Fibroma of tendon sheath is a rare, benign soft tissue tumor that grows along the lining of a tendon, most often in the fingers, hands, or wrists. It typically shows up as a slow-growing, painless lump attached to a tendon or its surrounding sheath, and the standard treatment is surgical removal. While the growth itself poses no risk of spreading to other parts of the body, the story behind what causes it, how it behaves in unusual locations, and whether it comes back after surgery is more nuanced than a simple “benign equals harmless” summary might suggest.
What a Fibroma of Tendon Sheath Actually Is
A fibroma of tendon sheath (FTS) is a well-defined, firm nodule made up of dense, collagen-rich tissue populated by scattered spindle-shaped cells that resemble fibroblasts. Under a microscope, these tumors show a hallmark feature: narrow, slit-like vascular channels running through a thick collagenous backdrop. The cells themselves look bland, with no signs of the abnormal or aggressive appearance you would see in a malignancy. Most of these lesions are relatively low in cellularity, though some show denser clusters of cells at their edges, a pattern that pathologists call “cellular” FTS.
When a surgeon removes one and cuts it open, it typically looks like a grayish-white, lobulated mass with a thin capsule around it, usually ranging from about one to a few centimeters across. The tumor was first described in the medical literature in 1949, and its clinical and pathological features were later fleshed out in more detail by researchers Chung and Enzinger.
Who Gets It
FTS occurs most commonly in people between the ages of 20 and 40, with a slight preference for men in many reported series. It is overwhelmingly a tumor of the upper extremities: the fingers, palm, and wrist account for the vast majority of cases. Finding one in a child is extremely rare, though isolated case reports exist, including a case in a three-year-old boy.
The tumor typically presents as a solitary nodule, and having more than one at the same time is very uncommon. A case report of multiple fibromas arising simultaneously on the palms and soles stands out precisely because of how unusual that presentation is.
Causes and the Reactive Versus Neoplastic Debate
For decades, the prevailing view was that FTS is not a true tumor at all but rather a reactive process, essentially scar-like tissue that forms in response to some kind of local irritation or minor trauma to the tendon sheath. Early pathological studies supported this interpretation, noting that the tissue looked more like an exaggerated healing response than an autonomous growth.
That picture has changed significantly in recent years thanks to genetic testing. Researchers discovered that many FTS specimens harbor rearrangements in a gene called USP6, a finding that links these tumors to a family of neoplasms that includes nodular fasciitis and aneurysmal bone cyst. In one study of 18 FTS specimens analyzed by RNA sequencing, 17 carried a USP6 fusion, regardless of whether the tumor was classified as cellular or classic in type. The fusion partners varied widely, with several different genes identified as the other half of the rearrangement.
The genetic story is not perfectly clean, though. A larger Chinese study of 134 cases found that USP6 rearrangements were much more common in the cellular subtype of FTS (found in about half of the cellular cases tested) than in the classic, low-cellularity subtype, where only about 3% tested positive. An earlier study focusing specifically on cellular FTS detected USP6 rearrangements in two-thirds of specimens.
What this means practically is that the old “purely reactive” explanation does not hold up for many cases, particularly the cellular variant. At the same time, the classic low-cellularity version of FTS may indeed behave more like a reactive fibrous process. The field has not fully settled on a unified explanation, and both mechanisms may contribute depending on the subtype.
Symptoms and How It Feels
The most common presentation is a firm, painless lump attached to a tendon in the hand. It grows slowly, often over months or even years, and many people ignore it until it becomes large enough to interfere with hand function or causes cosmetic concern. A typical scenario involves a patient noticing a hard nodular mass in the palm or along a finger that does not hurt but gradually limits how well the finger bends.
In some cases, however, the tumor does cause symptoms beyond a simple lump:
- Limited finger movement: When the mass sits near a pulley or along the flexor tendon, it can physically block full flexion of one or more fingers.
- Radiating pain: Pressure on nearby nerves can produce pain that travels along the finger or into the hand.
- Triggering or catching: A mass positioned at a pulley can mimic or cause trigger finger, where the tendon snags during movement.
One case report described a 35-year-old man who developed limited flexion and radiating pain in his middle and ring fingers over just a single month, caused by a hard nodular mass in his palm.
