I would say that its bonafide. Cancer does what cancer wants. Mammary tumors can be of different types, some more aggressive than others.
There is a syndrome that medical students are prone to, and that is thinking they have something they are studying about. Remember, very few things are so cut and dry that they are diagnostic by symptoms alone. For instance, if you get a head ache with nausea, it could be a migraine, a virus, something you ate or came in contact with or a number of other things. There is usually a cross over of symptoms from one cause to another.
If you think there may be a problem, the best thing to do is see a Dr. They have the diagnostic tools and are familiar with illnesses or diseases with the same symptoms. Although cancer or anything else may have enough symptoms for a Dr. to suspect a specific problem, but even they want to back up their diagnosis with tests.
By the way, you can still get breast cancer after top surgery. I was told after my surgery to do a breast exam now and then even with the majority of breast tissue removed. Tissue like breast tissue although different in looks to other tissue, its not like an organ that is well defined and removal is complete to the naked eye. Cis men can get breast cancer as well.
If you have breast cancer in your family, then definetly have yourself checked out. Some cis women actually get mastectomies when there is no sign of cancer if breast cancer is prevalent in their families. The lymph nodes in the area make matastasis common.
You will probably sleep better if you have been checked out by a Dr.
sam1234