sentinel lymph node biopsy

**Concept:** Lymphatic drainage from a tumor follows an orderly pattern to a first-echelon "sentinel" node. If that node is tumor-free, downstream nodes are very likely tumor-free (false-negative rate ~5-10%). If it is positive, the patient is upstaged and may need further management.

**Technique:** - Peritumoral or intradermal injection of a tracer: Technetium-99m sulfur colloid (radiocolloid, detected with a handheld gamma probe), isosulfan blue or methylene blue dye (visual detection), indocyanine green (fluorescence), or superparamagnetic iron oxide. - Dual tracer (radiocolloid plus blue dye) increases identification rate. Preoperative lymphoscintigraphy may map drainage, which is especially useful in melanoma of the trunk or head and neck. - The node(s) are excised and evaluated by frozen section, H&E, and sometimes immunohistochemistry (e.g., cytokeratin in breast, S-100/HMB-45/SOX10/Melan-A in melanoma).

**Indications:** - Breast cancer: clinically node-negative (cN0) invasive cancer. Trials such as ACOSOG Z0011 showed that for T1-T2 tumors with 1-2 positive SLNs treated with lumpectomy and whole-breast radiation, axillary lymph node dissection (ALND) does not improve survival. Newer data (e.g., SOUND, INSEMA) suggest SLNB can be omitted in selected low-risk older patients with small HR+ tumors. Axillary dissection is still needed for clinically positive nodes confirmed by biopsy, although targeted axillary dissection after neoadjuvant chemotherapy is an option. - Melanoma: Breslow thickness >1 mm (or 0.8-1 mm with ulceration or other high-risk features) and clinically negative nodes. MSLT-I showed that SLNB gives prognostic information and improves disease-free survival. MSLT-II showed that immediate completion lymph node dissection for a positive SLN does not improve melanoma-specific survival, so observation with ultrasound is now preferred. - Other uses: Merkel cell carcinoma, vulvar and penile cancer, some cutaneous squamous cell carcinomas, and selected endometrial, cervical, and oral cavity cancers.

**Contraindications / limitations:** - Clinically palpable or biopsy-proven nodal disease (go straight to ALND or therapeutic dissection). - Inflammatory breast cancer. - Prior extensive axillary surgery or radiation, which can disrupt lymphatic drainage and cause false negatives. - Pregnancy: avoid blue dye (anaphylaxis risk). Technetium is generally considered acceptable. - Large tumors or neoadjuvant therapy can reduce accuracy.

**Complications:** - Lower rates than ALND of lymphedema, shoulder dysfunction, seroma, and paresthesia from intercostobrachial nerve injury (numbness of the medial upper arm). - Anaphylaxis to isosulfan blue (rare). Blue dye also causes blue-green skin discoloration and falsely low pulse oximetry readings (methylene blue can also cause serotonin syndrome with SSRIs). - Tattooing or discoloration at the injection site.

**High-yield for boards:** - The SLN is the first node draining the tumor, not necessarily the closest one. - A positive SLN means metastasis and upstages the cancer (the N category in TNM staging). Nodal status is the most important prognostic factor in breast cancer and melanoma. - Axillary nodes are classified by their relationship to the pectoralis minor muscle: level I is lateral, level II is posterior, and level III is medial. - In a patient with a clinically negative axilla and breast cancer, SLNB is preferred over ALND to minimize lymphedema. - Know the two big melanoma trials: MSLT-I (SLNB is prognostic) and MSLT-II (no survival benefit from completion dissection).

**Clinical pearl:** SLNB is a staging procedure, not a treatment. Its main value is accurate staging with less morbidity than full lymphadenectomy.