Distinct clinicopathological and prognostic features of thin nodular primary melanomas: an international study from 17 centers.

BACKGROUND: Nodular melanoma (NM) is more likely to be fatal compared to other melanoma subtypes, an effect attributed to its greater Breslow thickness. METHODS: Clinicopathological features of NM and superficial spreading melanoma (SSM) diagnosed in 17 centers in Europe (n = 15), USA and Australia between 2006 and 2015, were analyzed by multivariate logistic regression analysis, with emphasis in thin (T1 ≤ 1.0 mm) melanomas. Cox analysis assessed melanoma-specific survival (MSS). All statistical tests were two-sided. RESULTS: In all, 20,132 melanomas (NM: 5,062, SSM: 15,070) were included. Compared to T1 SSM, T1 NM was less likely to have regression (OR: 0.46, 95% CI: 0.29-0.72) or nevus remnants histologically (OR: 0.60, 95% CI: 0.42-0.85), and more likely to have mitoses (OR: 1.97, 95% CI: 1.33-2.93) and regional metastasis (OR: 1.77, 95% CI: 1.02-3.05). T1 NM had a higher mitotic rate than T1 SSM (adjusted geometric mean 2.2 [95% CI:1.9-2.4] vs 1.6 [95% CI:1.5-1.7] per mm2, p < 0.001). Cox multivariate analysis showed a higher risk for melanoma-specific death for NM compared to SSM for T1 (HR: 2.10, 95% CI: 1.24-3.56) and T2 melanomas (HR: 1.30, 95% CI: 1.01-1.68), while after accounting for center heterogeneity, there was statistical significance only for T1 (HR: 2.20, 95% CI: 1.28-3.78). The NM subtype did not confer increased risk within each stratum (among localized tumors or cases with regional metastasis). CONCLUSIONS: T1 NM (compared to T1 SSM) was associated with a constellation of aggressive characteristics that may confer a worse prognosis. Our results indicate NM is a high-risk melanoma subtype that should be considered for inclusion in future prognostic classifications of melanoma.