You will have chemotherapy, right? This is done to make sure that if your cancer has spread anywhere in the body, it will be eradicated. There is really no point in scanning every inch of your body for metastatic cancer, because absolutely no test will pick up micro, or very small, cancer cells. So, the chemo is both curative and preventative.
just because your cancer has spread to your lymph nodes that does not mean you have metastatic, or stage 4 cancer.
like yours, my cancer was also described as metastatic because of the lymph node involvement. My cancerous lymph nodes also did not show up on MRI, Ultrasound and/or Mammogram. They showed up as the result of surgery and I had chemotherapy before surgery.
So, it does not really matter, since they are treating you as aggressively as they can, and also like they would if there was cancer in other parts iof the body.
But lets say, you had every imaginable test, and it showed all negative results, you could still have tiny cancer cells that did not show up. Do you understand what I mean?
Your situation is really not that rare, or that special, and not even the most knowledgeable doctor can give you exact answers.
Please consult your Oncologist regarding screening tests.