The importance of mitosis ≥2 in selecting patients with T1 cutaneous melanomas for sentinel lymph node biopsy.

The 8th Edition of the American Joint Committee on Cancer (AJCC) Staging Manual removed the mitotic rate (MR) as a staging criterion for T1 melanomas, thus leading to a debate on sentinel lymph node biopsy (SLNB) in thin melanomas. This study investigates whether MR plays a role in selecting patients with T1 melanoma for SLNB. We analyzed clinical and histological data from the Florence Melanoma & Skin Cancer Unit database for 313 patients with a single thin melanoma who had undergone SLNB. We determined sentinel lymph node (SLN) positivity percentages in T1 melanomas according to the AJCC 8th Edition focusing on MR. Of the 313 T1 patients, 108 had MR = 0, 127 had MR = 1 and 78 had MR ≥2. The overall SLN positivity rate was 8.6%, (5.6% with MR = 0, 6.3% with MR = 1 and 16.7% with MR ≥2). The SLNB positivity rate in T1b melanomas was 12.1%, (8.5% with MR = 0, 5.7% with MR = 1 and 24.4% with MR ≥2), whereas in T1a melanomas it was 5.8%, (3.3% with MR = 0, 6.8% with MR = 1 and 8.1% with MR ≥2). In a logistic regression analysis, MR ≥2 had an odds ratio of almost three in comparison with MR = 0/1 also adjusting for thickness. Thus, MR ≥2 significantly predicted SLN metastases in T1 melanomas. Of those patients with positive SLN, 37% were classified as T1a according to the AJCC 8th edition. These findings underline the importance of MR ≥2 in selecting patients with T1 cutaneous melanomas for SLNB.