Understanding the breast cancer related pathology would really help you out in deciding the future course of action. Breast cancer is considered a systemic disease since before it manifests, micro metastasis would have occurred. Hence just mastectomy is not sufficient. Following mastectomy you should undergo adjuvant chemotherapy alone or in combination with radiation. With a histopathology report of this kind (Positive for cancer) surgeon would prefer an radical approach and hence chances of recurrence is least provided the protocol is adhered to strictly.
I had a friend from high school with similar tumor. She had a lumpectomy, chemo, radiation and is fine at this point. I don't think she had lymph node involvement, though.
I had a larger tumor (5 cm) with 1 lymph node (2 mm) involved. I had a full mastectomy with lymph node removal. Chemo, radiation, reconstruction. All is good here, too.
We're both recent patients. My friend and I had discussed our situations. She seemed happy with her results, although she thought that the dimple from the lumpectomy was a bit awkward looking. My reconstruction looks a bit like frankenstein's boob, but is pretty under clothing. And I got a tummy tuck to boot! That was an additional plus since I had just had my last child.
If your surgeon is worth his weight, then he would know whether it would be better to remove a lump or the entire breast. If you are not certain of his qualifications on the answer to your question, ask your oncologist his opinion. They would be the best to judge what degree of removal is best in your situation. If you are still uncomfortable, talk to the surgeon about going ahead with the mastectomy.
Good luck on your surgery. I wish you a speedy recovery and an easy road back to complete health!