Classification and pathology of soft-tissue tumors and tumor-like lesions about the knee:
TUMOR-LIKE LESIONS:
SYNOVIAL CYST:
- Definiton: a herniation or continuation of the synovial membrane through the joint capsule
- POPLITEAL CYST / BAKER CYST is the most commonly recognized,
results from communication between knee joint and gastrocnemius – semimembranosus bursa
Fig. 3: Case1. Popliteal cyst: A fluid filled mass (purple arrows) on sagittal T1 weigthed image(wi), coronal (b) and axial (c) T2 with fatsat (T2FS)wi of MRI as result of communication between knee joint and gastrocnemius (*yellow) – semimembranosus (*red) bursa.
References: Hospital Clínico San Carlos, Madrid
Radiology:
- Fluid filled mass – US/CT/MRI
- May have septations
- Can have solid components if complicated (rupture) with hemorrhage,
dissection or superimposed infection
- Contrast enhancement of rim/septae
- Contrast enhancement of solid components of complicated cysts
- Complicated cysts difficult to exclude other causes of mass ; must look at morphology
MENISCAL CYST: Cystic masses related to meniscal tears.
Fig. 4: Case2. Meniscal cyst: A complex cystic mass (*) on coronal (a) and axial (b) T2FS wi. Location, adjacent to lateral meniscus, and communication with a meniscal tear (purple arrow) on sagittal gradient echo (GE) wi are diagnostic.
References: Hospital Clínico San Carlos, Madrid
Radiology:
- Radiographs – soft tissue mass
- CT/US/MRI – fluid collection adjacent to meniscus
- Lateral > medial
SOFT TISSUE GANGLION:
Pathology
- Thick walled unilocular/multilocular cystic spaces: Gelatinous – mucinous fluid rich in hyaluronic acid and mucopolysaccharides
Radiology:
- CT/US/MRI – cystic mass
- May have higher attenuation on CT or signal T1W MR image – high protein mucin
- Wall/septae may show mild enhancement
MYOSITIS OSSIFICANS:
HETEROTOPIC BONE FORMATION LOCATION
Clinical features:
- Young adults,
M > F,
usually trauma history
- Can involve muscles,
fascia,
tendons,
subcutaneous fat
- Initially pain/tenderness and localized mass; pain decreases with time
Pathology:
- Zonal pattern of maturation
a.
Central immature osteoid/fibroblastic tissue
b.
Periphery calcifying osteoid to mature lamellar bone
- Cortical bone with further maturation
Fig. 5: Case 3. Myositis ossificans in a 51 year-old male with calcified soft-tissue masses in the left calf on CR. A radiolucent cleft (string sign) separate the lesions from the cortex of the adjacent bone.
References: Hospital Clínico San Carlos, Madrid
Radiology:
- Radiographs:Circumferential calcification with a lucent centre and a radiolucent cleft (string sign) that separates the lesion from the cortex of the adjacent bone.
- MRI: Often misinterpreted as malignant tumor.
Fig. 6: Case 3. Myositis ossificans. On MRI, coronal T2 wi (a, B) show asymmetry between the legs, being dominant the right. On T2FS wi (c) anterior mass shows low signal intensity (late stage= purple arrows) while the mass within the calf shows high signal intensity related to an intermediate stage (orange arrows). On T1FS wi after contrast administration bothe masses show enhancement on a different way.
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1.
Early to intermediate
Normal with displaced fascial planes (T1W)
Increased intensity mass with prominent edema (T2W image)
2.
Late: heterogeneous well defined mass marrow fat on T1W/T2W MR images,
no edema,
low intensity rim
Treatment and prognosis
- May resorb or be asymptomatic
- Resect after maturation (12 – 18 months)
- Premature resection – recurrence with vengeance
- Rare report malignant transformation
- Malignant myositis (mucinous carcinoma)
TUMORAL CALCINOSIS:
- Large calcified paraarticular mass,
hip,
shoulder,
elbows,
and feet
- Can be associated with CPPD arthropathy,
pseudoxanthoma elasticum-like syndrome
Pathology:
- Gross-encapsulated multilocular mass,
filled with viscous calcium hydroxyapatite
- Fibrous septations
- May have inflammatory elements
BENIGN TUMORS
MYXOMA:
Clinical features
- Adults 40 – 70 years of age
- Painless palpable mass
Pathology: Intramuscular myxomas are benign soft-tissue tumours of mesenchymal origin that presents as a deeply seated mass confined to skeletal muscle.
