Sentinel Node Biopsy
Sentinel node biopsy removes the first lymph node or nodes that receive lymph from a tumour, to find out whether cancer has spread, without removing all the regional nodes. If the sentinel node is free of tumour, the other nodes in the basin are very likely free as well, and a full node dissection, with its risk of lymphoedema, is avoided.
The sentinel node is found with a tracer injected near the tumour. The tracer travels in the lymphatic channels to the first node. Detection uses a radioactive tracer (technetium-99m-labelled colloid or tilmanocept) with a gamma probe, a blue dye, a fluorescent dye (indocyanine green), or magnetic particles, alone or in combination.
The procedure is done under general or local anaesthesia, usually with the primary tumour excision, and takes 30 to 90 minutes. Lymphoscintigraphy the day before or on the day shows the draining basin and the number of nodes. Pathology reports in 3 to 7 days, or at the time of surgery if a frozen section is used.
Identification rates are over 95% with the combination of tracers. The false negative rate (a tumour free sentinel node when other nodes contain tumour) is about 5 to 10%. Sentinel nodes contain cancer in about 15 to 20% of melanomas with suitable thickness and about 25 to 30% of invasive breast cancers.
Indications.
- Melanoma: tumour thickness of 0.8 mm or more, or thinner with ulceration or other high risk features, with no clinical evidence of nodal disease.
- Breast cancer: invasive cancer with clinically node negative axilla (assessed by examination and ultrasound).
- Vulval and penile squamous cell carcinoma with clinically negative groins, in tumours beyond a stage.
- Early cervical and endometrial cancer, in selected centres.
- Oral cavity squamous cell carcinoma (early stage).
- Merkel cell carcinoma and some high risk skin squamous cell carcinomas.
- Selected colon, gastric and thyroid cancer in specialist settings.
Not indicated when nodes are clinically involved (the diagnosis is made by aspiration or core biopsy, followed by node dissection or systemic treatment), or in tumours with very low risk of spread.
Contraindications and precautions.
- Clinically positive nodes or proven nodal metastases.
- Previous surgery or radiotherapy that has disrupted lymph drainage in the area (wide excision, axillary surgery), which lowers the identification rate.
- Allergy to the blue dye (anaphylaxis in 0.1 to 2%), which needs observation and resuscitation facilities. Dye allergy is a reason for using radiotracer or fluorescent dye alone.
- Pregnancy: the radiotracer dose is small and the procedure can be done when needed, while the blue dye is avoided.
- Inflammatory breast cancer and large, locally advanced tumours.
- Known multifocal tumours, or prior excision biopsy: the accuracy is lower and local policy applies.
- Patients unable to have general anaesthesia: local anaesthesia and radiotracer alone are used.
Preparation and technique.
Consent includes the risks of dye reaction, lymphoedema, numbness and false negative results. The tumour site and the lymph nodes at risk are reviewed on imaging.
The radiotracer is injected into or around the tumour or the biopsy scar (intradermal for melanoma and breast, peritumoural or subareolar for breast) on the day before or on the morning of surgery. Lymphoscintigraphy images show the sentinel nodes and map them on the skin.
In theatre, blue dye or indocyanine green is injected, the area is massaged, and a gamma probe or fluorescence camera finds the nodes. A small incision is made over the signal, and the nodes that are radioactive, blue, fluorescent, or palpably abnormal are removed until the background count falls to a low level (a node is removed if its count is at least 10% of the hottest node).
The nodes are labelled by the site, sent fresh or in formalin, and marked as sentinel or non-sentinel. The wound is closed, and drains are rarely needed.
