Improving diagnostic accuracy in atypical melanocytic tumors using p16 immunohistochemistry and 9p21 fluorescence in situ hybridization: analysis of 206 second opinion cases.

Diagnosing atypical melanocytic tumors can be challenging without molecular characterization, necessitating simple tools to enhance diagnostic accuracy in daily practice. This study retrospectively analyzed the utility of p16 immunohistochemistry (IHC) and 9p21 fluorescence in situ hybridization (FISH) on 206 tumors referred for expert second opinion. The performance of p16 and 9p21 was compared to histological diagnosis (both initial and final respectively without and with p16 and 9p21 status), histological subtype, and follow-up data. Negative p16 immunolabelling detected 90% of malignant cases, while only 11% of benign tumors were p16 negative. Homozygous 9p21deletion detected 42% of malignant tumors and excluded 95% of benign ones. Heterozygous deletion showed no diagnostic value. Homozygous 9p21 deletion significantly improved diagnostic confidence (P < 0.001), leading to tumor upgrading (n = 23) or melanoma confirmation (n = 22). Among 97 patients with follow-up, 17 had adverse outcomes. Kaplan-Meier analysis showed no significant difference in progression-free survival between groups (P = 0.64). Combining both techniques ultimately enhanced histological diagnostic confidence in daily practice. However, in cases where p16 is negative without homozygous deletion, or where histological malignancy is uncertain and p16 positive, other p16-inactivation mechanisms or molecular anomalies should be considered, necessitating further molecular investigations.