Which lymph nodes need to be sampled in a patient with melanoma on their back?

Lymph Node Sampling in Melanoma of the Back

For a patient with melanoma on their back, sentinel lymph node biopsy (SLNB) should be performed for tumors ≥1 mm Breslow thickness, with lymphoscintigraphy used preoperatively to identify all draining basins, as drainage patterns from the back are highly unpredictable and can involve multiple unexpected nodal basins including the triangular intermuscular space, axillary, inguinal, and even para-aortic/retroperitoneal nodes. 1, 2

Indications for SLNB Based on Tumor Thickness

• Routine SLNB is NOT recommended for T1a melanomas (≤0.8 mm without ulceration) 1
• SLNB may be considered for T1b melanomas (0.8-1.0 mm or <0.8 mm with ulceration) after thorough patient discussion of risks and benefits 1
• SLNB is normally considered for melanomas ≥1 mm thickness, where approximately 20% are positive 1
• SLNB should be strongly considered for melanomas 1.2-3.5 mm thick, as this group shows improved distant metastasis-free survival with the procedure 1

Critical Importance of Lymphoscintigraphy for Back Melanomas

Preoperative lymphoscintigraphy is mandatory for trunk melanomas because clinical prediction of lymphatic drainage is impossible and old guidelines based on Sappey's lines should be abandoned 1, 2, 3. The back presents particularly complex drainage patterns:

• Drainage from the back can occur to unexpected sentinel nodes in the triangular intermuscular space (between trapezius, latissimus dorsi, and rhomboid muscles) 2, 3
• Some patients demonstrate drainage through the posterior body wall to para-aortic, paravertebral, and retroperitoneal nodes 2, 3
• Drainage across the midline is quite common in trunk melanomas 1, 2
• Interval nodes (lying along lymphatic vessels between the lesion and recognized node fields) are not uncommon, especially on the trunk 1, 2, 3
• The sentinel node is not always in the nearest node field and drainage patterns are highly variable even from the same skin area 2

Technical Requirements for SLNB

• Technetium-99m filtered sulfur colloid (0.5 mCi) should be injected intradermally around the melanoma or excision scar 10-15 minutes before surgery 4
• Small-particle radiocolloids are essential to visualize actual lymphatic collecting vessels draining into sentinel nodes 2, 3
• Intraoperative gamma probe detection is critical, with success rates directly related to probe use in all lymph node basins 5
• The average number of sentinel nodes removed is approximately 2 per patient 4

Evaluation of Identified Sentinel Nodes

• Multiple sectioning and immunohistochemistry are required for sentinel node evaluation 1
• Immunohistochemistry increases positivity detection by 40% compared to hematoxylin and eosin staining alone 5
• All true sentinel nodes must be biopsied regardless of location, as micrometastatic disease can be present in any sentinel node 2, 3

Management of Clinically Suspicious Nodes

Nodes clinically suspicious for melanoma should be sampled using fine needle aspiration cytology (FNAC) prior to formal block dissection 1. However:

• If FNAC is negative but lymphocytes were seen, core or open biopsy should be performed if suspicion remains 1
• Open biopsy incisions must allow subsequent complete formal block dissection without compromise 1
• Prior to lymph node dissection, staging by CT scan should be performed unless this would cause undue delay 1

Common Pitfalls to Avoid

• Never rely on clinical prediction of drainage patterns for back melanomas—lymphoscintigraphy is essential 2, 3
• Do not assume the nearest node field contains the sentinel node—drainage can skip expected basins 2
• Failure to identify all sentinel nodes occurs in approximately 5% of cases when radiocolloid fails to migrate 1
• Missing interval or unexpected nodes can result in understaging, particularly critical for trunk melanomas 2, 3
• Consider nodal basin ultrasound prior to SLNB for patients with equivocal regional lymph node physical exam, though this is not a substitute for SLNB 1

Expected Outcomes

• Sentinel node positivity correlates directly with Breslow thickness (p<0.001) 5
• Patients with melanoma 1.2-3.5 mm thickness and positive SLNB have 75% 5-year survival compared to 90% if SLNB is negative 1
• The procedure carries approximately 5% morbidity, less than complete nodal dissection 1
• Success rates for sentinel node localization improve with surgical experience 5