What Is Sentinel Lymph Node Biopsy?

Introduction

Have you ever wondered how doctors determine the spread of cancer in the body? One crucial technique that plays a pivotal role in this process is - the sentinel lymph node biopsy (SLNB). 

SLNB procedure identifies and examines the first node cancer may spread to - this medical procedure is a key player in the field of oncology, helping healthcare professionals identify and assess the extent of the spread of cancer in a patient's body and guiding medical decisions for effective treatment.¹ 

This article will explore the contribution and significance of SLNB in the fight against cancer. 

The lymphatic system: lymph nodes 

You need to first understand the role of lymph nodes in your body to understand sentinel lymph node biopsy. 

The lymph nodes are small oval-shaped structures within our body that play an important role in the lymphatic system and the body’s immune system. The lymphatic fluid travels throughout the body, containing white blood cells and other immune cells, that circulate through the lymphatic vessels. Lymph nodes (containing immune cells) work as filters that trap and eliminate harmful substances carried by the fluid - bacteria, viruses, and cancer cells.²

How does cancer reach and spread to lymph nodes?

Cancer often spreads to the lymph nodes first. 

Cancer spread (metastasis) begins when cancer breaks away and spreads from the primary tumour (i.e., where it initially began) to other body parts via the bloodstream or lymph system.²  

Purpose of SLNB

SLNB serves a dual purpose in the detection of the spread of cancer and staging. 

Cancer diagnosis

SLNB provides vital insights into whether cancer has started to spread (metastasised) to nearby lymph nodes by identifying and examining the sentinel lymph node - this is the initial node where cancer may have spread towards from the primary tumour.^1,3,4 

Cancer staging

Usually, cancer patients are clinically staged through imaging techniques that can include the following techniques: CT, ultrasound, MRI and ¹⁸F-FDG PET/CT (¹⁸F-flourodeoxyglucose PET/computed tomography).^8,9  

Clinical staging 

The TNM staging is the most common system to determine:⁶ 

• Tumour (T) is the primary tumour size.
• Nodes (N) are the number and location of lymph nodes containing cancer cells. 
• Metastasis (M) is whether cancer has spread to other body parts. 

Lymph nodes can be classified either by:^6,7

• X, not measurable.
• 0, no cancer in the lymph nodes.
• 1-3, the higher the number, the more lymph nodes containing cancer cells. 

Pathological staging

• Pathological staging with SLNB 
• SLNB is a key player in the staging of cancer and aids in this staging process by identifying the sentinel lymph node's involvement.
• The SLNB procedure may be recommended by the specialist based on your cancer type and the stage of your cancer. 
• The information collected from SLNB helps doctors accurately figure out how advanced the cancer is, guiding them in choosing the right treatments. This precision helps catch the cancer early and lets doctors create personalised treatment plans that match the specific traits of the cancer - SLNB contributes to better and more individualised patient care and improves patient outcomes.^1,3,4,5

SLNB procedure overview

The SLNB procedure consists of two primary stages: the preoperative process, where the goal is to identify and locate the sentinel lymph nodes before surgery, followed by the intraoperative process, which entails the removal of these identified sentinel lymph nodes for subsequent examination.

Preoperative process

• Before the biopsy, doctors use imaging tools called 'lymphoscintigraphy' combined with 'SPECT/CT' to locate sentinel lymph nodes.¹¹
• Patients are injected with safe amounts of radioactive substances that travel along the lymphatic system. This is then visualised by lymphoscintigraphy, which creates a map and takes pictures of the lymph node, and where these nodes are placed are marked by the radiographer.^3,10,11
• Lymphoscintigraphy captures a broad view, while SPECT/CT provides a detailed 3D image.^10,11

Intraoperative process

• During the biopsy, patients are injected with general anaesthesia. 
• Once the patient is asleep, the patient is injected with a radioactive substance, blue dye, or both. The radioactivity is detected by a scanner, and the doctor scans the lymph node that has taken up the blue dye, helping them identify the ‘sentinel lymph nodes’. 
• Sentinel lymph nodes and skin around the area are removed. 
• The sentinel lymph nodes are sent and examined by a pathologist to see for any presence of cancer cells. 
• Positive results may mean further removal of lymph nodes - either within the same procedure or in a subsequent session.  
• Doctors can also follow up with patients to discuss the results and further treatment.^2,3

Indications for SLNB

SLNB procedure can be widely employed as a diagnostic tool for various cancer types, with breast cancer and melanoma being the most common. 

Breast cancer

Breast cancer patients may be offered SLNB to stage the cancer and potentially help prevent regional recurrence of the disease.¹³ 

The National Institute for Health and Care Excellence (NICE) recommends:¹²

• SLNB for patients with invasive breast cancer when there is either no indication of lymph node involvement detected through ultrasound or when an ultrasound-guided needle biopsy yields negative results. 
• SLNB for patients having a mastectomy for ductal carcinoma in situ.  

Melanoma

Depending on the stage your melanoma is classified, you may be recommended this procedure.³ 

NICE recommends SLNB for melanoma patients with:¹⁴

• Breslow thickness (0.8-1.0mm) and with at least one of the following characteristics: ulceration, lymphovascular invasion, or a mitotic index of 2 or higher.  
• Breslow thickness greater than 1.0mm.

Other solid tumours

• Penile cancer^15,16
• Womb cancer¹⁸
• Emerging role of SLNB for head and neck cancers
  • NICE recommends SLNB for early oral cancer patients¹⁹
• Ongoing research for SLNB for gynaecological cancers (vulvar, cervical, endometrial) and colorectal.¹⁷

Advantages

• It is an easy process - only requires a quick visit or a short stay after the procedure. 
• SLNB is safer and minimally invasive than most surgeries and is associated with reduced side effects and complications.^1,2,3

Limitations and potential risks

• Allergic reactions may occur due to the dye used in this procedure.
• You require general anaesthesia for this procedure.
• You might feel some pain or numbness or both around where the surgery happened. 
• There is a risk of infection, tightness due to scar tissue in the arm/leg (cording), swelling (lymphedema), extra fluid (seroma) or bruises (haematoma) around the wound can occur.
  • These side effects do not last long and may go away after a few weeks. Doctors may drain any extra fluid or blood and may prescribe medication if required. 
• False-negative results can still occur therefore, follow-up and regular checkups are important. ^1,2,3,13

FAQs

What is a sentinel node biopsy and why is it done?

Sentinel node biopsy/sentinel lymph node biopsy is a surgical procedure that is conducted by a professional to examine whether cancer has spread. 

What is the difference between a lymph node and a sentinel node?

Lymph nodes are lumps of tissues that fight and filter bacteria, viruses, and cancer cells carried by the lymphatic fluid. The sentinel node is the first node where cancer cells from the primary tumour may spread. 

What happens if the sentinel node is positive?

If the sentinel node is positive, cancer cells are present and have started to spread to lymph nodes and other body parts. 

Summary

• Sentinel lymph node biopsy (SLNB) is a procedure that serves as both a diagnosis and staging tool. 
• It plays a crucial role in understanding the extent of cancer spread by identifying the first lymph nodes that may be affected - it provides further information about the cancer, aids treatment decisions and can help prevent cancer recurrence in nearby areas. 
• Minimally invasive and reduced side effects and complications compared to more extensive surgeries. 
• SLNB is commonly used for melanoma and breast cancer cases and is emerging as a valuable tool for head and neck cancers and other cancer types.