Desmoid tumor (aggressive fibromatosis) is a soft tissue neoplasm arising from fibroblast/myofibroblast proliferation that is locally aggressive but does not metastasize. In the chest wall, it may originate from intercostal muscles, pectoral muscles, or rectus abdominis and invades surrounding structures with infiltrative margins. It is characterized by somatic mutation in the beta-catenin pathway (CTNNB1 gene). It has a strong association with Gardner syndrome/FAP (familial adenomatous polyposis) — desmoids develop in 10-20% of FAP patients. Recurrence rate after surgery ranges from 20-70%. Low T2 signal intensity on MRI related to collagen content provides a diagnostic clue. Treatment approach spans a wide spectrum from active surveillance to wide surgical resection.
Age Range
15-60
Peak Age
30
Gender
Female predominant
Prevalence
Rare
Desmoid tumor is a monoclonal fibroblastic/myofibroblastic proliferation resulting from constitutive activation of the Wnt/beta-catenin signaling pathway. In sporadic cases, somatic gain-of-function mutation in the CTNNB1 gene (3p22.1) (most commonly T41A, S45F) prevents phosphorylation and proteasomal degradation of beta-catenin, leading to nuclear accumulation and transcriptional activation of target genes (c-myc, cyclin D1). In FAP-associated cases, germline mutation in the APC gene disrupts beta-catenin regulation. The tumor produces dense collagen (types I and III), which is the histological basis for the low T2 signal intensity on MRI. The infiltrative growth pattern results from tumor cell spread along muscle fascicles and aponeuroses — microscopic extension beyond macroscopic margins is the primary cause of recurrence. Hormonal sensitivity (estrogen receptor positivity) explains the pregnancy and oral contraceptive association.
The most diagnostic MR feature of desmoid tumor is the low signal intensity bands or areas seen within the tumor on T2-weighted sequences. These bands correspond to dense collagen deposition (type I and III collagen) and reflect the fibrous nature of the tumor. Collagen bands cause the tumor to show muscle-like or sub-muscle low T2 signal — this is distinctly different from the high T2 signal shown by most malignant soft tissue sarcomas. The combination of low T2 signal + infiltrative margins + young woman + chest wall location strongly suggests desmoid tumor. The density of collagen bands reflects the maturity of the tumor — low T2 signal is dominant in mature/treated tumors.
The most characteristic MR finding of desmoid tumor is areas of low signal intensity on T2-weighted sequences. These areas correspond to dense collagen deposition. The tumor generally shows heterogeneous signal: mature/fibrotic areas show low T2, active/cellular areas show intermediate-high T2 signal. Band-like low T2 signal areas may create a characteristic band pattern within the tumor.
Report Sentence
The mass shows heterogeneous signal on T2-weighted sequences with low signal intensity in collagen-dominant areas and intermediate-high signal in cellular areas; compatible with desmoid tumor (aggressive fibromatosis).
On T1-weighted sequences, desmoid tumor shows iso- or slightly hypointense signal relative to muscle. Margins are infiltrative — the tumor advances by separating muscle fascicles and forming irregular digitiform extensions. Tumor tissue extending between muscle planes is best evaluated on T1 sequence. Loss of intramuscular fat planes indicates infiltration.
Report Sentence
The mass shows isointense signal to muscle on T1-weighted sequences with infiltrative margins and loss of surrounding muscle planes.
Heterogeneous enhancement pattern is observed after gadolinium. Cellular/active areas show prominent enhancement while collagen-dominant fibrotic areas show minimal enhancement. The enhancement difference between these two components reflects the maturity heterogeneity of the tumor. Enhancement pattern is used in treatment response assessment — enhancement decreases with treatment and low T2 signal fibrotic areas increase.
Report Sentence
The mass shows heterogeneous enhancement on post-contrast series, with prominent enhancement in cellular areas while enhancement is minimal in fibrotic areas.
Soft tissue mass in the chest wall near muscle density or slightly hyperdense. Margins are infiltrative and surrounding muscle planes are lost. Enhancement is variable, generally moderate to prominent. May originate from intercostal muscles, pectoral muscles, or aponeuroses. Erosion or remodeling of adjacent bony structures may be seen but primary bone destruction is absent.
Report Sentence
A soft tissue mass measuring approximately _x_x_ cm with infiltrative margins and moderate to prominent enhancement is observed in the chest wall; surrounding muscle planes are lost, compatible with desmoid tumor.
Diffusion restriction varies depending on tumor cellularity. Cellular/active areas may show moderate diffusion restriction. Mature fibrotic areas show no diffusion restriction. ADC values are generally higher than malignant soft tissue tumors (1.0-1.8 × 10⁻³ mm²/s). ADC heterogeneity reflects the biological heterogeneity of the tumor.
Report Sentence
The mass shows heterogeneous signal on diffusion-weighted imaging, with moderate diffusion restriction in cellular areas (ADC: ~___ × 10⁻³ mm²/s) while restriction is not prominent in fibrotic areas.
Hypoechoic solid mass within or between chest wall muscles. Margins are irregular and infiltrative — differentiation from surrounding muscle structures is difficult. Internal structure is generally homogeneous hypoechoic but fibrotic bands may be seen as hyperechoic linear structures. Variable vascularity on Doppler — generally low to moderate flow.
Report Sentence
A hypoechoic solid mass measuring approximately _x_ cm with irregular margins is observed within/between chest wall muscles; low to moderate vascularity is present on Doppler examination.
