Urethral Melanoma - Clinical, Pathological and Molecular Characteristics.

Background: Mucosal melanoma involving the urethra is a rare disease with distinct clinical and molecular characteristics and poor outcomes. Our current knowledge is limited by the small number of reports regarding this disease.

Objective: To describe the clinical, pathological, and molecular characteristics of urethral melanoma.

Methods: We summarized the clinicopathologic data for 31 patients treated for urethral melanoma from 1986-2017 at our institution. Genomic data from our institutional sequencing platform MSK-IMPACT (n = 5) and gene-specific PCR data on BRAF, KIT, and/or NRAS (n = 8) were compared to genomic data of cutaneous melanomas (n = 143), vulvar/vaginal melanomas (n = 24), and primary non-melanoma urethral tumors (n = 5) from our institutional database.

Results: Twenty-three patients were diagnosed with localized disease, 7 had regional/nodal involvement and one had metastases. Initial treatment included surgery in 25 patients; seven had multimodal treatment. Median follow-up was 46 months (IQR 33-123). Estimated 5-year cancer-specific survival was 45%. No significant change in survival was observed based on a year of treatment.Primary urethral melanomas showed a higher frequency of TP53 mutations compared to cutaneous (80.0% vs. 18.2%, p = 0.006) and vulvar/vaginal melanomas (80.0 vs. 25.0%, p = 0.04). BRAF mutations were absent in urethral primaries (0% vs. 46% in cutaneous melanoma, p = 0.02). Tumor mutation burden was higher in cutaneous than urethral melanomas (p = 0.04). Urethral melanomas had a higher number of somatic alterations compared to non-melanoma urethral tumors (median 11 vs. 5, p = 0.03).

Conclusions: Our findings support a unique mutational landscape of urethral melanoma compared to cutaneous melanoma. Survival remains poor and is unchanged over the time studied.