The most common technique when performing a SLN biopsy is to utilize both blue dye and radiotracer for lymphatic mapping. Although either technique alone is reliable when performed by experienced surgeons, the combination may be the most accurate for surgeons early in their SLN learning curve [13]. Injection of the radioactive tracer, typically technetium-99 m (^99mTc) sulfur colloid, should occur within 24 h of surgery and may be followed by lymphoscintigraphy to confirm localization. Injections are most commonly peritumoral or subareolar, with high identification and concordance rates between the two injection sites [14]. In addition, subareolar injection is generally favored for injection of non-palpable tumors and multicentric disease.

Injection of blue dye occurs in a similar fashion as radiocolloid. Either 1 % isosulfan blue (5 cc) or dilute methylene blue (1–2 cc diluted to 5 cc with normal saline) should be injected 5–15 min prior to the procedure in the periareolar or peritumoral location. Methylene blue should not be injected too superficially due to risk of skin or nipple necrosis. Furthermore, the anesthesiologist should be informed when injection of the blue dye occurs, as this can cause decreased oxygen saturation, mild blue rash or hives (0.4 %), or in rare cases a severe anaphylactic reaction (0.2 %) [12]. Gentle massage is applied to the breast for 3–5 min to facilitate drainage through the lymphatic channels to the nodal basin.

The majority of SLN biopsies can be conducted under regional block. If general anesthesia is required, paralytics are typically avoided. The patient should be positioned with the involved arm extended out on an arm board; some prefer to place a roll under the ipsilateral shoulder to elevate the axillary contents into the field. After the blue dye has been injected, the ipsilateral chest and involved axilla are prepped. The hand-held gamma probe is then used to localize the area of maximal radioactive uptake within the axilla, and this spot is marked. For patients undergoing mastectomy, the SLN can be identified through the mastectomy incision. For breast conservation patients, it is helpful to consider what the optimal ALND incision would look like when planning the SLN incision (see ALND section next). A small portion of this ALND incision which overlies the area of greatest radioactivity can then be used for the SLN biopsy. Most commonly, this is an approximately 3 cm incision just inferior to the axillary hairline. If the radiotracer has failed to identify a SLN, the incision should be made at the base of the axillary hairline, just posterior and perpendicular to the pectoralis major.

A scalpel is used to make the skin incision, and electrocautery used to dissect down through the subcutaneous tissue and clavipectoral fascia to expose the axillary contents. Utilizing the blue lymphatic channels and gamma probe as a guide, a combination of blunt dissection and electrocautery is used to localize SLNs. All nodes should be harvested that meet the following criteria: the “hottest” (most radioactive) node, nodes with ex vivo counts >10 % of the hottest node, any blue node, any node at the end of a blue lymphatic channel, or suspicious nodes. All lymph nodes removed should have an ex vivo count taken using the gamma probe. Once all SLNs have been excised, a final background count of the axilla should be obtained. If this final count is higher than 10 % of the “hottest” node, a search for additional SLNs should be performed. Dissection should be performed along lymphatic channels and close to the SLNs to minimize damage to surrounding tissues. Although rarely visualized during the SLN biopsy procedure, care should be taken to avoid injury to the thoracodorsal or long thoracic nerve, as the SLN dissection is sometimes quite deep in the axilla.

The options for intraoperative pathologic evaluation of a SLN includes frozen section or touch cytology (i.e. stamping the cut SLN pieces onto a glass slide and performing hematoxylin and eosin stain). Intraoperative pathologic evaluation is controversial in patients undergoing partial mastectomy, as not all patients may require a full ALND per the American College of Surgeons Oncology Group (ACOSOG) Z0011 trial [15]. However, if planning a completion axillary dissection in the case of a positive SLN, intraoperative evaluation may allow for completion ALND in a single operation.