Lumpectomy and sentinel biopsy

Why would a sentinel biopsy not be done first & then a decision made as to whether it should be a mastectomy or a lumpectomy?

[Cleveland Clinic]

Dear wanabehelper: A sentinel node biopsy is done to evaluate the lymph nodes for evidence of cancer.  This is part of the staging process which gives information that helps in determining treatment after surgery.  The finding of the sentinel node is done in conjunction with the surgery to remove the primary tumor.  The results of the sentinel node biopsy does not determine whether a mastectomy or lumpectomy is performed.  

[japdip]

I would assume that you had some sort of biopsy of the lump/tumor since a lumpectomy or mastectomy is now being considered. Nodes are never biopsied before the surgery. This is only to ascertain if the cancer has invaded the lymphatic system.

[wanabehelper]

I do not have cancer.  I am trying to help a very good friend in doing her research.  Yes, she had a biopsy with the results showing a stage 11 malignancy in her breast ~ 2 cm with clear margins.   Thanks for all the replies.