Desmoid tumour (aggressive fibromatosis)
A desmoid tumour is a growth of fibrous tissue that invades locally but never spreads to distant organs. It occupies a genuinely unusual position: it is not a cancer in the conventional sense, yet it can be locally destructive and difficult to control. What has changed most in the past decade is not a new drug but a reversal of the whole approach to treatment. Surgery, once automatic, is now the last resort; active surveillance — deliberately doing nothing and watching — has become the recommended first step, because a substantial proportion of these tumours stop growing or shrink on their own.
How they form
Desmoid tumours arise from myofibroblasts, the cells responsible for wound contraction and scar formation. The underlying fault is in the Wnt/β-catenin signalling pathway, which normally controls when fibroblasts proliferate and when they stop.
- Sporadic desmoids (the great majority) carry an activating mutation in CTNNB1, the gene encoding β-catenin itself. The protein escapes degradation, accumulates in the nucleus, and continuously drives transcription of growth genes. Specific mutations — particularly the T41A and S45F variants — are detectable and have prognostic value; S45F is associated with a higher risk of progression and recurrence.
- FAP-associated desmoids arise in patients with familial adenomatous polyposis, who have an inherited APC mutation. APC normally forms part of the complex that degrades β-catenin, so its loss produces the same end result by a different route. Desmoid tumours are a leading cause of death in FAP patients after colectomy, and they are characteristically intra-abdominal, arising in the small bowel mesentery.
Two further associations are clinically important:
- Trauma and surgery. Desmoids frequently arise in surgical scars and at sites of injury — which makes sense given their origin in the cells of wound healing. This is also why operating on a desmoid can provoke a larger and more aggressive recurrence, and it is one of the strongest arguments against reflexive surgery.
- Oestrogen. Desmoids occur predominantly in women of reproductive age, commonly arise during or after pregnancy, and frequently regress after the menopause. Oestrogen receptors are present in many tumours, which underlies the historical use of anti-oestrogen treatment.
Sites reflect these mechanisms: the abdominal wall (classically in young women after pregnancy), the intra-abdominal mesentery (particularly in FAP), and the extremities, chest wall and shoulder girdle.
Why surveillance works
The natural history of desmoid tumours is unpredictable and, importantly, often favourable. Studies of patients managed by observation alone show that a large proportion — roughly half or more — either stabilise spontaneously or regress without any treatment. A smaller proportion progress. There is currently no reliable way to predict which at the outset, though S45F mutation and intra-abdominal location are adverse features.
Given that every treatment carries real cost — surgery risks provoking recurrence and causes morbidity, radiotherapy causes late fibrosis and second malignancy, systemic therapy has side effects — and that half the tumours will settle by themselves, a period of observation is a rational first step rather than neglect. This is a genuine reversal of previous practice, and patients who were told years ago that a desmoid must be removed are entitled to an explanation of why the advice has changed.
Symptoms
- A firm, fixed mass, often painless at first
- pain, which is common and can be significant and out of proportion to the size
- restricted movement where a limb or joint is involved
- neurological symptoms from nerve compression
- Intra-abdominal desmoids — the most troublesome — cause bowel obstruction, ureteric obstruction with hydronephrosis, and compression or encasement of the mesenteric vessels, which is the mechanism by which they threaten life
- a mass at a previous surgical scar
How the diagnosis is made
- MRI — the investigation of choice. Desmoids are characteristically T1 isointense and variably T2 hyperintense with prominent low-signal bands of dense collagen. A useful practical point: a tumour that becomes progressively low signal on T2 over time is maturing and becoming inactive, even if its size has not changed — signal change is therefore as informative as size on surveillance.
- CT — for intra-abdominal and mesenteric disease, defining the relationship to bowel, ureters and mesenteric vessels.
- Core biopsy — essential for diagnosis, since desmoids are readily mistaken for sarcoma, and the management is entirely different. Histology shows bland spindle cells in dense collagen with infiltrative margins.
- Nuclear β-catenin staining on immunohistochemistry, and CTNNB1 mutation analysis, which both confirm the diagnosis and provide prognostic information.
- Colonoscopy and consideration of FAP in any patient with an intra-abdominal desmoid, or a desmoid at a young age, or with a suggestive family history. Identifying FAP has major implications for the patient and their family.
Treatment
Active surveillance — the first line
Recommended for most newly diagnosed desmoids that are not immediately threatening a vital structure. MRI is repeated at short intervals initially — typically three months, then lengthening — with treatment introduced only if the tumour progresses or symptoms develop. Pain is managed actively during this period; surveillance does not mean leaving the patient unsupported.
Systemic treatment
Preferred over surgery for progressive or symptomatic disease.
- Nirogacestat — a gamma-secretase inhibitor targeting Notch signalling, which showed substantial improvement in progression-free survival and pain in a randomised trial and has become a standard option. Ovarian toxicity in women of childbearing age is an important consideration to discuss.
