Background: T1 melanoma substaging was recently modified by the American Joint Committee on Cancer (AJCC). Although sentinel lymph node (SLN) positivity is the most important prognostic factor in melanoma, there is a lack of consensus on whether SLN biopsy should be performed in patients with thin melanoma (≤1 mm).
Objective: The main aim of this study was to investigate predictors of SLN positivity in patients with thin melanoma, with a special emphasis on mitotic rate. A secondary aim was to evaluate survival in this group of patients.
Materials and methods: Retrospective multicenter observational study with analysis of age, sex, tumour location, thickness, mitotic rate, regression and microscopic satellites. Predictive factors were identified using a classification and regression tree (CART) approach. Melanoma-specific survival according to SLN status was estimated using Kaplan-Meier curves.
Results: We analysed 203 patients with a melanoma ≤1 mm. Using the new AJCC staging criteria, the CART algorithm identified a 7.5% likelihood of SLN positivity in T1a patients. In the case of T1b melanoma, there was a 14.3% likelihood of SLN positivity in patients with a mitotic rate >1 mitosis/mm2 and a 3.2% likelihood in those with ≤1 mitoses/mm2 . None of the patients with T1b disease who had ≤1 mitoses/mm2 and regression had SLN positivity. In T1b patients, 5-year melanoma-specific survival was 98.7% in the SLN-negative group and 75% in the SLN-positive group (P = 0.05). When stratified by mitotic rate, survival was 100% for patients with a mitotic rate of ≤1 mitoses/mm2 and 91.4% for those with >1 mitosis/mm2 (P = 0.022). There were no deaths in the T1a subgroup.
Conclusions: Sentinel lymph node metastasis was less common in patients with T1b melanoma who had a mitotic rate of ≤1 mitoses/mm2 . Performance of SLN biopsy should be carefully considered in this subgroup of patients, particularly considering the good prognosis.
© 2017 European Academy of Dermatology and Venereology.