It is classically described as hypocellular and hypovascular,
and is composed of cytologically bland stellate and bipolar fibroblasts separated by abundant extracellular myxoid matrix.
Radiology:
- A well-circumscribed intramuscular mass.
- US: Hypoechoic to the surrounding soft tissue,
posterior acoustic enhancement,
bright cap sign.
Fig. 7: Case 4. An intramuscular myxoma in a 37 year-old female with a painless mass in the right thigh. A well-circumscribed hypoechoic intramuscular mass with posterior acoustic enhancement on ultrasound.
- MRI: Intra-tumoural cysts may be present in small proportion of cases,
pseudocapsule,
perilesional / rim of fat and perilesional oedema may be present.
Fig. 8: Case 4. Intramuscular myxoma. On axial T1wi (a), the well-circumscribed intramuscular shows similar signal intensity to the muscle. On sagittal T2 wi (b), a small amount of perilesional edema mimicking splint fat sign. On axial T1FS wi (c), the mass shows slight high signal intensity in comparison to the muscle. On axial T1FS wi after contrast media administration (d), the lesion shows a ring enhancement.
Fig. 9: Case 4. Intramuscular myxoma. Core biopsy guided by ultrasound.
PERIPHERAL NERVE SHEATH TUMOURS (PNST'S) are a group of primary tumours that arise from nerve sheaths.
Classification
benign
- schwannoma (peripheral nerve sheath schwannoma)
Fig. 10: Case 5. Schwannoma in a 60 year-old man with a painful mass in the right popliteal fossa. A well-circumscribed hypoechoic fusiform-shaped mass (*) with tapered ends (arrows) on ultrasounds (a) and hypovascular on Doppler-US (b).
References: Hospital Clínico San Carlos, Madrid
- neurofibroma (peripheral nerve sheath neurofibroma)
a.
localised neurofibroma (~90%)
b.
diffuse neurofibroma
c.
plexiform neurofibroma: arborising tumour of peripheral nerves that is pathognomonic of NF1
malignant
- malignant peripheral nerve sheath tumour (MPSNT)
Radiology (MRI)
Fig. 11: Case 5. Schwannoma. On axial (a) and coronal (b) T2FS wi a fusiform-shaped mass (*) shows an heterogeneous and high signal intensity with tapered ends (orange arrows). On axial (c) and sagittal (d) T1FS wi after Gadolinium administration, the lesion (*) shows an intense enhancement.
Incidental Baker cyst (green arrow) shows only a peripheral enhancement(c).
References: Hospital Clínico San Carlos, Madrid
- fusiform-shaped mass with tapered ends
- a “split-fat” sign
- atrophy of the muscles supplied by the involved nerve
- a “fascicular sign”
- a “target sign”
Fig. 12: Case 5. Schwannomas are benign encapsulated neoplasms of schwann cells (WHO grade I). They arise eccentrically from their parent nerve. They are composed of two cell types: Antoni A and Antoni B.
References: Hospital Clínico San Carlos, Madrid
SYNOVIAL LIPOMA:
Two types
1.
Diffuse form –lipoma arborescens
Fig. 13: Case 6. Lipoma arborescens (Diffuse form) secondary to trauma in a 58 year-old female with severe osteoarthritis on CR.
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- Diffuse infiltration of synovium by fat
- Monoarticular – knee most common
- Often secondary (but can be primary) to chronic arthritis from trauma or inflammatory disease
- Radiographs –soft tissue swelling
- CT fatty infiltration
Fig. 14: Case 6. Lipoma arborescens. On sagittal (a) and coronal (d) T1 wi diffuse infiltration of synovium by fat. On axial (b) and coronal (c) T2FS wi (b) joint effusion with synovial proliferation are evident.
