Background — The 8th edition UICC/AJCC TNM8 (Tumour, Nodes, Metastasis) melanoma staging system introduced several modifications from the 7th 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.
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