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Popliteal fossa masses - a pictorial review
Poster No.:
P-0055
Congress:
ESSR 2015
Type:
Educational Poster
Authors:
A. Shah , R. Botchu , S. L. J. James ; Leicester/UK, Kettering/
1
2
3 1
2
3
UK, Birmingham/UK
Keywords:
Neoplasia, Education and training, Cysts, Diagnostic procedure,
Ultrasound, MR, CT, Musculoskeletal system, Musculoskeletal soft
tissue, Anatomy
DOI:
10.1594/essr2015/P-0055
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Page 1 of 70
Learning objectives
•
•
•
To recap the pertinent anatomy of the popliteal fossa.
To present a pictorial review of the common pathologies encountered in the
popliteal fossa and present less common causes.
Popliteal masses can be defined by its origin:
1.
2.
3.
4.
5.
6.
7.
8.
Synovial
Bursal
Soft tissue
Vascular
Nerve
Lymphoid
Muscle/Tendon
Bone
Background
•
•
•
•
A wide variety of masses may be visualized in the posterior knee some of
which present as palpable masses or are found incidentally on MRI.
We present a pictorial review of the most common lesions found in the
popliteal fossa as well as several of the less common entities we have
encountered.
Thorough understanding of the appearance and location of the popliteal
fossa masses one can reach a specific differential diagnosis in the majority
of cases.
This will eliminate unnecessary additional investigations and enable prompt
and appropriate treatment.
Imaging findings OR Procedure Details
POPLITEAL FOSSA ANATOMY:
Page 2 of 70
Fig. 1: Axial PD MRI of the popliteal fossa.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Page 3 of 70
Fig. 2: Coronal PD MRI of the popliteal fossa.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
SYNOVIAL
Pigmented vilonodular synovitis (PVNS):
•
•
•
•
•
PVNS: Knee (80%) > ankle > hip > shoulder > elbow
Unknown aetiology - possibly a reactive inflammatory process
Abnormal synovium prone to haemorrhage
Haemosiderin deposits in synovium and with nodule formation due to
repeated haemorrhagic effusions
Managed as low-grade, locally aggressive neoplasm
Page 4 of 70
Fig. 3: MRI PD FS sagittal (a) and T1 axial (b) demonstrating a large posterior intraarticular lobulated mass with villonodular proliferation of haemorrhagic synovium
demonstrated areas of low signal intensity representing haemosiderin (red arrows).
Typical imaging appearances of PVNS which was histologically confirmed.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
MRI diagnostic in 95% of cases:
•
•
•
Gradient echo imaging shows "blooming" phenomenon of haemosiderinladen nodules Fig. 4 on page 40
Large effusion +/- erosions
Intense inhomongenous enhancement Fig. 5 on page 40
Synovial chondromatosis:
•
•
•
Knee (50-65%) > hip > shoulder > elbow
Benign neoplastic synovial proliferation and formation of cartilaginous
(chondromatosis) or osseous bodies (osteochondromatosis)
Malignant transformation is extremely rare and no reliable distinguishing
feature
Page 5 of 70
Fig. 6: Lateral radiograph of the knee (a) and sagittal gradient echo T2* (b)
demonstrates innumerable tiny ossified bodies both in the anterior and posterior joint
space. Numerous bodies can become conglomerate, appearing as a "mass" within the
joint as in this case. This was a case of biopsy proven synovial osteochondromatosis.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
•
Multiple, round, similar-sized calcified bodies seen on radiograph
80% have erosions detectable by MR
Bodies are of variable MR signal, depending on proportion of calcium,
chondroid, and mature ossific tissue
BURSAL:
Baker's cyst:
•
•
Fluid-filled sac with neck arising from interspace between gastrocnemius
muscle and semimembranosus tendon Fig. 7 on page 41
Contralateral subclinical Baker cysts common
Page 6 of 70
Fig. 8: Axial PD MRI of the popliteal fossa demonstrating the characteristic
appearance of a Baker's cyst. A cystic structure with a 'talk bubble' configuration
is seen arising between the tendons of the medial head of the gastrocnemius and
semimembranosus muscles.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
Must see neck arising from medial gastrocnemius-semimembranosus
tendon interspace to make diagnosis of Baker cyst Fig. 9 on page 43
Characteristic "talk-bubble" configuration on transverse scans
Page 7 of 70
•
•
Free fluid tracking adjacent to cyst indicates recent leakage Fig. 10 on page
44
Acute pain suggetive of rupture or haemorrhage into cyst Fig. 11 on page
44
SOFT TISSUE:
Ganglion:
•
•
•
Myxoid tissue encased in a fibroblastic collagenous wall (pseudocapsule)
with no synovial lining
Arises from joint or tendon but intraosseous (Muir et al) or intramusclular
(Parks et al) locations have been reported
In the knee, commonly arises from cruciate ligaments or joint capsule
Fig. 12: MR STIR axial (a), coronal (b) and sagittal (c) of the knee demonstrating
a uniformly T2 hyperintense multilobulated soft tissue cystic mass arising from the
posterior knee joint capsule which was histologically proven as a ganglion.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
T2: Uniformly hyperintense soft tissue cystic mass
Lipoma:
•
•
Benign lipomatous tumour composed of mature adipose tissue representing
almost 50% of soft tissue masses.
