Sentinel lymph node biopsy in patients with thin melanomas: Frequency and predictors of metastasis based on analysis of two large international cohorts

Background: Sentinel lymph node (SLN) metastasis in patients with thin melanomas (≤1 mm) is uncommon but adverse prognostic factors may indicate an increased risk. We sought to determine how often SLN biopsy (SLNB) was performed in patients with thin melanomas, establish the frequency of SLN metastasis and evaluate the predictive value of ulceration, tumor mitotic rate, and thickness for SLN involvement. Methods: Melanoma patients with a Breslow thickness greater than or equal to 0.5 to less than or equal to 1 mm, diagnosed 2009-2016, were identified in the Swedish Melanoma Register (SMR) and the Melanoma Institute Australia (MIA) Database. Results: In total 8165 patients were included from the SMR and 1603 from MIA. SLNB was performed in 9.5% and 16.2% of patients, respectively. Corresponding figures for T1b (American Joint Committee on Cancer [AJCC] 7th Edition) were 19.5% and 24.6%. The SLN positivity rate were 4.4% (Sweden) and 5.8% (MIA). SLN metastasis was more frequent in tumors with ulceration, mitoses, and Breslow thickness greater than or equal to 0.9 mm but none were statistically significant. Younger age was identified as a significant risk factor for SLN positivity at MIA. Conclusions: A minority of patients with thin melanomas had SLNB performed and the SLN positivity rate was low. This study did not confirm tumor ulceration, mitoses, or thickness as statistically significant predictors for SLN metastasis.