I can not give any definitive answer, but logically breasts are made up of breast tissue, mammary glands, blood vessels, etc. I would believe that there is no difference between breast cancer and breast tissue cancer. Specifically, my DX was invasive mammary carcinoma since I had so many different types of cancer cells involved. It was still breast cancer.
Maybe some of our more experienced ladies will weigh in for you on this issue.
P.S. If you are talking about a breast cancer metastisis to a distant site, it is still breast cancer, and will probably be treated as such.