The prognostic role of mitotic rate in cutaneous malignant melanoma: Evidence from a multicenter study on behalf of the Italian Melanoma Intergroup.

BACKGROUND: This study aimed to improve the understanding of the prognostic value of tumor mitotic rate (TMR) in cutaneous melanoma and assessed its significance as a predictor for overall, melanoma-specific, and recurrence-free survival.

PATIENTS AND METHODS: This is a retrospective multicenter Italian cohort study of 13,016 patients diagnosed with and treated for invasive primary melanoma between 2005 and 2020 with median follow-up of 5.5 years. The survival probability was assessed by Kaplan-Meier method, hazard ratios (HRs), and corresponding 95% confidence interval (CI) of all-cause mortality and recurrence/death by multivariable Cox proportional hazards models.

RESULTS: Higher dermal mitoses number was associated with decreased overall survival. Among patients with TMR 0/mm2 , 1/mm2 , 2/mm2 -3/mm2 , 4/mm2 -10/mm2 , and >10/mm2 , 5-year overall survival (OS) was 97.3%, 93.6%, 88.3%, 73.0%, and 60.9%, respectively. In multivariate analyses, compared to TMR of 0/mm2 , HRs for all-cause mortality were 1.35 (95% CI, 1.08-1.68), 1.70 (95% CI, 1.40-2.07), 2.04 (95% CI, 1.67-2.49), and 2.39 (95% CI, 1.90-3.00) for 1 mitoses/mm2 , 2 mitoses/mm2 -3 mitoses/mm2 , 4 mitoses/mm2 -10 mitoses/mm2 , and >10 mitoses/mm2 , respectively. A similar increase in risks was observed in melanoma-specific survival (MSS) and recurrence-free survival (RFS). The HRs for MSS and RFS for the highest compared to the lowest TMR category were 3.01 (95% CI, 2.20-4.11) and 2.26 (95% CI, 1.88-2.73), respectively. Sentinel lymph-node biopsy positivity was significantly associated with TMR increase even with adjustment for several potential confounders.

CONCLUSIONS: A clear association was demonstrated between an increasing TMR and decreased OS, MSS, and RFS, suggesting a reconsideration of TMR prognostic role for future inclusion in the melanoma staging system.

PLAIN LANGUAGE SUMMARY: The 8th American Joint Committee on Cancer for melanoma staging removed tumor mitotic rate (TMR) from the staging criteria for T1 melanomas, giving way to ulceration and tumor thickness as stronger prognostic predictors. However, it is still recommended that TMR should be assessed and recorded in all primary invasive melanomas. In a large retrospective multicenter study on primary invasive melanomas, we investigated the prognostic value of TMR to assess its significance as survival predictor. Our results showed a clear association between increasing TMR and decreased patients' survival, suggesting that TMR should be considered for inclusion in the melanoma staging system.