References: Hospital Clínico San Carlos, Madrid
- MRI best to identify frond - like fatty projections
Fig. 15: Case 6. The normal synovium is replaced by hypertrophied villi demonstrating marked deposition of mature lipocytes within them.
References: Hospital Clínico San Carlos, Madrid
2.
Localized form
- Rare – knee most frequent
- Solid fatty intraarticular mass
- Filling defect on arthrogram
- CT/MRI – lipomatous tissue
SYNOVIAL HEMANGIOMA
Clinical features
- A rare benign vascular malformations that occur in relation to the joint.
- Most lesions tend to occur around the knee.
- Children and young adults.
- Occasionally patients can have recurrent haemarthroses.
Fig. 16: Case 7. Synovial hemangioma. On MRI, a lobulated intra-articular mass is seen with an intermediated signal intensity on axial T1 wi (a), high on axial (b) and sagittal (c) T2FS wi. On sagittal GE wi (d) magnetic susceptibility artefact due to hemosiderin is evident. On sagittal T1FS wi with Gadolinium administration (f) enhancement is intense in comparison with sagittal T1FS without (e).
References: Hospital Clínico San Carlos, Madrid
Radiology:
- Plain film findings are generally nonspecific and may be seen a soft tissue mass adjacent to the knee.
Accompanying phleboliths may be present.
- MRI: Typically seen as a lobulated intra-articular mass.
SYNOVIAL CHONDROMATOSIS:
Clinical features
- Formerly synovial osteochondromatosis
- Cartilage metaplasia in synovium
-
Can also involve tendons and bursa
-
Secondary chondromatosis – trauma,
OA,
RA,
AVN,
osteochondritis dissecans
Pathology
- Hyaline cartilage metaplasia in synovium
- Cartilage nodules (2 –3 cm) can break away into joint,
grow,
reattach to synovium
- Hypercellularity and nuclear atypia simulate cartilage malignancy
Radiology
- Radiographs – calcified bodies (70% – 75%),
may ossify,
extrinsic erosions,
joint widened,
OA changes
Fig. 18:
Case 8. Synovial chondromatosis. On US, an heterogeneous mass partially calcified and increased vascularization is depicted.
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Fig. 19: Case 8. Synovial chondromatosis. CR show calcified bodies within the popliteal fossa.
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- CT thickening about joint,
effusion often small if present,
calcification/ossification
Fig. 20: Case 8. Synovial chondromatosis. CT shows calcified bodies within the popliteal fossa.
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- MRI – variable depending on degree of mineralization,
some hyperintensity T2W images
Fig. 21: Case 8. Synovial chondromatosis. On MR, the mass shows areas of low, intermediate and high signal intensity, specially on T1 wi (a, c) and enhances after Gadolinium administration (d).
References: Hospital Clínico San Carlos, Madrid
SOFT TISSUE CHONDROMA:
Less common than synovial chondromatosis
Pathology
- Usually < 3 cm,
often attached to tendon
- Mature hyaline cartilage lobular pattern
- Can show ossification
- Fibrous capsule not tenosynovium unlike synovial chondromatosis
Fig. 22: Case 9. Soft-tissue chondroma. Calcified bodies are seen within infrapatellar space on CR.
References: Hospital Clínico San Carlos, Madrid
Fig. 23: Case 9. Soft-tissue chondroma. On MR, the mass shows areas of low, intermediate and high signal intensity, specially on T1 wi, on sagittal (a) and coronal (d) views related to chondroid matrix which may ossify.
References: Hospital Clínico San Carlos, Madrid
Radiology
- Nonspecific soft tissue mass related to IP joint
- Also common in infrapatellar fat
- Chondroid matrix,
can ossify
- Unusual to erode underlying bone
Fig. 24: Case 9. A soft-tissue chondroma is defined a fibrous capsule unlike synovial chondromatosis.
