General Comment: Cancer stage definitions are disease specific.  Without putting more details in the narrative, you force every sender/receiver to research what is meant by the individual stage values to determine if they are sending the right data when they develop their FHIR submissions.  And you run the risk that someone interprets it differently. By putting more detail in the narrative, you give the development teams (Data Architects, Data Analysts, Quality Analysts) an understanding of what to look for in their source systems, how to set up tests, etc.  Otherwise, any errors might not be found until Biostatisticians wonder why the data looks so weird. 

Specific comment:

Stage definitions from NCI Thesaurus
I Localized disease, excised.
II Total gross resection with regional disease spread.
III Gross residual disease.

IV Distant metastases