What the operation does
A sentinel lymph node biopsy (SLNB) is a minimally invasive procedure to check whether breast cancer has started to spread to the lymph nodes under the arm. It samples only the first one to four nodes that the breast drains into (usually one or two nodes are removed) — the sentinel nodes — rather than removing the whole bed of axillary lymph nodes.
SLNB is now the standard of care for assessing the armpit in patients with breast cancer whose lymph nodes appear normal before surgery. It has largely replaced full axillary clearance as the first-line operation, because it gives the same prognostic information with a much lower risk of long-term side effects — particularly lymphoedema (arm swelling).
Why the armpit is assessed
In invasive breast cancer, knowing whether the cancer has reached the lymph nodes is a critical piece of prognostic information. It influences:
- The stage of the cancer
- Decisions about chemotherapy, radiotherapy, and hormone therapy
- Long-term surveillance planning
- In some situations, whether further axillary surgery is needed
Assessing the nodes does not cure the cancer on its own — but it is essential to building the right post-operative treatment plan.
How it works
The sentinel nodes are the first nodes the breast drains into, and the logic is straightforward: if cancer has not reached them, it has almost certainly not reached the nodes further up the chain. If it has reached them, the next steps depend on how much cancer is found.
The sentinel nodes are identified during the operation using:
- A radioactive tracer — a small amount of radioactive material injected into the breast the day before or on the morning of surgery, which drains into the sentinel nodes and can be detected with a small hand-held probe.
- A blue dye — injected at the start of the operation, which also drains into the sentinel nodes and makes them visible.
- Alternative tracers — some centres use a magnetic tracer (Magtrace, used with a magnetic probe; NICE-recommended, MTG72, 2022) or indocyanine green fluorescence, which avoid the need for radioactive material. The choice depends on what is available locally and on the surgeon’s preference.
Most UK surgeons use a dual-tracer technique (radioisotope plus blue dye) to maximise the chance of finding every sentinel node.
When SLNB is not the right first step
A sentinel lymph node biopsy is not used when:
- There is already biopsy-confirmed cancer in the lymph nodes before surgery — in which case the discussion is between a targeted axillary dissection, a full axillary clearance, or neoadjuvant chemotherapy first with re-assessment afterwards.
- The patient has had previous axillary surgery that has disrupted the normal lymphatic drainage, making a sentinel node harder to identify reliably.
For DCIS (non-invasive cancer) treated with lumpectomy, SLNB is not routinely done. For DCIS treated with mastectomy, SLNB is usually done at the same operation because a future sentinel biopsy becomes impossible after the breast has been removed.
Results and what they mean
The sentinel nodes are examined in detail by the pathologist after surgery. The detailed pathology result usually comes back about 1–2 weeks after surgery, and falls into one of the following categories:
- Node-negative (N0) — no cancer cells found. No further axillary surgery needed.
- Isolated tumour cells (ITC) or micrometastases — very small deposits of cancer. Usually no further axillary surgery needed; the finding is factored into the adjuvant treatment plan.
- One or two sentinel nodes with macrometastases — a larger deposit. In many current UK protocols, especially with radiotherapy planned to the breast or chest wall, no further axillary surgery is needed. In specific situations an axillary node clearance or targeted axillary radiotherapy may be recommended.
- Three or more sentinel nodes involved — more often an indication for full axillary clearance; axillary radiotherapy may be recommended instead in selected cases. Your MDT will advise.
Your MDT will review the result and any decision about further surgery is discussed with you, not imposed.
Lymphoedema risk
A small risk of lymphoedema (long-term arm swelling from lymphatic disruption) exists after SLNB, but it is much lower than after full axillary clearance — in the order of around 5% after SLNB versus 20–25% after axillary clearance, across published series.
Precautions to reduce risk include avoiding blood pressure readings, blood tests, or vaccinations on the affected arm where possible, and reporting any new or unexplained arm swelling or heaviness promptly.