- Sorafenib — a tyrosine kinase inhibitor, also supported by randomised evidence, with high rates of disease control.
- Low-dose chemotherapy — methotrexate with vinblastine or vinorelbine, well tolerated and effective, particularly in younger patients.
- Anthracycline-based chemotherapy — reserved for rapidly progressive, life-threatening disease.
- Tamoxifen and NSAIDs — historically used, with modest and inconsistent evidence; still occasionally employed.
Local treatment
- Cryoablation and other ablative techniques — increasingly used for accessible extra-abdominal desmoids, giving good symptom and size control without an operation.
- Radiotherapy — effective but used sparingly, because these are young patients and the late effects, including second malignancy, are a real concern over decades.
Surgery
Now reserved for selected cases: an abdominal wall desmoid that can be resected with reconstruction and low morbidity, or disease causing obstruction that cannot be managed otherwise.
Two points about surgery deserve stating plainly. First, recurrence after resection is common — a substantial proportion recur regardless of margin status, and the relationship between microscopic margin and recurrence is weaker than in true sarcoma. Second, surgery for mesenteric desmoids in FAP is hazardous: the tumour encases the mesenteric vessels, resection risks catastrophic loss of small bowel, and recurrence is frequent. In that setting, systemic treatment is strongly preferred.
Recovery and living with a desmoid
For patients on surveillance, the challenge is psychological as much as physical — living with an untreated tumour requires a clear explanation of why that is the right approach, and reliable access to review. Pain management, physiotherapy and, where relevant, psychological support are part of care rather than optional extras.
After surgery, recovery depends on the site and the reconstruction; abdominal wall resection with mesh typically means a week in hospital and two to three months to full activity.
Follow-up
MRI surveillance at intervals determined by behaviour, continuing for years. Recurrence and progression can occur late, and equally, long-term stabilisation is common. Regression after the menopause is well described.
When to seek an opinion
Any firm soft tissue mass, particularly one arising in a surgical scar or in a young woman after pregnancy, should be imaged and biopsied before removal. A desmoid diagnosis should be managed by a sarcoma multidisciplinary team, and patients with intra-abdominal desmoids or a young age at onset should have FAP considered.
Common questions
Is this cancer?
Not in the usual sense. It does not spread to other organs, which is the feature that makes most cancers dangerous. But it grows into surrounding tissue rather than pushing it aside, and it can come back after removal, so it is not simply benign either. It sits between the two, and it is managed by cancer specialists for that reason.
Why am I being told to do nothing?
Because roughly half of these tumours stop growing or shrink by themselves, and every treatment carries a cost — surgery can provoke a more aggressive regrowth, radiotherapy has long-term effects in young patients, and drugs have side effects. Watching with regular scans, and treating only if it progresses, spares many people treatment they never needed. This is a deliberate strategy, not neglect, and your pain should still be treated actively during it.
My scan shows it is the same size. Is that good or bad?
Often good. Beyond size, the MRI appearance shows how active the tumour is — as it matures and becomes inactive it turns darker on particular sequences, even without shrinking. A stable, darkening tumour is behaving well, and this is one of the things your radiologist is assessing.
Could surgery make it worse?
It can. These tumours arise from the cells responsible for wound healing, and they frequently appear in surgical scars, so an operation can stimulate a larger recurrence. This is a major reason surgery has moved from being the first treatment to being one of the last.
Did my pregnancy cause it?
Pregnancy is a recognised trigger, and these tumours occur predominantly in women of reproductive age and often appear during or shortly after pregnancy. Hormones appear to be involved, which is also why they frequently shrink after the menopause. It is not something you did.
Can I have another pregnancy?
This needs individual discussion rather than a blanket answer. Many women have successful pregnancies after a desmoid diagnosis, though the tumour may grow during pregnancy and then settle. Some treatments affect fertility, and this should be discussed before starting them. Raise it early rather than afterwards.
Is it connected to bowel polyps in my family?
It can be. Desmoid tumours, particularly those inside the abdomen, occur in a hereditary condition causing large numbers of bowel polyps. Anyone with an abdominal desmoid, or one at a young age, should have this considered, as it has important implications for you and your relatives.
Will it ever go away?
Quite possibly. Spontaneous stabilisation is common and genuine regression occurs in a proportion of patients, sometimes years later and often around the menopause. This is a condition where patience frequently succeeds where intervention does not.
Related conditions
Other conditions of the sarcoma & retroperitoneal covered on this site:
- Retroperitoneal liposarcoma
- Leiomyosarcoma
- Soft tissue sarcoma of the trunk and abdominal wall
- Benign retroperitoneal tumours and masses
This page provides general information and does not replace an individual medical consultation. Desmoid tumours should be managed by a specialist sarcoma multidisciplinary team.