Well-defined, oblong-shaped encapsulated mass with fine linear striations
parallel to long axis of tumour
Page 8 of 70
•
Characterised by location as:
1.
2.
3.
superficial to the deep fascia
deep to the deep fascia
intramuscular
Fig. 13: Axial (a) and coronal (b) T1 and corresponding axial (c) and coronal (d) fat
suppressed MRI sequences demonstrating the fatty lesion seen in the radiograph
of Figure 10. The lesion is high signal on T1 in keeping with fat and demonstrates
complete suppression in keeping with a lipoma. Due to its size biopsy was performed
confirming a simple lipoma
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
Page 9 of 70
•
Should be similar to subcutaneous fat on any imaging modality Fig. 14 on
page 46
Fibromatosis:
•
•
Spectrum of benign fibroblastic disorders with tendency to infiltrate adjacent
tissues
Tendency for local recurrence (65%) without metastasis
Page 10 of 70
Page 11 of 70
Fig. 15: Axial T1 (a), axial STIR (b) and corresponding T1 and STIR coronal MRI
images (c) and (d) respectively, demonstrate a soft tissue mass hypointense to skeletal
muscle (blue arrow) that was low signal on all sequences. There curvilinear areas of
low signal represent areas of mature collagen.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
•
Intense enhancement post contrast
Extends along fascial planes
Locally infiltrative
•
US: Nonspecific soft tissue mass, variable echogenicity, smooth well-defined
margins Fig. 16 on page 49
non-contrast CT: Soft tissue mass hyperdense to muscle
MRI: T1 - Hypointense to muscle; T2 - Hyperintense with bands of
hypointense signal regions (fibrous components) Fig. 15 on page 47
•
•
Fasciitis:
•
•
•
Benign mass-forming fibrous proliferation
Extension of mass along fascia suggests diagnosis
Three types:
•
•
•
Subcutaneous = well-circumscribed round nodule attached to fascia
extending into superficial fat
Fascial = poorly circumscribed fascial mass with stellate growth
pattern
Intramuscular = ovoid intramuscular mass attached to fascia
Page 12 of 70
Fig. 17: Sagittal T1 (a) demonstrates a soft tissue lesion anterior to the popliteal
vessels which is similar intensity to skeletal muscle on T1 and high signal intensity
on axial STIR (b). Pre (c) and post (d) contrast T1FS demonstrates predominantly
peripheral enhancement. Biopsy of the lesion confirmed proliferative fasciitis. MR
appearances will depend on the amount of collagen, myxoid and cellular component.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
Page 13 of 70
•
•
Similar intensity to skeletal muscle on T1, intermediate to high signal
intensity on T2
Enhancement depends on myxoid and fibrous content
Epidermal inclusion cyst:
•
•
benign lesion formed by the inclusion of dermal squamous epithelium with
retention of keratinous debris and cholesterol or sebaceous materials
Well-circumscribed subcutaneous mass
Fig. 18: Axial MR images of the knee. Dermal soft tissue sesion seen in figure 15 is
low on T1 (a) and high on T2 (b) with internal low signal debris. Post contrast T1FS (c)
demonstrates only subtle peripheral enhancement. Biopsy proven epidermal inclusion
cyst.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
"Pseudotestis" appearance on ultrasound
Page 14 of 70
Fig. 19: Transverse ultrasound image of a superficial subepidermal lesion
with internal speckles and posterior acoustic enhancement. This lesion was
histologically confirmed as an epidermal inclusion cyst - though the classic
'pseudotestis' appearance is not seen.
