diagnosis without biopsy

I had a large lymph node under my arm biopsied and it came back positive for cancer.  Without an MRI or breast biopsy to confirm, I was told I had breast cancer and was given two options for treatment -- lumpectomy or mastectomy followed by a course of chemotherapy.  How can I be sure I have breast cancer?  Isn't a biopsy of the spots found in my breast by the follow up MRI  required for an accurate diagnosis?  The 4 spots ranged between 0.7 cm and 2.5 cm.  Trying to figure out what my next steps should be.

[Cleveland Clinic]

Dear jmcb1:  If the biopsy of the lymph node shows malignant breast cells, it is positive for breast cancer and one can assume that the origin is the "spots" in the breast.  The question if there are multiple spots in the breast is whether lumpectomy could remove all of the areas and leave enough healthy breast tissue or whether mastectomy would be necessary.