Sentinel lymph node biopsy (SLNB) is the minimally invasive technique used to determine whether cancer has spread to the axillary lymph nodes, by identifying and removing only the first one to three nodes most likely to be affected rather than removing all axillary lymph nodes.

It is now the standard of care for axillary staging in early breast cancer and has transformed the management of the axilla over the past two decades.

Prior to the development of SLNB, axillary staging required the removal of all lymph nodes in the armpit — a procedure called axillary dissection — even in patients whose nodes turned out to be cancer-free. This exposed a large proportion of patients to the risks and long-term consequences of extensive axillary surgery, most significantly lymphoedema (chronic arm swelling), when they would never have benefited from it. Sentinel node biopsy has spared the majority of patients from this outcome.

Identifying the Sentinel Node

Identifying the sentinel node requires mapping the lymphatic drainage of the breast before and during surgery, using a combination of tracer techniques.

Lymphoscintigraphy — on the day before surgery or on the morning of the operation, a small amount of radioactive tracer (a weakly radioactive colloid) is injected near the nipple or tumour site under local anaesthetic. The tracer travels through the lymphatic system to the sentinel nodes. A nuclear medicine scan is then performed to create a map showing the location and number of sentinel nodes, which is reported by a nuclear medicine specialist and guides the surgical team.

Blue dye injection — at the start of the operation, a small amount of blue dye is injected into the breast under general anaesthetic. The dye travels through the lymphatic channels and stains the sentinel nodes blue, providing a real-time visual guide to their location during surgery. Patients should be aware that blue dye can temporarily discolour urine and skin for 24 to 48 hours, and very rarely causes an allergic reaction.

Gamma probe detection — during the operation, a handheld gamma probe detects the radioactive signal from the tracer, allowing the surgeon to precisely locate the sentinel nodes through the overlying tissue, make a small targeted incision in the axilla, and remove only the nodes that are radioactive, blue-stained, or both. The combination of radiotracer and blue dye achieves the highest sentinel node identification rate and is the standard dual-tracer approach used in our practice.

The Sentinel Node Biopsy Procedure

Sentinel lymph node biopsy is performed under general anaesthetic as part of your main breast cancer operation, alongside lumpectomy or mastectomy. It does not require a separate hospital admission.

A small incision — typically 2 to 4 cm — is made in the natural skin crease of the axilla, although in many cases the sentinel node excision can be performed via the lumpectomy or mastectomy incision. The surgeon uses the gamma probe and visual identification of blue-stained lymphatic channels to locate and remove the sentinel node(s), typically between one and four nodes. The axillary incision is closed with dissolving sutures and heals with a small, inconspicuous scar.

The sentinel nodes are sent to the pathology laboratory for detailed microscopic examination, with results typically available within 48 to 72 hours.

What the Sentinel Node Result Means

Sentinel nodes clear (negative) — if no cancer cells are found in the sentinel nodes, this indicates with high confidence that the remaining axillary lymph nodes are also free of cancer, and no further axillary surgery is required. This is the result for the majority of patients with early breast cancer and spares them from the risks and recovery of full axillary dissection. A negative sentinel node result is an important positive prognostic indicator.

Sentinel nodes positive — micrometastases and isolated tumour cells — when only very small deposits of cancer are found in the sentinel node (micrometastases less than 2mm, or isolated tumour cells less than 0.2mm), evidence from major clinical trials has established that in many patients undergoing breast-conserving surgery followed by radiotherapy, further axillary dissection does not improve survival or regional control and can be safely omitted. The decision is always made in the context of the full clinical picture and discussed at the multidisciplinary team meeting.

Sentinel nodes positive — macrometastases — when clearly involved cancer deposits larger than 2mm are present in the sentinel node(s), further axillary management is typically recommended. The approach depends on the extent of nodal involvement, the planned adjuvant treatments, and other clinical factors. Options include completion axillary lymph node dissection and, in selected patients, management with axillary radiotherapy rather than surgery. Your surgeon will discuss the current evidence and what is recommended for your specific situation.

Risks of Sentinel Node Biopsy

Sentinel lymph node biopsy is a safe and minimally invasive procedure with a significantly lower complication profile than full axillary dissection. Potential side effects include temporary bruising, swelling, and tenderness at the axillary incision site; seroma (fluid collection), which is common and usually resolves without intervention; numbness or altered sensation in the inner upper arm from disturbance of the intercostobrachial sensory nerve; lymphoedema, with a risk of approximately 5% after sentinel node biopsy alone; and the very small risk of an allergic reaction to blue dye.