When the Tumor Presses on a Nerve
A particularly troublesome complication occurs when an FTS compresses the median nerve at the wrist, producing carpal tunnel syndrome. This is not the typical cause of carpal tunnel that most people think of, which involves repetitive strain, but the symptoms can be identical: numbness, tingling, and weakness in the thumb and first few fingers. Case reports have documented FTS triggering carpal tunnel symptoms during wrist movement, and in one unusual case an FTS within the radiocarpal joint compressed the median nerve and caused secondary changes in the radius and the scapholunate joint.
These nerve-compression presentations are uncommon, but they matter because they can lead clinicians down the wrong diagnostic path. A patient with classic carpal tunnel symptoms might be treated with splinting or steroid injections for months before imaging reveals a mass as the actual culprit.
Diagnosis and Imaging
FTS is not something you can diagnose reliably by touch alone. The lump feels firm and fixed, but so do several other hand tumors. Imaging and ultimately pathology are needed to pin down what it is.
On ultrasound, FTS typically appears as a uniformly hypoechoic (dark) nodule, most often found on the flexor side of the hand, frequently near an annular pulley. Ultrasound is useful as a first-line tool because it is quick, inexpensive, and can show the mass’s relationship to the tendon.
MRI provides more detail. FTS shows up as a well-defined nodular mass next to a tendon sheath, with low to intermediate signal on most imaging sequences. How bright or dark it looks on T2-weighted images and how much it enhances with contrast depend on the proportion of fibrous tissue versus cellular tissue within the tumor. One feature that occasionally shows up on plain radiographs or MRI is mild scalloping or erosion of the adjacent bone cortex, caused by long-standing pressure from the mass sitting against the bone surface.
Telling It Apart from Giant Cell Tumor of Tendon Sheath
The single biggest diagnostic challenge is distinguishing FTS from its look-alike, giant cell tumor of the tendon sheath (GCTTS, also known as localized tenosynovial giant cell tumor). Both present as firm lumps near tendons in the hand, both are benign, and they can even look similar on gross pathology when a surgeon opens them up.
On MRI, though, several features help separate the two. FTS tends to be round or oval, while GCTTS more often has a lobulated or irregular, casting-mold shape that conforms to surrounding structures. GCTTS commonly shows a dark ring on proton-density-weighted images due to hemosiderin deposits from old bleeding within the tumor, a feature largely absent in FTS. GCTTS also tends to cause more cortical bone erosion on adjacent bones and is more likely to show susceptibility artifacts on gradient echo sequences, again because of hemosiderin.
A study comparing MRI features between the two found that lesion shape, signal homogeneity, the presence of a hypointense ring, and the pattern of contrast enhancement all showed significant differences. Combining these features achieved diagnostic accuracy ranging from about 60% to 89% depending on which feature was evaluated. The signal intensity ratio on T2-weighted images has also been proposed as a useful standalone differentiator.
The distinction matters because GCTTS has a notably higher recurrence rate after surgery and sometimes requires a more aggressive surgical approach. Confusing the two could mean either unnecessary worry about an FTS or insufficient surgery for a GCTTS.
Treatment by Surgical Excision
The standard treatment for FTS is marginal excision, meaning the surgeon removes the mass along with a thin rim of surrounding tissue while preserving the tendon and nearby structures. No chemotherapy, radiation, or other adjuvant treatment is used. The procedure is typically performed under local or regional anesthesia as a day surgery.
While the concept sounds straightforward, the actual surgery can be more challenging than it appears on paper. These tumors are often firmly adherent to the tendon, tendon sheath, and sometimes neurovascular bundles, making clean dissection tricky. One study specifically noted that FTS in the hand was more difficult to excise without causing surgical complications than other common soft tissue hand tumors, precisely because of how tightly it sticks to surrounding structures.
When the tumor sits in the hand or fingers, the surgeon typically makes a zigzag or mid-lateral incision, identifies the mass, and carefully peels it away from the tendon and sheath. If the mass wraps partially around a digital nerve, the dissection requires magnification and patience. In most series, patients recover well with standard wound care and gentle mobilization, and major complications are uncommon.
