Sentinel lymph node mapping

What is a sentinel lymph node?

Lymph nodes are small parts of the immune system that filter fluid draining from tissues. Some cancers can spread through these drainage channels to a lymph node.

The first node receiving fluid from the tumour area is called a sentinel lymph node. There may be more than one. The draining node is not always the closest node. A node can contain cancer cells even when it feels normal in size.

Mapping finds which node to check. It does not show whether cancer is present. To answer that question, a sample of cells or tissue must be examined in the laboratory.

Why might your pet need it?

Checking how far a cancer has spread is called staging. A lymph-node result can change which operation is recommended, the need for other treatment and what to expect afterward.

Dr. Wood plans mapping as part of tumour surgery when the tumour can spread to lymph nodes and the result would change treatment. Dr. Wood uses mapping mainly for mast cell, mammary and toe or nail-bed tumours. It may also help in selected dogs with other tumours, including anal sac cancer.

The choice depends on the tumour type, its location, existing test results and your goals for treatment. A lump’s location alone does not mean it needs mapping.

Tell the surgical team if the lump has already been removed. Surgery and scar tissue can change its drainage routes. Mapping may still be possible, but the surgical team needs that history to plan it and interpret the results.

How do you find the node?

Mapping and node sampling may be done during the same anaesthetic as tumour surgery.

1. Trace the drainage. A small amount of a tracer—a substance that can be followed on images or during surgery—is injected around the tumour or, in selected cases, its scar. The tracer travels along the fluid drainage channels.
2. Locate the node or nodes. Images or a signal from the tracer help show where the fluid goes. More than one node may need checking.
3. Examine a sample. Depending on the plan, the surgical team takes a needle sample or removes a node for laboratory examination. A removed node is labelled separately from the tumour so each result can be matched to its site.

If a node is removed, it may be reached through the cut made to remove the tumour, called an incision, or need a separate incision.

There are different mapping methods. Some use a substance that shows on X-rays or a CT scan, which takes detailed X-ray pictures. Others use ultrasound or a dye detected during surgery.

Before the appointment, the surgical team explains the mapping method, the anaesthetic and whether mapping happens before or during tumour surgery. Sedation makes your pet sleepy; general anaesthesia keeps them fully asleep. Some methods involve preparation the day before surgery, while others can be done on the same day.

Mapping can sometimes fail to show a clear route. If that happens, Dr. Wood considers the other findings before deciding which nodes to sample or whether another test would help.

How can the result change treatment?

The laboratory checks the sample for tumour cells. Examination of cells is called cytology. Examination of tissue from a removed node is called histopathology. Sometimes a needle sample does not give enough information, and tissue examination is recommended.

After the laboratory report, the next step may be monitoring, further surgery, or discussion with a cancer care team about radiation or medicines. Not every pet needs further treatment. Spread to a node does not automatically mean treatment cannot help.

A negative result means cancer was not found in that sample. It cannot rule out every small deposit or spread elsewhere in the body. Mapping does not replace other staging tests your pet needs.

The surgical team reviews the node and tumour reports together. Removing a node does not guarantee that all cancer has been removed.

What care will your pet need afterward?

The home-care plan depends on whether your pet had mapping alone, a needle sample or an operation to remove a node, as well as any tumour surgery.

At consultation, discuss the expected hospital stay, any help needed at home and the recheck plan with the surgical team.

Possible problems after node removal include pain, bruising, bleeding, infection, a pocket of fluid, swelling or delayed wound healing. Discuss the risks for the node or nodes being removed with the surgical team.

Follow the written instructions for wound protection, medicines, activity and rechecks. Contact the team about worsening pain or swelling, discharge or a wound opening.

Use the contact details on your discharge sheet. If you do not have them, call the team caring for your pet. For an urgent or worsening concern, call rather than waiting for an email reply.