FDG uptake is variable and generally low to moderate (SUVmax typically 2-5). Cellular/active tumors may show higher FDG uptake. Fibrotic/mature tumors show minimal uptake. PET-CT is not a primary diagnostic tool for desmoid tumor but can be used for treatment response monitoring and active disease assessment.
Report Sentence
The chest wall mass demonstrates low to moderate FDG uptake (SUVmax: ___); compatible with the metabolic activity spectrum of desmoid tumor.
Criteria
Most common type. Occurs in chest wall, shoulder girdle, back, thigh, and head-neck region. Carries CTNNB1 mutation in sporadic cases. May be triggered by surgical scar, trauma, or pregnancy. 2-3 times more common in women than men. In the chest wall, originates from intercostal or pectoral muscles.
Distinct Features
Trunk or extremity location, may not be FAP-associated, CTNNB1 somatic mutation, history of surgery/trauma
Criteria
Originates from the anterior abdominal wall (rectus sheath, oblique muscles). Usually in women of childbearing age, often post-pregnancy or surgery (cesarean section). Presents with abdominal wall irregularity and palpable mass. May extend inferiorly to the chest wall.
Distinct Features
Anterior abdominal wall location, pregnancy/surgery association, rectus sheath origin
Criteria
Occurs in familial adenomatous polyposis (FAP) / Gardner syndrome patients. Most commonly mesenteric-origin intra-abdominal desmoid. Usually develops after colon surgery (colectomy). Carries APC germline mutation. Develops in 10-20% of FAP patients and is the second most common cause of death in FAP. May be multiple. Chest wall extension is rare but possible.
Distinct Features
FAP/Gardner syndrome history, APC mutation, mesenteric location, post-colectomy, may be multiple
Distinguishing Feature
Malignant — has metastatic potential (absent in desmoid). Generally higher T2 signal on MRI (different from desmoid's low T2 signal). More prominent enhancement and necrosis areas. ADC values generally lower. Faster growth. Peritumoral edema more prominent. Diagnosis is made histopathologically — distinction from desmoid is critical on biopsy.
Distinguishing Feature
Benign, self-limiting myofibroblastic proliferation. Usually <3 cm, rapid growth (<4-6 weeks). Fascia-related but infiltrative margins not as prominent as desmoid. T2 hyperintense on MRI (different from desmoid's low T2). USP6 gene rearrangement is diagnostic. Spontaneous regression may occur in 2-3 weeks.
Distinguishing Feature
Subscapular location (infraserratus region) is specific. Common in elderly women (>55 years). Alternating muscle and fat tissue ('layered cake' appearance) is pathognomonic MRI finding. May be bilateral (10-60%). No treatment needed if asymptomatic. Infiltrative margins not as aggressive as desmoid.
Distinguishing Feature
Usually well-defined mass (different from desmoid's infiltrative margins). Low T2 signal on MRI (collagen — similar to desmoid) but margins are smooth. Prominent enhancement (hypervascular). STAT6 nuclear positivity is diagnostic. May be of pleural origin (different from chest wall). Variants with malignant potential exist.
Urgency
moderateManagement
Treatment approach has undergone a paradigm shift in recent years — observation (active surveillance/wait-and-see) has become the primary approach for asymptomatic or stable tumors, as spontaneous regression is seen in 20-30% of cases. Surgical resection is considered for symptomatic, growing, or critical structure-compressing tumors — but positive surgical margins increase recurrence risk to 50-70%. Wide negative margins (≥1-2 cm) are targeted. Medical treatment: tamoxifen (anti-estrogen), NSAIDs (sulindac — especially FAP-associated), low-dose chemotherapy (methotrexate + vinblastine), tyrosine kinase inhibitors (sorafenib — 33% partial response). Radiotherapy may be used for unresectable or recurrent cases (50-56 Gy). Gamma-secretase inhibitors (nirogacestat) are the first FDA-approved systemic treatment (2023).Biopsy
NeededFollow-up
In active surveillance approach: MRI assessment of size and signal every 3-6 months for first 1-2 years. If stable or regressing, interval may be extended (6-12 months). Treatment is re-evaluated if growth or symptom increase occurs. Post-surgery: MRI every 3-6 months for first 2 years (recurrence screening), recurrence usually occurs within first 2 years. Increased enhancement and T2 signal change on MRI may indicate recurrence or reactivation. In FAP patients, regular screening continues for colon polyps and other FAP manifestations (osteomas, epidermoid cysts, thyroid carcinoma).Desmoid tumor is a locally aggressive but non-metastasizing soft tissue neoplasm. Active surveillance (wait-and-see) has become the primary strategy for asymptomatic/stable tumors in the treatment approach — spontaneous regression is observed in 20-30% of cases. Biopsy is mandatory for histological diagnosis and distinction from malignant soft tissue sarcomas is critical. FAP/Gardner syndrome association should be queried and APC gene testing performed if indicated. MRI is the preferred modality for follow-up — T2 signal and enhancement changes reflect disease activity. Nirogacestat (gamma-secretase inhibitor) is the first systemic treatment to receive FDA approval in 2023, expanding treatment options.
Desmoid tumors, despite not metastasizing, display locally aggressive behavior with a high post-surgical recurrence rate (20-70%). Treatment requires a multidisciplinary approach: active surveillance (some lesions may spontaneously regress), surgical excision (with wide margins), radiation therapy, or medical therapy (antiestrogens, NSAIDs, chemotherapy). Screening is recommended in patients with Gardner syndrome (FAP/APC mutation). Close follow-up with MRI is required.