How staging of thin melanoma is changed after the introduction of TNM 7th edition: a population-based analysis

In 2009, the American Joint Committee on Cancer (AJCC) incorporated the tumor mitotic rate in the melanoma pathological TNM staging system. To investigate the effect of this change on the pT1 substaging of primary cutaneous melanomas, we reclassified the cases collected by a cancer registry according to the 6th and the 7th editions of AJCC melanoma staging.

Patients with pathological T1 melanoma diagnosed in the period 2000-2008 were selected from Tuscan Cancer Registry. The histological reports were reviewed and pT1 melanomas classified according to both the 6th and the 7th editions of the AJCC staging system. The shift of melanomas between pT1 substages was analyzed.

Among the 242 pT1 melanomas collected in the study period and with mitotic index available, there were 202 (83 % of all pT1) and 175 (72 %) pT1a, according to the 6th and the 7th editions of the AJCC melanoma staging, respectively. When the 7th edition was used, 20 % of all pT1a melanomas shifted to pT1b, and 32 % of all pT1b melanomas shifted to pT1a. A poor level agreement between the two TNM staging systems, measured by the Cohen's kappa coefficient, was found (K = 0.37).

The addition of mitotic activity to the pathological staging resulted in an increase in pT1b proportion and in a change in the classification of some cases. This modification could influence the clinical approach, with a different use of the sentinel lymph node biopsy, and underlines the role of mitosis evaluation in the management of thin melanoma patients.