From the Guidelines
PET-CT cannot fully replace sentinel lymph node biopsy (SLNB) in melanoma staging. While PET-CT is valuable for detecting distant metastases in advanced melanoma (stages III-IV), it lacks the sensitivity needed to detect microscopic lymph node metastases that SLNB can identify. SLNB remains the gold standard for regional lymph node assessment in patients with clinically node-negative melanoma, particularly for intermediate-thickness melanomas (1-4mm) 1. The procedure involves injecting a radioactive tracer and sometimes blue dye near the primary tumor site to identify and remove the first draining lymph node(s) for histopathological examination. This provides crucial prognostic information and guides treatment decisions regarding completion lymphadenectomy and adjuvant therapy.
Key Points
- PET-CT's spatial resolution (approximately 5-7mm) simply cannot detect the small tumor deposits that SLNB can identify, which is why major melanoma treatment guidelines continue to recommend SLNB as the standard approach for nodal staging in appropriate candidates 1.
- PET-CT is better utilized for baseline imaging in high-risk patients and for surveillance after treatment of advanced disease.
- The decision to perform SLNB should be based on the patient's risk of a positive SLNB, with a risk of <5% not recommending SLNB, and a risk of 5%-10% discussing and considering SLNB 1.
- SLNB is an important staging tool, and although it has not been shown to improve disease-specific survival, a positive SLNB would upstage a patient to stage III, and adjuvant therapy has been shown to improve recurrence-free survival and overall survival in selected high-risk patients with stage III disease 1.
From the Research
PET CT vs SLNB in Melanoma Staging
- The use of PET CT as a replacement for Sentinel Lymph Node Biopsy (SLNB) in melanoma staging has been investigated in several studies 2, 3.
- According to a systematic literature review, SLNB is still considered the gold standard for assessing local lymph nodes in patients with melanoma, and PET CT has not been shown to be superior to SLNB for identifying occult locoregional lymph node metastases 2.
- Another study found that ultrasound is superior to computed tomography (CT) and/or positron emission tomography (PET)-CT for the detection of lymph node metastases, whereas PET-CT was superior for the detection of distant visceral metastases 3.
- The false-negative rate of SLNB is a concern, with several large-scale studies reporting a relatively high false-negative rate (5.6-21%) 4.
- Ultrasound-guided fine needle aspiration cytology (FNAC) prior to SLNB has been shown to be a useful tool in identifying metastases in the lymph nodes, with a sensitivity of 59% and a specificity of 100% 5.
- Ultrasound-guided core needle biopsy (US-CNB) has also been investigated as an alternative to FNAC for the diagnosis of lymph node metastasis in cutaneous melanoma, with high sensitivity, specificity, and positive predictive value 6.
Limitations of PET CT
- The likelihood of PET/CT identifying distant metastases in patients with intermediate-risk melanoma is equally low due to the small risk of having distant metastases at diagnosis 2.
- Further study is required, including larger multicenter prospective trials, to determine the role of PET CT in melanoma staging 2.
Alternative Diagnostic Tools
- Ultrasound has been shown to be superior to physical examination for the detection of lymph node metastases, and has been incorporated into many national guidelines for the follow-up of melanoma patients 3.
- US-CNB has diagnostic value similar to that of FNAC, and represents a reliable alternative method in melanoma lymph node metastasis to avoid surgery in patients who will not benefit from it 6.