BACKGROUND: The definition of T1b cutaneous melanoma was changed in the 8th edition of the American Joint Committee on Cancer (AJCC) staging system based on survival differences but not risk of sentinel lymph node (SLN) metastases. We sought to evaluate changes in SLNB use and rates of positive SLNB in response to updated staging criteria, and to evaluate the incidence of high-risk features in T1a melanoma in whom SLNB is now recommended. STUDY DESIGN: The 2021 National Cancer Database Melanoma Participant User File was used to obtain SLNB use and positivity rates in T1 (thin) melanoma (thickness 1.0 mm or less) from 2012 to 2021. Rates were compared between AJCC 7th (2012 to 2017) and 8th editions (2018 to 2021). Factors associated with SLNB use in nonulcerated T1 melanoma were evaluated. The presence of high-risk features in T1a melanoma was identified and SLN positivity rates were reviewed. RESULTS: A total of 136,966 cases were included with 76,485 (55.8%) cases diagnosed under the AJCC 7th edition era (2021 to 2017). The overall proportion of patients with thin melanoma undergoing SLNB was relatively stable over the time periods, roughly 30%. In the AJCC 8th edition era, the overall SLNB positivity rate slightly increased from 4.5% to 6.6% (p <
0.001). There was increased SLNB use in melanoma with a thickness of 0.8 to 1.0 mm (T1b: odds ratio 2.53 [95% CI 2.31 to 2.78]) and decreased use when the thickness was less than 0.8 mm (T1a: odds ratio 0.80 [0.76 to 0.85]). The rate of SLNB positivity increased in both thickness groups over time. CONCLUSIONS: After implementation of the AJCC 8th edition staging criteria, surgeons have become more selective in SLNB use with a resulting increase in SLNB positivity rate. Fewer SLNBs in T1a and more SLNBs in nonulcerated T1b are being performed.