Background: Tumor (Breslow) thickness at diagnosis is an important prognostic variable in localized melanoma. Thin melanoma (T1: ≤1.0 mm) is the most common melanoma, and increasing T1 incidence has been observed in many countries. T1 is subclassified at 0.8 mm by including information on ulceration:  T1a (<0.8 mm without ulceration) and T1b (<0.8 mm and ulcerated, or ≥0.8–1.0 mm).
Objectives: The aim of this study is to investigate the risk of melanoma-specific and other cause death in patients with T1 melanoma, and how these risks change when the thickness threshold classifying T1a versus T1b varies between 0.5 and 1.0 mm.
Methods: Data for the patients diagnosed with a T1 melanoma in Norway, 1983-2019, were extracted from the Cancer Registry of Norway and the Norwegian Melanoma Registry. First, cumulative incidence of melanoma-specific (CI-MS) and other cause (CI-OC) death was estimated by thickness categories (≤0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 mm), by sex and overall. Then, we created six different subclassifications, T1a* and T1b*, using information on ulceration and six different tumor thickness thresholds (0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm).  Sex-specific and overall CI-MS and CI-OC death were estimated for each new classification of T1a* and T1b*.
Results: Among the 19,710 patients (aged 15-90) diagnosed with a T1 melanoma in 1983-2019, 756 died from melanoma and 3474 from other causes during a mean follow-up of 10.5 years (standard deviation 8.8). Overall and for each sex, CI-MS death increased with increasing thickness from ≤0.4 to 1.0 mm, while CI-OC death did not change by thickness category. CI-MS and CI-OC death were consistently higher in men than women. In men and women, 10-year CI-MS death was higher for melanoma diagnosed in 1983-2007 than 2008-2019 in all the thickness categories. The sex difference was smaller in the most recent period.
For all six thickness thresholds, CI-MS death was higher in T1b* than T1a* while CI-OC death remained similar, overall and by sex. For T1a*, CI-MS death was similar for all six thickness thresholds, while for T1b*, CI-MS death increased with increasing thickness threshold and more for men than women. No thickness threshold is standing out in our preliminary analysis.
Conclusions/Implications: In this ongoing study of T1 melanoma, risk of melanoma-specific death increased with increasing thickness. When classifying T1 melanoma based on different thickness threshold and ulceration, the risk of melanoma-specific death increased with increasing thickness threshold in patients classified as T1b* but not in those classified as T1a*.