A population-based validation study of the 8th edition UICC/AJCC TNM staging system for cutaneous melanoma

Background — The 8^th edition UICC/AJCC TNM8 (Tumour, Nodes, Metastasis) melanoma staging system introduced several modifications from the 7^th edition (TNM7), resulting in changes in survival and subgroup composition. We set out to address the limited validation of TNM8 (stages I-IV) in large population-based datasets.

Methods — This retrospective cohort-study included 6,414 patients from the population-based Ontario Cancer Registry diagnosed with cutaneous melanoma between January 1, 2007 and December 31, 2012. Kaplan–Meier curves estimated the melanoma-specific survival (MSS) and overall survival (OS). Cox proportional hazard models were used to estimate adjusted hazard ratios for MSS and OS across stage groups. The Schemper-Henderson measure was used to assess the variance explained in the Cox regression.

Results — In our sample, 21.3% of patients were reclassified with TNM8 from TNM7; reclassifications in stage II were uncommon, and 44.1% of patients in stage III were reclassified to a higher subgroup. Minimal changes in MSS curves were observed between editions, but the stage IIB curve decreased and the stage IIIC curve increased. For TNM8, Stage I (n = 4,556), II (n = 1,206), III (n = 598), and IV (n = 54) had an estimated 5-year MSS of 98.4%, 82.5%, 66.4%, and 14.4%, respectively. Within stage III, IIIA 5-year MSS was 91.7% while stage IIID was 23.5%. HRs indicated that TNM8 more evenly separates subgroups once adjusted for patient- and disease-characteristics. The variance in MSS explained by TNM7 and TNM8 is 18.9% and 19.7%, respectively.

Conclusion — TNM8 performed well in our sample, with more even separation of stage subgroups and a modest improvement in predictive ability compared to TNM7.