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Author: Dr Sarah Hill, Dermatology Registrar, Waikato Hospital, Hamilton, New Zealand, 2007. Updated by Chief Editor, Hon A/Prof Amanda Oakley, July 2015.
Staging a cancer involves evaluating the primary tumour (the T stage), spread to the lymph nodes (the N stage) and spread of metastases elsewhere (the M stage). Sentinel node biopsy, often abbreviated to SNB, is a surgical procedure used to determine the N stage soon after the cancer has been diagnosed. It is often used in patients with cutaneous melanoma.
Cancer can spread through the body (metastasise) by three common routes:
Lymph vessels transport milky fluid called lymph, which drains through lymph nodes. Lymph nodes entrap foreign or potentially harmful substances, such as bacteria and cancer cells. Groups of lymph nodes are easily felt around the neck when they become enlarged and sore during an upper respiratory tract infection or sore throat. Other main groups of nodes are located in the groin (draining the leg), armpits (draining the arm), deep in the chest, and in the abdomen.
If the cancer has spread to the nearby lymph nodes, they may be obviously enlarged and are firm or hard on palpation. This may be detected by the patient or by a doctor on routine follow-up examination. Other reasons for enlarged lymph nodes include infection, inflammatory skin disease or a haematological cause (eg lymphoma).
Tests to find out if the nodes contain cancer cells include:
A pathologist examines FNA or biopsy tissue under a microscope and reports the findings to the surgeon. If the biopsy is positive for cancer, the surgeon may recommend that all the lymph nodes in the area be removed (completion lymphadenectomy).
Examining lymph nodes around the site of the primary cancer can help determine the extent of cancer spread via the lymph vessels. Spread to the nearby lymph nodes may reduce the chance of survival (prognosis), and the cancer may require different treatment.
Lymphadenectomy or lymph node dissection refers to the surgical removal of cancerous lymph nodes, usually after confirmation that they contain cancer cells.
Patients may be advised to have the draining lymph nodes removed ‘just in case’ they have collected cancer cells; this prophylactic procedure is known as elective lymph node dissection. In melanoma, there is no evidence for a survival benefit from elective lymph node dissection, ie, melanoma patients who have this procedure do not live any longer than those who do not.
Lymphadenectomy may lead to serious side effects including:
Sentinel node biopsy involves biopsy of a lymph node soon after the primary cancer is diagnosed. A sentinel lymph node is the very first lymph node to which lymph from the cancer site is likely to drain, but is not yet enlarged. A negative sentinel lymph node biopsy indicates a lower risk that the cancer has spread than if the biopsy shows cancer cells.
Sentinel node biopsy is performed by cancer surgeons as a staging procedure in some patients with skin cancer. Diagnoses include:
Melanoma specialists vary in their opinion regarding the value of the procedure. The use of sentinel node biopsy in melanoma patients has created much controversy in the medical literature. The clinical practice guidelines for the diagnosis and management of melanoma (Australia) updated in 2017, state:
Sentinel node biopsy provides:
Arguments against sentinel node biopsy in melanoma include:
The largest trial of sentinel node biopsy (MSLT-1 [1,2]) reported in 2014 that the procedure provides useful prognostic information in patients with intermediate-thickness or thick primary melanomas. The authors considered the data also showed a survival benefit for node-positive patients. The scientific conduct and reporting of this trial have been criticised, with some authors providing an argument against recommending the procedures [3,4,5,6]. A further study (MSLT-II ) published in 2017, found that immediate completion lymph-node dissection increased the rate of regional disease control and provided prognostic information but did not increase melanoma-specific survival among patients with melanoma and sentinel-node metastases.
Lymphatic draining patterns vary and it is not always obvious where to explore for metastatic cancer cells. To find the sentinel lymph node a doctor or technician injects a small amount of either blue dye or radioactive tracer near the original cancer. The success rate is best if both are used.
The radioactive tracer is an isotope of technetium (Tc). It results in less exposure to radiation than that received during a standard X-ray. It disperses over a short time. Often, a scan of all lymph node basins is performed soon after the tracer is injected, using a special camera (lymphoscintigraphy).
Twenty minutes or up to a few hours after the injection, a radioactive scanner (Geiger counter) is held over the skin and makes a noise when it encounters the radioactive tracer. Approaching from the direction of the original cancer, the first ‘hot spot’ encountered is the sentinel lymph node. A small cut is made into the skin overlying the area and the sentinel lymph node is removed. It is easier to find the node if dye has been injected, as it is stained blue. Sometimes more than one sentinel lymph node is detected in one or more body sites.
Sometimes the sentinel lymph node may not contain cancer while other lymph nodes do. If a surgeon is suspicious during the procedure, other lymph nodes may be removed for pathological examination. If the original or primary cancer has not yet been removed, the surgeon will do so after stitching up the sentinel node biopsy wound. In some cases, the primary has been removed previously, but the surgeon performs a wider excision after the sentinel node biopsy. How much extra tissue is removed depends on the nature and thickness of the original cancer.
In most cases, the patient may go home within a few hours of the surgery.
After the procedure the blue dye is excreted from the body in the urine, which may appear green for few days. The stain at the excision site fades away over a few months. The radioactive tracer dissipates over a short period of time.
A pathologist examines the sentinel lymph node, sometimes providing a report within an hour or so of the procedure, with the help of frozen sections. However, the report is more accurate if the tissue is fixed and processed in the normal way, which takes at least a day or so to complete. The diagnosis of cancer is not always easy, and special stains and/or other opinions may be required.
If the pathologist finds cancer on frozen section, the surgeon may remove other lymph nodes during the same procedure. If it is reported later, this surgery (called completion lymphadenectomy) will be rescheduled for another time. These other nodes may or may not also contain cancer cells. If pathology report is negative, wide excision of the lymph nodes is not necessary.
Sentinel node biopsy is unsuitable if:
Complication rates are 10% for sentinel node biopsy, and 37% for completion lymphadenectomy, if required.
Delayed wound healing, bleeding from the wound, wound infection, cellulitis, and other complications of sentinel node biopsy are more likely in patients with diabetes, obesity or heart disease, and in smokers. Other risks are listed below.
It is usual for the surgeon to arrange follow-up of all patients with melanoma and other high-risk skin cancers, whether or not the sentinel node biopsy was positive.
New targeted and immunotherapy drugs are leading to longer survival than historically expected in patients prescribed these for advanced or widespread melanoma. However, there is as yet no cure. Sentinel lymph node biopsy does not alter the prognosis of this disease and some authors question the usefulness of the procedure.
Immediate completion lymph-node dissection increased the rate of regional disease control and provided prognostic information but did not increase melanoma-specific survival among patients with melanoma and sentinel-node metastases. Faries, MB, et al. Completion Dissection or Observation for Sentinel-Node Metastasis in Melanoma. N Engl J Med 2017; 376:2211-2222 June 8, 2017 DOI: 10.1056/NEJMoa1613210
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