Giant cell tumor of the tendon sheath (GCTTS) is a benign neoplasm arising from the tendon sheath synovium and is the second most common soft tissue tumor of the hand (after ganglion cyst). It is the localized extra-articular form of PVNS (pigmented villonodular synovitis). Most commonly found in the fingers (85%) and dorsum of the hand; more frequent in women aged 30-50 (F:M = 1.5:1). Presents as a painless, slowly growing, firm mass adjacent to the tendon sheath. US is the primary diagnostic modality, appearing as a peritendinous, well-defined, homogeneous hypoechoic solid mass. Low signal on MRI T2 due to hemosiderin deposition is characteristic, and prominent blooming artifact on GRE/SWI sequences is the signature MRI finding of GCTTS.
Age Range
20-60
Peak Age
40
Gender
Female predominant
Prevalence
Uncommon
GCTTS results from neoplastic proliferation of cells in the tendon sheath synovium. The tumor consists of multinucleated giant cells (osteoclast-like), foamy macrophages (lipid-laden, xanthoma cells), hemosiderin-laden macrophages (siderophages), and mononuclear stromal cells. Translocation of the CSF1 (colony stimulating factor 1) gene (t(1;2)(p13;q35-37)) plays a central role in the tumor's growth mechanism — CSF1 overexpression attracts monocytes and macrophages to the tumor microenvironment, forming the reactive component. Hemosiderin deposition results from recurrent microhemorrhages: erythrocytes leaking from neovascular structures within the tumor are phagocytosed by macrophages and hemoglobin is converted to hemosiderin (iron-III oxyhydroxide). This hemosiderin contributes to the hypoechoic-isoechoic heterogeneous appearance on US and causes dramatic signal changes on MRI. On MRI, hemosiderin markedly reduces T2/T2* signal through its superparamagnetic effect — local magnetic field inhomogeneity accelerates phase dephasing. This effect is extreme on GRE/SWI sequences, producing 'blooming' artifact. The peritendinous location on US reflects the tumor's tendon sheath origin — the mass encases the tendon (splaying pattern) or sits on one side, potentially forming a semilunar (crescent) shape over the tendon.
Blooming artifact on GRE/SWI sequences from the superparamagnetic effect of hemosiderin is the signature finding of GCTTS. Signal loss area appears larger than actual lesion size. This finding is not seen in other peritendinous lesions without hemosiderin (schwannoma, fibroma, ganglion) and provides definitive differential distinction. Low SE T2 signal + GRE blooming combination confirms GCTTS.
On B-mode US, GCTTS appears as a well-defined, oval or lobulated, homogeneous hypoechoic solid mass adjacent to or surrounding the tendon sheath. Usually 1-3 cm in size. The relationship with the tendon sheath is a critical diagnostic clue: the mass may push the tendon to one side (eccentric position) or encase it (splaying). Internal structure is generally homogeneous, but fibrous septations or cystic degeneration areas in large lesions may create heterogeneity. The hypoechoic appearance reflects dense cellular composition — the mixture of giant cells, macrophages, and stromal cells creates lower acoustic impedance than surrounding tendon/fat. Bone erosion in adjacent phalanx is rare but possible (10-15%).
Report Sentence
A well-defined hypoechoic solid mass adjacent to the tendon sheath is seen; consistent with GCTTS.
GCTTS generally shows low or minimal internal vascularity on color and power Doppler. In some cases, punctate vascular signals may be seen at the lesion periphery or within internal septa. This low vascularity reflects the slow-growing benign nature of the tumor. Increased vascularity is rare and should raise suspicion for more aggressive behavior or malignant transformation (though malignant transformation in GCTTS is extremely rare). Power Doppler is more sensitive than color Doppler for detecting low-velocity flow and may demonstrate small intratumoral vessels.
Report Sentence
Minimal vascular signal is detected within the mass on Doppler; consistent with a benign solid lesion.
GCTTS grows by pushing the tendon to one side (eccentric) or encasing it (splaying). On US transverse section, the tendon is seen surrounded by the mass or displaced to one edge. The splaying pattern is characteristic of GCTTS and confirms the tendon sheath origin of the tumor. Tendon integrity is preserved — the mass does not invade the tendon but compresses and displaces it. With dynamic US during finger flexion-extension, tendon movement beneath or beside the mass is observed — this maneuver clarifies the tendon-mass relationship. In large lesions, bone erosion (cortical notching in phalanx) may be added.
Report Sentence
The mass encases the tendon (splaying pattern), consistent with a tendon sheath-origin lesion.