References: Hospital Clínico San Carlos, Madrid
PIGMENTED VILLONODULAR SYNOVITIS (PVNS):
Clinical features
- Proliferative disorder of synovium of joint,
tendon or bursa
- Young adults 3rd and 4th decades
- Symptoms – pain,
swelling,
ranged of motion loss
Two types: diffuse (15% – 25%) and localized (75% – 85%)
1. Intra-articular:
a. Difusse 50%: knee (60% – 80%) hip,
ankle,
shoulder,
elbow
- Radiology: Erosive bone lesion
- Geographic IA lytic lesion – extrinsic erosion
- Joint effusion
- CT – soft tissue mass,
increased attenuation
Fig. 25: Case 10. Difusse Pigmented VilloNodular Synovitis (PVNS). On sagittal (a, b) and axial (d, e, f) PDFS wi , joint effusion and synovial proliferation are seen. Coronal GE wi (c) shows “blooming” effect caused by magnetic susceptibility artifact due to hemosiderin.
References: Hospital Clínico San Carlos, Madrid
- MRI – Low- to intermediate on T1 and low signal intensity mass on T2-wi.
- “blooming” on gradient-echo images,
caused by magnetic susceptibility artifact (hemosiderin).
b. Localized
- Almost exclusively involves the knee: infrapatellar >> suprapatellar > posterior intercondylar.
Fig. 26: Case 11. Localized and intra-articular PVNS. Inespecific knee CR.
Fig. 27: Case 11. Localized and intra-articular PVNS.
On axial T1 wi, a posterior intercondylar nodule with extrinsic erosion of medial condile. On axial T1FS wi with Gadolinium administration (b), the lesion enhances in comparison to axial T1FS without (c). Blooming effect is visible on coronal GE wi (d) although much less extensive than those seen in diffuse intraarticular disease.
- Low- to intermediate-signal-intensity soft-tissue masses with well-defined nodular or lobular margins on T1-wi.
- Heterogeneous and variable signal intensity was seen on T2wi
- Joint effusion
- Hemosiderin deposition much less extensive than those seen in diffuse intraarticular disease.
2. Extra-articular and localized: giant cell tumor tendon sheath (GCT – TS) hand (80%),
feet,
knee (12%)
- Second most common mass hand/wrist
- Lobulated soft tissue mass < 2 cm
- More common volar surface
- Osseous erosion uncommon 10% – 15%
MALIGNANT TUMORS:
SYNOVIAL SARCOMA:
Clinical features
- Malignant mesenchymal tumor
- Young adults 15 – 40 years of age
- Often indolent slow growing mass (4 years to diagnosis)
Pathology: Two cell lines: Monophasic/biphasic
- Epithelial (keratin positive)
- Spindle cells
- Variable – calcification,
hemorrhage
Radiology
Radiographs
- Soft tissue mass near joint
- Bone erosion or periosteal reaction (15% – 20%),
bone invasion (5%)
- Soft tissue calcification up to 30%–best by Bone scan –increased activity
Fig. 28: Case 12. Monophasic synovial sarcoma in a 57 year-old female. On US, large and heterogenous mass in the distal thigh with increased vascularization.
References: Hospital Clínico San Carlos, Madrid
Fig. 29: Case 12. Monophasic synovial sarcoma. On sagittal (b) and axial (c) T2FS wi, marked heterogeneity and multilobulation (bowl of grapes sign) with the triple sign (areas of high [*red], intermediate [*blue], and low signal intensity). On sagittal T1FS wi +Gd (d), heterogeneous enhancement.
References: Hospital Clínico San Carlos, Madrid
Fig. 30: Case 12. Monophasic synovial sarcoma. Initial PET-CT in same patient with synovial sarcoma in the right lower extremity and a left inguinal linph node involved.
