I had to read about this kind of tumor https://www.mayoclinic.org/diseases-conditions/desmoid-tumors/diagnosis-treatment/drc-20446388

So it is non-cancerous in the sense that it does not spread to other parts of the body, but can grown where it is located and affect organs. Surgery, radiation and chemo may be used. It says that if it has grown to involve organs, some tissue may be left during surgery and those other treatments might help.

I don't know if anyone can chime in with actual experience. You might also try breastcancer.org, to get more opinions.