On T2-weighted MRI, GCTTS shows markedly low signal — the superparamagnetic effect of hemosiderin deposition shortens T2 relaxation. This low T2 signal is the most characteristic MRI finding of GCTTS and plays a critical role in differential diagnosis. Low-intermediate signal on T1-weighted images. Hemosiderin density may be variable in some parts of the lesion → heterogeneous signal possible. Enhancement is usually mild-moderate. In large lesions, cystic degeneration areas may appear as T2 hyperintense foci.
Report Sentence
A peritendinous solid mass showing markedly low signal on T2 is seen; consistent with hemosiderin deposition, suggesting GCTTS.
On GRE (Gradient Echo) or SWI (Susceptibility Weighted Imaging) sequences, GCTTS shows prominent 'blooming' artifact — the signal loss area appears larger than the actual lesion size. This blooming results from the superparamagnetic effect of hemosiderin and is the signature MRI finding of GCTTS. Blooming degree is directly proportional to hemosiderin concentration. The combination of low signal on SE T2 + dramatic blooming on GRE confirms GCTTS diagnosis. This finding is the most reliable criterion for differentiating from other hypoechoic peritendinous lesions (schwannoma, fibroma, synovial cyst).
Report Sentence
Prominent blooming artifact is seen in the lesion on GRE/SWI sequences; confirming hemosiderin deposition and strongly supporting the diagnosis of GCTTS.
In large GCTTS lesions, pressure erosion (cortical notching) may be seen on the adjacent bone surface (phalanx). On US, it appears as focal irregularity or depression of the hyperechoic cortical line. This erosion results from chronic pressure effect rather than direct bone invasion — benign remodeling. Incidence is 10-15%, more common in larger, longstanding lesions. CT or MRI can detail the erosion. Cortical erosion does not indicate aggressive behavior and does not change treatment planning.
Report Sentence
Pressure erosion (cortical notching) of the adjacent phalanx cortex is seen; consistent with GCTTS.
On strain elastography, GCTTS appears stiff (blue coding) compared to surrounding tissue — cellular composition and hemosiderin deposition create stiff tissue. Strain ratio generally >3. This stiffness aids differentiation from cystic lesions (ganglion cyst — soft) and lipomas (soft). Shear wave values typically 30-80 kPa.
Report Sentence
The mass appears stiff compared to surrounding tissue on elastography, consistent with solid composition.
Criteria
Most common form (75%). Single nodular mass, well-defined, encapsulated. Adjacent to tendon sheath.
Distinct Features
Homogeneously hypoechoic on US, well-defined. Low T2 on MRI. Recurrence 10-20% (after incomplete excision).
Criteria
Rare (15%). Multiple nodules or diffuse infiltrative growth. Larger, less well-defined.
Distinct Features
Heterogeneous on US, irregular margins. Higher recurrence risk (30-50%). More complex surgical planning.
Criteria
Extremely rare (<2%). Histologically sarcomatous transformation, high mitosis, necrosis.
Distinct Features
Rapid growth, increased vascularity, irregular margins on US. Peritumoral edema on MRI. Hematogenous metastasis risk exists.
Distinguishing Feature
Schwannoma shows tail sign along nerve course, prominent posterior enhancement, and high T2 signal (myxoid component). GCTTS is tendon sheath-associated, low T2 signal (hemosiderin), and blooming on GRE.
Distinguishing Feature
Fibromatosis is fascia/aponeurosis-associated, fusiform shape. T2 signal is stage-dependent but no GRE blooming. GCTTS is tendon sheath-associated with pathognomonic GRE blooming.
Distinguishing Feature
Ganglion cyst is anechoic/hypoechoic cystic structure with prominent posterior enhancement, compressibility, and connection (stalk) to joint/tendon sheath. GCTTS is solid, non-compressible, no posterior enhancement, with hemosiderin findings on MRI.
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
12-monthGCTTS treatment is marginal excision (complete tumor removal). Biopsy is not needed with typical US and MRI findings — preoperative diagnosis is confidently made. Recurrence rate is 10-20% in localized type (incomplete excision most common cause), 30-50% in diffuse type. Preoperative MRI detailing tendon/nerve/artery relationship is needed before surgery. Annual US for recurrence surveillance — early recurrence may appear as a small hypoechoic nodule at the original excision site. CSF1R inhibitors (pexidartinib) are FDA-approved (2019) for medical treatment in recurrent or inoperable cases. Malignant transformation is extremely rare (<2%), usually after multiple recurrences.
GCTTS is benign but local recurrence rate after surgical excision is 15-25%. Marginal excision may be insufficient, and surgery can be challenging due to adhesion to adjacent structures. Malignant transformation is extremely rare. Adjacent bone erosion indicates chronicity of the lesion, NOT malignancy.