Tenosynovial Giant Cell Tumor (Giant Cell Tumor of Tendon Sheath): Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
October 3, 2026

A tenosynovial giant cell tumor is a noncancerous tumor that develops from the synovium. The synovium is the thin lining of joints, of the sheaths that surround tendons, and of the small fluid-filled sacs called bursae.

The form that develops along a tendon, most often in a finger, is still widely called a giant cell tumor of tendon sheath. Both names describe the same tumor, and you may see either on your report.

This tumor does not spread to other parts of the body. It can grow into nearby tissue and come back after surgery, and how often that happens depends on which of the two types you have.

This article explains how a tenosynovial giant cell tumor is diagnosed and what the findings in your pathology report mean.

What are the types of tenosynovial giant cell tumor?

Your report will usually state which type you have. The difference matters more than anything else in the report, because it determines the chance of the tumor coming back and the treatment options:

Localized type. A single, well-defined nodule. This is by far the most common type and is usually called a giant cell tumor of the tendon sheath. It most often develops along a tendon in a finger or hand, and less often near the wrist, ankle, foot, or knee. Surgery usually cures it.
Diffuse type. Spreads throughout the lining of a large joint, most often the knee, and less often the hip, ankle, or elbow. It is less common, accounting for roughly 1 in 10 of these tumors. This type was previously called pigmented villonodular synovitis, and older reports may still use that name. It comes back after surgery much more often.

Who gets a tenosynovial giant cell tumor?

These tumors usually develop in adults in their 30s and 40s, although they can occur at any age. The localized type is about twice as common in women. The diffuse type affects men and women about equally.

What are the symptoms of a tenosynovial giant cell tumor?

Symptoms depend on the type:

Localized type. A firm, slow-growing lump, usually painless, along a finger or on the hand. It may make it harder to fully bend the finger.
Diffuse type. Swelling, stiffness, and pain in a large joint, often the knee. The joint may lock or catch, and repeated bleeding into the joint can fill it with fluid.

What causes a tenosynovial giant cell tumor?

Most of these tumors result from a change involving a gene called CSF1. The gene moves to a new position on a chromosome, causing it to make far too much of a protein that attracts immune cells.

Only a small proportion of tumor cells carry this change. Most of the tumor is made of immune cells drawn in by the extra protein, which is why the tumor contains such a mixture of cell types.

This change develops in the tumor during a person’s lifetime. It is not inherited and cannot be passed on to children. Doctors do not know what triggers it.

How is the diagnosis made?

A pathologist makes the diagnosis after examining tumor tissue under a microscope. The tissue may come from a biopsy or from surgery that removes the tumor in an excision or resection. MRI is often done first, especially for the diffuse type, because it shows how far the tumor extends through the joint.

Under the microscope, the tumor is made of several kinds of cells mixed:

Mononuclear cells. Small round cells, each with a single nucleus. Some of these are the tumor cells that carry the CSF1 change.
Osteoclast-like giant cells. Very large cells with many nuclei. They resemble osteoclasts, the cells that break down bone, and they give the tumor its name. Your report may call them multinucleated giant cells.
Foamy cells. Immune cells filled with fat, which pathologists call foamy histiocytes.
Iron-containing cells. Immune cells containing a brown iron pigment left behind from old bleeding. This pigment explains the brown color of the tumor, and the word pigmented in the older name.

Additional tests are not usually needed. The combination of these cells, along with the tumor’s location, is enough for the diagnosis.

Is a giant cell tumor ofthe tendon sheath the same as a giant cell tumor of bone?

No. Despite the similar name, a giant cell tumor of bone is a different tumor. It starts inside a bone, has a different genetic cause, and is treated differently. Both contain large cells with many nuclei, which is where the shared name comes from.

Margins

A margin is the edge of tissue cut by the surgeon. Your report may note whether the tumor reaches the cut edge.

For a localized tumor, a positive margin raises the chance that the tumor comes back in the same spot. For the diffuse type, complete removal is often not possible, because the tumor spreads through the joint lining. A positive margin is common in that situation and is one reason the diffuse type recurs more often.

What is the prognosis?

The outlook is very good. A tenosynovial giant cell tumor is noncancerous, does not spread, and does not become cancer.

The main issue is that it can come back. After surgery, the localized type comes back in fewer than 15% of people. The diffuse type comes back much more often, in up to roughly half of people in some studies. Repeated recurrence can damage the joint over time, which is why newer medications have become an important option for the diffuse type.

What happens after the diagnosis?

Surgery for the localized type. Removing the tumor is the usual treatment and is curative for most people.
Surgery for the diffuse type. The goal is to remove as much of the abnormal joint lining as possible, either through small incisions using a camera or through an open operation.
Medication for the diffuse type. When surgery would be difficult or would cause significant loss of function, medications that block the CSF1 signal can shrink the tumor and ease symptoms. Pexidartinib and vimseltinib are FDA-approved for this use. In their main trials, the tumor shrank in roughly 40% of people treated. Availability differs between countries.
Follow-up. People with the diffuse type are usually followed with periodic MRI, because the tumor can come back years later.

Questions to ask your doctor

Do I have the localized type or the diffuse type?
Was the whole tumor removed, and did it reach a margin?
What is the chance this tumor will come back?
If it comes back, what would the next step be?
Would medication be an option instead of more surgery?
Do I need follow-up imaging, and how often?
Has the tumor caused any damage to the joint?