Clinical and Pathological Predictors of Unfavourable Outcomes in Thin and In Situ Melanomas: A Retrospective Cohort Study from Taiwan.

Thin melanoma, defined as tumour thickness ≤1.0 mm, and melanoma in situ are increasingly diagnosed. However, acral-predominant Asian populations appear to experience higher rates of adverse outcomes than Western cohorts. This study aimed to identify prognostic factors among Taiwanese patients with early-stage melanoma. We reviewed 178 melanoma in situ/thin melanoma cases diagnosed from 1995 to 2025 at a tertiary centre and analysed overall survival, melanoma specific survival, recurrence-free survival, and distant metastasis-free survival using Cox and Fine-Gray competing-risk models. During a mean follow-up of 81 months, adverse events were not uncommon, including recurrence (8.8%), distant metastasis (4.7%) and melanoma-specific mortality (5.1%). Age was the most consistent predictor across endpoints in both Cox and competing-risk analyses. A Breslow thickness ≥0.8 mm independently increased the risk of melanoma-specific mortality, whereas ulceration and mitotic activity were not significant predictors. In conclusion, melanoma in situ and thin melanoma in this acral-predominant Asian cohort are not uniformly low risk. Older patients or those with tumours approaching or exceeding 0.8 mm in thickness warrant closer, risk-adapted surveillance.