Identifying the high risk group in stage pT1bN0 cutaneous melanoma.

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9583 Background: In recent years, there has been a substantial improvement in prognosis for patients diagnosed with stage II or III melanoma due to the advent of adjuvant and neoadjuvant therapy. At the same time, significant percentage of melanoma deaths are caused by prevalent cases of thin melanoma. In this work, we aimed to identify the stage pT1bN0 melanoma subgroup with the worst prognosis. Methods: All cases of stage pT1b melanoma (with negative sentinel node biopsy) treated between 1998 and 2022 in high-throughput reference center were analyzed. Patients were followed for ten years or up to lost to follow-up or death. Patients alive at 10 year mark were censored. The patients were divided into groups with BOTH pT1b features (i.e., Breslow thickness > 0.8mm AND ulceration) vs those with just ONE of those features. Additionally, patients’ sex, the tumor’s pathological subtype, and mitotic index were analyzed. The Kaplan-Meier and the Cox proportional model were used in statistical analysis. Results: There were 665 cases identified. 63% of patients were female, median age was 49 (IQR: 38-60). 70% of patients were classified as pT1b due to Breslow thickness, 21% due to ulceration and 9% due to BOTH features. 48% of patients had superficially spreading melanoma, 3% - acral, 9% - nodular while other patients had other/unspecified subtype. The median mitotic index was 1/mm2, but this feature was missing in 46% of cases. Overall, 61 patients died during the follow-up. The 10-year overall survival (OS) rate for the whole group was 86% (95% CI: 83-90%). In a multivariable model, patients with BOTH IB features (HR 2.1, 95%CI:1.1-4.2), nodular or acral melanoma (HR 2.2, 95%CI:1.2-4.1) and males (HR 2.6, 95%CI:1.6-4.4) had worse prognosis; the mitotic index was not predictive for OS in this model. In patients with BOTH pT1b features the 10 year survival rate was 75% (95%CI: 62-89) and 87% (95%CI: 84-91%) in case of just one feature (log-rank p=0.008). Conclusions: There is a specific subgroup of patients diagnosed with stage IB who have a relatively bad prognosis. Focusing on perioperative treatment or on a more strict follow-up schedule in this group may prove beneficial.