When expanding CA (for instance, in diagnosis where it would normally be expanded), is cancer/carcinoma interchangeable? 

Dorland’s:

Cancer:  A neoplastic disease, the natural course of which is fatal.  The term includes the 2 broad categories of carcinoma and sarcoma, but is often used synonymously with the former.

Appreciate your thoughts on this.

Brenda, when you don't know positively whether the dictator intends "cancer" or "carcinoma" in using the abbreviation "CA," the conservative or safe choice for expansion would be "cancer."  Because, as you've discovered, according to the exact definitions, all carcinomas are cancer, but not all cancers are carcinoma.  So "cancer" encompasses "carcinoma" as well as sarcoma, and I would expand "CA" into "cancer" unless instructed otherwise by the author/dictator. 

Thank you very much for your response.