The identification of the lymph node status is an integral component of the surgical primary therapy of breast cancer. Lymph node status still remains a significant prognosis parameter today, considerably influencing the decision on adjuvant therapy. For over 4 decades the complete removal of all axillary lymph nodes was the only option available to identify the lymph node status. Axillary dissection was a diagnostic and (in the case of lymph nodes with tumours) therapeutic measure to ensure local monitoring in the region of the lymph drainage area.
With sentinel lymph node excision (SLNE), a new procedure was introduced a few years ago which met the demands for high diagnostic precision and at the same time drastically reduced the morbidity of conventional axilla dissection. SLNE offers a targeted, minimally invasive surgical procedure with which the lymph node status can be precisely determined. The method is based on the fact that solid tumours show regular lymph drainage and are, in the first instance, drained via one (or a few) reproducible lymph nodes, the sentinel lymph nodes (SLN).
In the case of lymphogenic metastasation, the SLN are affected first, before other lymph nodes are involved. Therefore with a negative SLN, the removal of the other lymph nodes can be dispensed with.  With a positive SLN, a conventional axilla dissection must be carried out. Sentinel node excision is included in the S3 Leitlinie für die Diagnostik und Therapie des Mammakarzinoms (S3 Guidelines for the Diagnosis and Therapy of Breast Cancer) as follows:

The identification of the histological nodal status (pN status) is a constituent part of the surgical therapy of invasive breast cancer. This should be performed with the aid of sentinel node excision. For patients displaying a positive SLN, an axillary dissection with a distance of at least 10 lymph nodes is recommended. SLNE should not be carried out after neoadjuvant chemotherapy. This applies in particular to patients who show suspect lymph nodes prior to chemotherapy. Suspicion of advanced lymph node involvement in the axilla is a strong contraindication against SLNE. Lymphoscintigraphic marking is the obligatory standard procedure for the designation of the SLN. As radiopharmaceutical substance, 99m Tc marked colloids with a particle size of 20-100 nanogramm is used. Lymph drainage of the breast as a whole, including the covering skin, takes place via the axillary SLN on the same side.  It is only the region close to the breast wall which may have additional lymph drainage via the mediosternal lymph nodes. Peri-areolar intradermal localisation has proved beneficial as an injection site for the radiopharmaceutical substance.

During the operation the sentinel lymph nodes are first identified transcutaneously using a hand Geiger counter. After incision of the skin the Geiger counter is used further to locate the sentinel lymph nodes, which are then identified, removed and sent for rapid examination. In about15 % of cases more than 3 lymph nodes present as sentinel lymph nodes, so that correspondingly more lymph nodes have to be removed. If the removed lymph nodes are tumour-free, further axilla dissection is not necessary. In the event that the sentinel lymph nodes show tumour infiltration, a Level I or Level II  axilla dissection follows in order to be able to make a reliable statement about the lymph node status (pN status).
Rapid incision examination of the sentinel lymph nodes is an extremely safe procedure.

Despite initially negative lymph nodes in the rapid incision, in 5% of cases pathologists still susequently identify tumour infiltration in the final histological examination of the lymph node tissue, so that an axilla dissection has to be carried out again in a second surgical procedure. Nevertheless, SLNE is the standard procedure today for axillary staging in breast cancer.