Aims: Merkel cell carcinoma (MCC) is a rare, highly aggressive neuroendocrine skin malignancy with high-risk of locoregional spread. Adjuvant or definitive radiotherapy (RT) to the primary site is often required. Nodal status at time of diagnosis is a key prognostic marker for disease-free survival (DFS) in all-stage MCC. Sentinel lymph node biopsy (SLNB) availability is increasing, and studies have shown it improves ability to detect microscopic nodal disease over other imaging modalities. In SLNB node-negative MCC, the role of adjuvant nodal RT in early-stage MCC remains controversial due to the lack of prospective trial data. This prospective study aims to evaluate technical aspects of SLNB, and DFS following RT to primary site alone in node-negative early-stage MCC.
Method: This multi-centre non-randomised prospective cohort study recruited patients with stage cT1N0 MCC from 2017 to 2021. At time of diagnosis, participants were offered SLNB and allocated primary site RT alone if SLNB showed no nodal metastases. Patients had follow-up every 4 months over 2 years.
Results: 30 patients were eligible with 1 patient withdrawing consent (n=29). SLNB failed to map lymph nodes in 2/29 (6.9%). SLNB detected nodal micrometastases in 3/29 (10.3%). 24/29 (82.7%) were node negative. Following RT to the primary site, 3/24 (12.5%) had recurrence within the first echelon lymph nodes, 1/24 (4.16%) had distant metastases. DFS was 20/24 (83.3%) Acute toxicity from SLNB was mainly Grade 1 seroma. Late toxicity was uncommon (8.3%, 2/24), from persistent seroma and mild lymphoedema.
Conclusion: SLNB in early-stage MCC has a technical failure rate of 6.9%, micrometastatic detection rate of 10.3% and false-negative rate of 12.5%. SLNB side-effects were well-tolerated and there were uncommon late effects (8.3%). DFS with RT to the primary site alone was 83.3%. SLNB could be considered in select patients with early-stage MCC to de-escalate unnecessary nodal RT.