Breast cancer affects many women. One of the places to which it can spread is the lymph glands under the arm. The type of treatment offered to patients often will depend on whether those lymph glands have cancer in them or not. For this reason, a standard recommendation is that women with breast cancer have these lymph glands removed with surgery. This cancer causes side effects including numbness, pain, decreased ability to move the arm and arm swelling. A new type of surgery which looks only at the first gland that a cancer drains to (sentinel node biopsy) may help to avoid having to remove the glands under the arm. Also, a new way of imaging the glands under the arm called Positron Emission Tomography (PET) scanning may also give a better idea of the chance that these glands have cancer in them. This study is determining whether PET scans before surgery and sentinel node biopsy can decrease the need for a complete axillary dissection.