Extension tumor vs. Metastatic

I was recently diagnosed with stage IV metastatic breast cancer. Because I had a double mastectomy 3 years ago after I was first diagnosed with stage IIa breast cancer, followed by 4 rounds of chemo therapy, clean margins and no sign of cancer in lymph nodes, I believe my chances of the cancer spreading were 2%. At my consultation with the radiation oncologist she wondered if my tumor was an extension of my original tumor rather than metastatic. The tumor is only in my sternum. My original breast cancer was at the 9 o'clock position on my left breast, very close to the sternum. It is a large tumor, indicating it has been there for a while, but I thankfully do not have cancer anywhere else in my body.

From what I've learned so far if I am lucky enough that this is an extension of my original tumor, it is a very rare scenario. Because of that I'm having a really hard time finding doctors who are familiar with this and having a hard time putting together a plan moving forward for my care. Any information anyone has would be so gratefully appreciated. This is such a scary time. I know it is for everyone. Information is helping me, so anything is welcome. Thank you!