Page 15 of 70
•
•
•
References: Radiology, NHS Trust, University Hospitals of Leicester Leicester/UK
T1 isointense to muscle with mild heterogeneous signal ranging from low to
high signal
T2 Hyperintense signal with low or high signal debris Fig. 18 on page 50
Absent to low-level central contrast enhancement with enhancing capsule
Heterotopic ossification/Myositis ossificans:
•
•
•
Benign Heterotopic formation of bone and cartilage in soft tissue
Stem cells for osteoid production exist within affected soft tissues
Stimulus is usually traumatic though it may be inapparent or forgotten by
patient
Page 16 of 70
Fig. 20: AP and lateral radiographs of the knee demonstrates ossification of the
soft tissues at the posterior aspect of the knee. Importantly, no connection to the
femur is seen and periosteal reaction. Axial CT (b) and MR STIR (d) demonstrates
bone density within the soft tissues posterior to the medial femoral condyle with no
aggressive features. Differentials include heterotopic ossification but a parosteal
Page 17 of 70
osteosarcoma (Figure 40) cannot be entirely excluded although less likely. Biopsy
confirmed heterotopic ossification.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
Evolving radiographic stages with stages of bone development (indistinct
soft tissue through to mature bone formation)
Peripheral bone formation with central immature bone (versus parosteal
osteosarcoma which has organised bone centrally, less mature bone
peripherally)
Soft tissue sarcoma:
•
•
•
•
Malignant tumour that arises from connective (mesenchymal) tissue other
than bone
Classified according to tissue type rather than anatomical origin.
Grow centrifugally from single focus
Suspicious features include a soft tissue mass that is increasing in size, has
a size >5cm or is deep to the deep fascia, whether or not it is painful
Imaging features:
•
•
•
•
•
Features depend on the type of tumour and may have, internal cystic
component, myxoid tissue, fat, calcification or necrosis
Large heterogeneous hypoechoic mass
Usually located in deep (subfascial) tissues
Well-encapsulated
Usually hypervascular with disorganized vascular pattern on colour Doppler
imaging
± Myxoid tissue: Well-defined, intra-tumoural anechoic or hypoechoic areas.
± Necrosis: Poorly-defined hypoechoic areas.
± Calcification: Discrete intra-tumoural echogenic foci with acoustic shadowing.
± Haemorrhage: Ill-defined intra-tumoural echogenic areas without acoustic shadowing
Page 18 of 70
Fig. 21: Axial STIR (a) demonstrates a complex soft tissue lesion superficial to
the deep fascia. Pre (b) and post (c) contrast T1Fs demonstrates heterogenous
enhancement. Soft tissue sarcomas are tumors without specific imaging findings
beyond solid enhancing, soft tissue mass and needs biopsy for histological diagnosis.
Biopsy confirmed undifferentiated pleomorphic sarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
UNDIFFERENTIATED PLEOMORPHIC SARCOMA (formerly malignant fibrous
histiocytoma):
•
•
5% are extensively haemorrhagic, presenting as fluctuant mass often
misdiagnosed as haematoma
Underlying malignancy should be excluded in patients thought to have
spontaneous musculoskeletal haemorrhage
Fig. 22: Axial T2FS (a) and sagittal T1 (b) MRI demonstrates a high T2/low T1 signal
mass in the posterior aspect of the knee with internal areas of intermediate and low
Page 19 of 70
signal which bloom on sagittal gradient echo T2* (c) in keeping with calcification. This
lesion was histologically proven as undifferentiated pleomorphic sarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
•
Calcification present peripherally in 5-20%
Similar signal intensity to muscle on T1 MRI
Heterogeneously hyperintense to muscle on fluid-sensitive sequences
LIPOSARCOMA:
•
•
•
Liposarcomas account for approximately 10% of all soft tissue sarcomas
Liposarcomas arise from fat cells and can occur anywhere in the body.
