraphy

• Performed on the day before or day of surgery.
• A gamma camera produces dynamic and static images identifying lymphatic channels and sentinel nodes.
• Especially important for melanoma on the trunk, head, or neck - where lymphatic drainage is unpredictable and may drain to more than one basin.
• Can identify interval/in-transit nodes (found between the primary and the main nodal basin), which carry the same risk of harboring metastasis and must be removed.

Preoperative lymphoscintigraphy. A: Melanoma of the back draining to the axilla. B: Periumbilical melanoma draining to the left inguinal nodes. (Sabiston Textbook of Surgery)

Surgical Technique (Breast Cancer)

1. Radioactive colloid (0.5 mCi Tc-99m sulfur colloid for same-day; 2.5 mCi if injected the day before) is injected around the tumor or in the subareolar region.
2. In the OR, 3-5 mL of blue dye is injected peritumorally or subareolarly.
3. A hand-held gamma probe is used transcutaneously to locate the "hot" node.
4. A 3-4 cm incision is made in the lower axilla; blue lymphatic channels are followed to the SLN.
5. All nodes with radioactivity counts >10% of the hottest node ("10% rule") are removed.
6. Careful manual palpation throughout minimizes the false-negative rate.

Lymphatic mapping and SLN biopsy for breast cancer. A: Peritumoral injection of blue dye. B: Blue dye draining to the SLN (blue node, arrowed). (Schwartz's Principles of Surgery)

Histopathological Evaluation of the SLN

• Serial sectioning
• H&E staining (hematoxylin and eosin)
• Immunohistochemistry (IHC):
  • S-100 protein and HMB-45 for melanoma
  • Cytokeratin for breast cancer
• Molecular methods (RT-PCR) for ultrastaging are under investigation.

Efficacy Metrics

1. Identification rate - proportion of patients in whom an SLN was successfully found and removed. Should be >95% with combined technique.
2. False-negative rate (FNR) - proportion of patients with actual nodal metastases in whom the SLN was reported negative. Ranges 0-11% in study series; improves with surgical experience.

• Schwartz's Principles of Surgery, 11th Ed.

Clinical Applications by Tumor Type

1. Breast Cancer

• Standard of care for clinically node-negative (N0) early breast cancer.
• Also accurate for larger T3N0 tumors (but ~75% of these will have nodal metastases on histology, so preoperative identification is preferred).
• Accurate for staging after neoadjuvant chemotherapy (in patients who were clinically N0 at presentation).
• Contraindications to SLNB:
  • Inflammatory breast cancer
  • Biopsy-proven nodal metastasis
  • DCIS without mastectomy
  • Prior axillary surgery
• Safe in pregnancy when performed with radioactive colloid alone (without blue dye).

2. Melanoma

• Required by AJCC for all melanomas >1.0 mm in thickness.
• SLN status is the single most important prognostic factor in melanoma without clinical nodal disease.
• Considered for T1 (thin) melanomas with high-risk features: ulceration, mitotic rate ≥1/mm², age <40 years, thickness 0.8-1.0 mm.
• Results directly guide adjuvant therapy decisions (e.g., immunotherapy, targeted therapy).

3. Merkel Cell Carcinoma (MCC)

• MCC has a much higher rate of occult nodal involvement (~30% for a 1.7 cm MCC) compared to melanoma.
• SLNB is a sensitive staging tool and is routinely recommended.

4. Head and Neck Cancer

• Used for oral cavity squamous cell carcinoma with clinically N0 neck.
• Lymphatic drainage patterns are especially unpredictable; lymphoscintigraphy is indispensable.

5. Other Cancers

• Gynecological cancers (endometrial, cervical): SLNB is increasingly used and validated.
• Penile cancer and some GI malignancies: emerging applications.

Complications

• Allergic reaction to blue dye (isosulfan blue): rare (1:10,000); can be anaphylactic. Some centers premedicate with antihistamines + steroids + H2 blocker.
• Skin tattooing or necrosis if methylene blue injected subdermally (use peritumorally instead).
• Lymphedema: significantly less than with full axillary dissection.
• Radiation exposure from Tc-99m is minimal and considered safe.

Recent Evidence (2025-2026)

• The 2025 ASCO Guideline Update on SLNB in early-stage breast cancer reviewed evidence supporting current practice (Park KU et al., J Clin Oncol, 2025) - PMID 40209128.
• A 2025 systematic review and meta-analysis found that SLNB alone (without completion axillary dissection) in patients with residual nodal disease after neoadjuvant chemotherapy has acceptable oncological outcomes - PMID 40811983.
• A 2026 Lancet Oncology meta-analysis assessed survival benefit from SLNB for melanoma, adding to the evidence that SLN status guides meaningful treatment decisions - PMID 41796592.

Summary Table