Sentinel Lymph Node Biopsy for Back Melanoma
Sentinel lymph node biopsy should be performed for back melanomas based on Breslow thickness: routinely recommended for intermediate-thickness lesions (>1.0 to 4.0 mm), may be offered for T1b thin melanomas (0.8-1.0 mm or <0.8 mm with ulceration), and may be considered for thick melanomas (>4.0 mm) primarily for staging purposes. 1
Algorithmic Approach by Tumor Thickness
Thin Melanomas (<1.0 mm)
Do NOT perform routine SLNB for T1a melanomas (≤0.8 mm without ulceration), as the yield is too low to justify the procedure. 1
Consider SLNB for T1b melanomas (0.8-1.0 mm regardless of ulceration status, OR <0.8 mm with ulceration), as these patients have a slightly higher rate of sentinel node metastases. 1 The overall risk of nodal involvement in thin melanomas is approximately 5.1%, though subsets with ulceration and/or mitotic rate >1/mm² may have up to 20% positivity rates. 2
Additional high-risk features that may warrant SLNB in the 0.75-1.0 mm range include:
- Mitotic rate ≥1/mm² 1, 3
- Younger patient age (particularly ≤40 years) 4
- Presence of regression (which may increase sentinel positivity by nearly 6-fold) 5
For melanomas <0.75 mm, SLNB is generally not justified as no positive sentinel nodes were found in this subgroup in multiple studies. 6
Intermediate-Thickness Melanomas (>1.0 to 4.0 mm)
SLNB is strongly recommended for all T2 and T3 melanomas at any anatomic site, including the back. 1, 2 This provides accurate staging with high sensitivity and guides treatment decisions regarding adjuvant therapy. 1
The 10-year follow-up data from MSLT-I demonstrated significantly better disease-free survival in the SLNB group compared to nodal observation, and overall survival was improved when nodal metastases were diagnosed via sentinel node biopsy rather than clinical detection. 1
Risk factors for sentinel node positivity in this group include:
- Melanoma thickness 1.50-4.00 mm versus 1.00-1.49 mm 1
- Younger patient age (<60 years) 1
- Lack of tumor-infiltrating lymphocytes 1
- Lymphovascular invasion 1
- Microsatellitosis 1
Thick Melanomas (>4.0 mm)
SLNB may be recommended for T4 melanomas after thorough discussion with the patient about potential benefits and risks. 1 The probability of a positive sentinel node in thick melanomas is 30-40%, and SLN status is a strong independent predictor of outcome. 1
The primary value is for staging purposes and to facilitate regional disease control, though the evidence quality is lower than for intermediate-thickness melanomas. 1
Management After Positive SLNB
For patients with positive sentinel nodes, either completion lymph node dissection (CLND) or careful observation are acceptable options for low-risk micrometastatic disease. 1, 2
The MSLT-II and DeCOG-SLT randomized trials demonstrated no difference in melanoma-specific survival between CLND and observation groups, but lymphedema incidence was significantly higher with CLND. 1, 2
High-risk features that may favor CLND include:
- Extracapsular spread/extension 1
- Concomitant microsatellitosis of the primary tumor 1
- More than three involved nodes 1
- More than two involved nodal basins 1
- Immunosuppression 1
If CLND is not performed, the risk of regional nodal metastasis as first recurrence is approximately 15-20%, compared to 4.2% with CLND. 2
Critical Technical Considerations
Perform SLNB before or concurrent with wide local excision to avoid disrupting lymphatic drainage patterns. 7 The validity of sentinel node biopsy after prior wide excision is unknown, though patients may be considered on an individual basis. 1
Remove at least two sentinel nodes when possible, as false-negative rates are 31% with single-node sampling versus 12% with two nodes. 2
Verify residual radioactivity intraoperatively—it should be less than one-tenth that of the excised node with lowest radioactivity to confirm complete sentinel node removal. 2
Common Pitfalls to Avoid
The presence of one mitosis/mm² alone is no longer accepted as a primary indication for SLNB in thin melanomas by NCCN, as it is not a sufficiently powerful predictor when used as the sole criterion. 1, 5
Tumor regression does not consistently predict SLN metastasis across all studies, though some data suggest it may be an independent prognostic factor in specific subgroups. 4, 5
The anatomic location (back versus other sites) does not independently alter these recommendations—Breslow thickness and other histopathologic features remain the primary determinants. 1