There are 6 different sub-types; the most common are:
1.
2.
3.
well-differentiated liposarcoma,
pleomorphic liposarcoma
myxoid liposarcoma
Fig. 23: Transverse ultrasound of the knee demonstrates a fatty lesion with increased
doppler signal (a). Panoramic ultrasound view (b) of the lesion demonstrates a large
fatty lesion. Axial STIR MR (c) and T2 sagittal (d) demonstrates mixed signal intensity
Page 20 of 70
mass with heterogeneous enhancement (e) post contrast (pre contrast T1FS not
shown). Biopsy confirmed a myxoid liposarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
Non-specific soft tissue mass with myxoid and fat tissue ~< 25% fat
SYNOVIAL SARCOMA:
•
•
Contrary to name, synovial sarcoma is not does not arise intra-articular or
from synovium
Malignant soft tissue tumour with predilection for juxtaarticular regions of
young patients especially popliteal fossa of knee or tendon sheath
Fig. 24: Transverse ultrasound (a) demonstrates a soft tissue lesion with increased
doppler signal. Axial T1 MR (b) and sagittal T2FS (c) demonstrates a juxtaarticular low
T1/high T2 soft tissue mass which enhanced post contrast (images not shown). Lesion
proved to be a synovial sarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
Enhancing soft tissue mass near a joint
Calcification, heamosiderin, cystic change, fluid-fluid levels relatively
common
VASCULAR:
Popliteal artery aneurysm:
Page 21 of 70
•
Popliteal artery is most common lower extremity location (70% of all
peripheral aneurysms).
Fig. 25: Longitudinal panoramic view of the popliteal artery demonstrating a focal
fusiform aneurysm with mural thrombus.
References: http://www.ultrasoundcases.info/ Dr. Taco Geertsma, Hospital Gelderse
Vallei in Ede, The Netherlands
Imaging features:
•
Fusiform or saccular arterial enlargement, often with laminar thrombus within
Fig. 26: CT axial (a) and coronal reformat (b) demonstrating a right popliteal aneurysm
(yellow arrow) which is completely occluded with thrombus.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Page 22 of 70
Fig. 27: MR axial (a) and coronal T1 (b), coronal STIR (c) demonstrating the occluded
right popliteal artery which contains hyperintense T1 mural thrombus with central low
signal material in keeping with chronic blood products. Image (d) is a zoomed up STIR
coronal image demonstrate layers of differing blood products within the aneurysm.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Vascular malformations and haemangiomas (gresham et al):
•
•
•
Haemangiomas are vascular tumours that are rarely apparent at birth,
grow rapidly during the first 6 months of life, involute with time and do not
necessarily infiltrate but can sometimes be destructive
Vascular malformation/arteriovenous malformation (AVM) is a diffuse mass
of dilated tortuous serpiginous vessels involving both arteries and veins.
In contrast to haemangiomas, they are present at birth, slow growing,
infiltrative, and destructive
Almost all AVMs and nearly 40% of haemangiomas eventually require
intervention
Page 23 of 70
Fig. 28: MR Axial T1 (a), T2FS (b) and post contrast T1FS (c) demonstrating a low T1/
High T2 soft tissue mass with intense enhancement. Intralesional low signal areas are
suggestive of flow voids. Biopsy confirmed a haemangioma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
•
Flow voids in high flow AVM
High signal intensity on fat-suppressed T2 : Bright "bag of worms"
Low T2 signal intensity foci may reflect phleboliths, scars or haemosiderin
deposition best seen on gradient echo sequences
Haemangiopericytoma (HPC):
•
•
•
•
Entity arising from pericytes with 70% being benign
Characterised by its branching vascular pattern that (although can be seen
in numerous benign and malignant tumours).