Recurrence and Long-Term Outlook
FTS has never been reported to metastasize, so in terms of cancer-like behavior, the risk is zero. The relevant long-term question is whether it comes back after removal.
Most published surgical series report low recurrence rates. One series of 20 patients with 23 tumors found no recurrences at two to five years of follow-up. Another study using marginal excision reported no recurrences after a mean follow-up of eight years. These numbers sound reassuring, and they are, but there is a catch.
A review of the literature raised the hypothesis that recurrence may be underestimated because many studies have relatively short follow-up periods. If a slow-growing tumor takes several years to become noticeable again, a study that follows patients for only two or three years might miss it entirely. The authors of that review specifically argued that longer surveillance periods could reveal a higher true recurrence rate than what current data suggest.
Data from FTS arising in the knee, where the tumor is less common, support that concern. A literature review of knee FTS found a recurrence rate of about 24% across reported cases. Whether this higher rate reflects something inherently different about knee FTS or simply the difficulty of achieving clean margins in a complex joint is unclear.
Fibroma of Tendon Sheath Outside the Hand
Although FTS overwhelmingly favors the hand and wrist, it occasionally turns up in unexpected locations including the knee, ankle, elbow, and shoulder. When it arises around large joints, the clinical picture changes considerably.
In the knee, the tumor tends to present differently than in the hand. While hand FTS is usually painless, about 71% of knee FTS cases in one review presented with pain or discomfort, and only about a third had a palpable mass. Physical exam findings in knee FTS commonly included painful range of motion and decreased range of motion, making it easy to confuse with internal derangements like meniscal tears, loose bodies, or synovial cysts.
Imaging in these atypical locations follows the same MRI principles, showing a well-defined mass with low T1 signal and variable T2 signal, but the differential diagnosis shifts. In the knee, clinicians have to think about pigmented villonodular synovitis, synovial chondromatosis, and ganglion cysts in addition to GCTTS. One case report described an FTS originating from the posterior joint capsule of the knee in a 50-year-old man, a location so unusual that it was not suspected until the MRI results came back.
The key takeaway for patients with a mass near a large joint is that FTS should be on the list of possibilities even though it is uncommon outside the hand. Biopsy or excision with pathological examination remains the definitive way to confirm the diagnosis.
What Pathologists Look for Under the Microscope
For pathologists, confirming the diagnosis requires recognizing the characteristic histological pattern: spindle or stellate-shaped cells embedded in dense collagen, with those distinctive slit-like vascular spaces running through the tissue. The cells should look bland, without the cytological atypia that would suggest a more worrisome process.
Immunohistochemical staining helps further. The tumor cells are typically focally positive for smooth muscle actin, consistent with their fibroblastic or myofibroblastic character. Stains for CD34, S-100 protein, cytokeratin, and beta-catenin are typically negative, which helps rule out other tumors in the differential. The negativity for beta-catenin is particularly useful because desmoid-type fibromatosis, a more aggressive fibrous tumor, stains positive for it.
In ambiguous cases where the tumor looks unusually cellular or has features overlapping with nodular fasciitis, molecular testing for USP6 rearrangements can help. A positive result supports an FTS or nodular fasciitis diagnosis rather than a desmoplastic fibroblastoma, which tested negative for USP6 in one study.
Living with an FTS Diagnosis
If you have been told you have a fibroma of tendon sheath, the practical reality is mostly good news. The tumor is benign, it does not become cancerous, and surgery is usually curative. Most people return to normal hand function after recovery from the excision. The main things worth knowing going forward are that the lump can come back, that follow-up beyond the first year or two is reasonable, and that any new lump in the same area deserves a return visit to the surgeon rather than a wait-and-see approach.
For those with a mass in an atypical location like the knee, recovery may involve a longer rehabilitation period simply because surgery near a weight-bearing joint is a bigger deal than a small hand procedure. The principles, however, remain the same: complete excision, pathological confirmation, and periodic follow-up to watch for recurrence.
One common source of anxiety is the word “tumor” itself. In everyday language, people associate tumors with cancer, but in medical terminology a tumor simply means an abnormal growth. FTS sits firmly in the benign category, with no reported cases of metastasis across the entire published literature on the condition.