Differential expression analysis for genes in different stages of cancer by TCGA clinical data

Question

[Zahra]

Hi all, I want to perform differential expression analysis for some genes in different stages of cancer by TCGA RNA-Seq data. I use the TCGA clinical data to separate patients according to their cancer stage, but the staging system of TCGA clinical data is TNM. How can I use this staging system to my aim, or How to convert this staging to the common staging system? (I, II, III, IV)?

Thanks for any help