Is mitotic rate still useful in the management of patients with thin melanoma?

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).

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.

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.

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.

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.