Treatment

Yes ask for a second opinion but on answer to your question how do they decide on treatment based on no results. When a lump is visualised on ultrasound they sometimes can tell from its appearance whether it is a cyst, benign or malignant it isn’t used as the definitive diagnosis but it gives them a good indication so they will follow up with a mri and biopsy results it is good that they are being proactive it isn’t negative, I presume you are young and this is also a reason why they will be aggressive with their treatment 

Do you have the biopsy results yet? If so, the biopsy could tell them what is in there and point to a certain treatment path. Still, no harm in seeking a second opinion, and certainly you could ask for clarification of what you have been told so far.

They tailor your treatment plan to you so there will be a reason they want to give you chemo, maybe the type of cancer it is if they have your biopsy results.

I'm mid-40s and when they told me I needed chemo (after first being told just lumpectomy and rads), my surgeon said they wanted to chuck everything at me to make sure I live for decades to come. 

I was all for it!

It sounds like you don't have your full treatment plan yet and they haven't finished all their diagnostic tests (I think you're saying you still need MRI?) so I think it is a bit early to worry about 2nd opinions. But if you're not happy with final treatment plan, then no harm in asking again.

Worth remembering that none of the people you see act alone, they are part of a multi-disciplinary team who discuss your individual case.

Beatthebreast  I agree.  My treatmet plan changed once they had the results of all my diagnositc tests.  Lumpectomy, reduction and hormone tabs it was initially, then changed to mastectomy, chemo, radio and hormones. I too am in my 40's, and they have thrown everything at me and I have taken it with open arms and put my trust in the doctors xx