References: Hospital Clínico San Carlos, Madrid
MRI
- T1W image – similar to muscle
- T2W image – usually high intensity
- Triple sign on T2W MR (30%) (nonspecific)
- Not uncommonly well defined with pseudocapsule – simulates benign characteristics
- Fluid – fluid level 10% – 25% (hemorrhage) worse prognosis in highly vascular lesions
Fig. 31: Case 12. Monophasic synovial sarcoma. Initial PET-CT in same patient with synovial sarcoma in the right lower extremity and a left inguinal linph node involved.
References: Hospital Clínico San Carlos, Madrid
Fig. 32: Case 12. Monophasic synovial sarcoma. (b Three months after chemotherapy, PET-CT confirmed good response.
References: Hospital Clínico San Carlos, Madrid
CLEAR CELL SARCOMA
Clinical features
- Arise in tendons/aponeurosis
- Deep tissue without skin involvement
- Foot/ankle (43%),
knee,
thigh,
hand
Pathology
- Cells with clear cytoplasm
- Framework of fibrocollagenous tissue
- Intracellular melanin 60% – 75%
- Hemosiderin also present
Radiology
- Soft tissue mass at/in tendon/aponeurosis
- Bone erosion/destruction
- CT/MRI – infiltrative soft tissue mass
- T1W image – intermediate intensity
- T2W image – may be low intensity
PLEOMORPHIC SARCOMA :
Clinical features:
- Formerly known as fibrosarcoma and malignant fibrous histiocytomas (MFH).
- It is an aggressive tumour which accounts for 25-40% of all adult soft tissue sarcomas.
- A painless,
enlarging palpable mass.
Radiology:
- Plain radiographs will demonstrate a soft tissue mass and if arising from bone,
then an aggressive destructive bony lesion.
In some cases,
curvilinear or punctate regions of calcification may be demonstrated.
Fig. 33: Case 13. Pleomorphic sarcoma. On coronal T2 wi (a), the mass shows and intermediate signal intensity similar to the muscle. With FS, on axial T2FS wi (b), the lesion is hyperintense. After Gadolinium administration, on axial T1FS wi (d), the tumor shows an intense and heterogeneous enhancement in comparison with T1FS without (c).
- On MRI, these tumours are typically relatively well circumscribed,
located within or adjacent to muscle,
exerting positive mass effect on surrounding structures due to their (usual) large size at presentation.
- On T1wi,
intermediate (to low) signal intensity,
similar to adjacent muscle heterogeneity if hemorrhage,
calcification,
necrosis,
myxoid material are present and prominent enhancement of solid components.
- On T2 wi, intermediate to high signal intensity and heterogeneity if haemorrhage,
calcification,
necrosis,
myxoid material are present.
Role of imaging techniques:
Evaluation of a patient who presents with a soft tissue mass about the knee often includes radiography,
ultrasonography (US),
and magnetic resonance (MR) imaging.
Conventional radiography and CT are useful for evaluating an underlying focal bone lesion,
for analyzing bone destruction and assessment of mineralization within the mass.
Fig. 34
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US can be particularly useful for distinguishing between cystic and solid lesions.
In addition to categorizing the lesions as truly cystic or solid,
the differential diagnosis may be further narrowed by considering the anatomic location of the lesion or characteristic imaging features.
Ultrasound guided biopsy is a common form of image guided biopsy,
offering convenience and real time dynamic observation with echogenic markers on cannulae allowing for precise placement.
Fig. 35
MR imaging is well-suited for the evaluation of soft-tissue tumors and tumorlike lesions because of its intrinsically high soft-tissue contrast.
Evaluation with MR images allows tumor staging,
detection of neurovascular involvement,
identification of tumor necrosis,
and preoperative planning.
Fig. 36
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In cases of malignat soft-tissue tumors and in the particular case of synovial sarcoma and clear-cell carcinoma in which metastatic disease is common,
PET/CT accurately depict metastatic disease and lymph nodal involvement.
In follow-up imaging,
PET-CT allows assessment of response and disease recurrance.
Fig. 37
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Proposed diagnostic algorithm
Fig. 38: Proposed diagnostic algorithm
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