Sentinel lymph node biopsy (SLNB) is not routinely recommended for T1a cutaneous melanoma due to the overall low risk of positivity. Prognostic factors for SLN positivity (SLN+) in this population are poorly characterized.

Determine factors associated with SLN+ in patients with T1a melanoma.

Patients with pathologic T1a (<0.80 mm, non-ulcerated) cutaneous melanoma from five high volume melanoma centers from 2001-2020 who underwent wide local excision with SLNB were included in the study. Patient and tumor characteristics associated with SLN+ were analyzed by univariate and multivariable logistic regression analyses. Age was dichotomized into ≤42 (25% quartile cutoff) and >42 years.

Of the 965 patients identified, the overall SLN+ was 4.4% (N=43). Factors associated with SLN+ were age ≤42 years (7.5% vs. 3.7%, odds ratio [OR] 2.14, p=0.03), head/neck primary tumor location (9.2% vs. 4%, OR 2.75, p=0.04), lymphovascular invasion (LVI) (21.4% vs. 4.2%, OR 5.64, p=0.01), and ≥2 mitoses/mm² (8.2% vs. 3.4%, OR 2.31, p=0.03). Patients <42 years with ≥2 mitoses/mm2 (N=38) had a SLN+ rate of 18.4%.

Retrospective Study CONCLUSION: SLN+ is low in patients with T1a melanomas, but younger age, LVI, mitogenicity, and head/neck primary site appear to confer a higher risk of SLN+.