Can you say
Despite initial success, method A has not found its way into the canon of cancer treatments.
What I mean by "canon" is simply that method A is not one of those that a normal doctor at a normal hospital would suggest or would have learned about during his studies.
Looking at Merriam-Webster's definition, I think that the 4.th meaning might cover my usage, but I am not sure because the focus there is on 'principles', not 'methods'. Similarly, most other definitions that I looked at are even more limited to religious, poetic, or musical meaning.