Solitary fibrous tumour and haemangiopericytoma are histologically similar
and currently used almost interchangeably and are thought of as being part
of the same histopathologic spectrum (Bruder et al)
Lower extremity > retroperitoneum > head/neck > trunk > upper extremity
Page 24 of 70
Fig. 29: Longitudinal ultrasound image (a) demonstrating a vascular soft tissue mass.
Axial MR T2 (b) and sagittal T1 (c) demonstrates a predominantly hyperintense lesion
with internal branching flow-voids (red arrows) in keeping with vessels. Biopsy proved a
haemangiopericytoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
Haemangiopericytomas have almost no distinguishing characteristics
radiologically.
Myopericytoma:
•
•
•
Benign neoplasm thought to represent a subset of haemangiopericytomas
(HPCs)
WHO defines myopericytoma as a distinct entity encompassing
myopericytoma and glomangiopericytoma (flethcer)
Arises from perivascular myoid cells
Fig. 30: MR axial STIR (a) and pre (b) and post (c) axial T1FS demonstrates
an intermuscular maass more conspicuous post contrast. Biopsy confrmed a
myopericytoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
•
•
Typicallly well-defined subcutaneous mass typically occuring in the lower
extremities.
Isointense to muscle on T1
Hyperintense on T2 weighted sequences
Intense enhancement post contrast
NEURAL:
Page 25 of 70
Peripheral nerve sheet tumour (PNST):
•
•
•
Benign tumour arising from nerve; histologically classified as either
schwannoma or neurofibroma
Any peripheral nerve can be affected
Intermuscular > subcutaneous > intramuscular
Fig. 31: MR sagittal gradient echo T2 (a) and T2FS (b) demonstrates a hyperintense
fusiform mass oriented along long axis of nerve which is arising from the common
peroneal nerve. The mass can be clearly seen arising from the nerve (blue arrow) with
the nerve entering the mass. Biopsy confirmed a peripheral nerve sheath tumour.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
•
Typically fusiform or oblong-shaped and oriented along long axis of nerve
Fig. 31 on page 58
Nerve entering or exiting from tumor is seen in majority (90%)
Posterior enhancement is common (70%)
Page 26 of 70
•
Fig. 32: Coronal (a) and axial (b) MR T2FS demonstrating the classical
fusiform 'target' lesion with the nerve seen entering and exiting from the
lesion in keeping with a peripheral nerve sheath tumour (PNST).
References: Radiology, NHS Trust, University Hospitals of Leicester Leicester/UK
"Target" sign: Representing central fibrocollagenous component (low T2)
surrounded by peripheral myxoid component (high T2) commonly seen in
neurofibroma > schwannoma
Page 27 of 70
Fig. 33: Axial STIR MR of a neurofibroma which demonstrates the
typical 'target' appearance of a nerve sheath tumour. Central (red arrow)
fibrocollagenous component (low T2) surrounded by peripheral (blue arrow)
myxoid component (high T2).
References: Radiology, NHS Trust, University Hospitals of Leicester Leicester/UK
Malignant peripheral nerve sheet tumour:
•
•
•
•
Malignant spindle cell sarcoma of neural origin
25-70% associated with Neurofibromatosis 1
10-20% radiation-induced
5-10% of all soft tissue sarcomas
Page 28 of 70
Fig. 34: MR sagittal T1 (a) and axial STIR (b) demonstrates a large fusiform T2
hyperintense mass in the region of the common peroneal nerve. Biopsy confirmed a
malignant peripheral nerve sheath tumour.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
Infiltrative, ill-defined, fusiform mass > 5 cm
LYMPHOID:
Non Hodgkins Lymphoma:
•
•
•
Extranodal lypmhoma soft tissue lymphoma is reported as <0.01%
Must indicate to pathology if lymphoma is suspected to allow appropriate
immunohistochemical analysis to be performed.
Staging and management of lymphoma is drastically different compared to
sarcomas
Page 29 of 70
Fig. 35: Axial MR T1 (a) and STIR (b) demonstrates an ill-defined infiltrative soft tissue
mass encasing the popliteal artery - typical of lymphoma. Axial CT angiogram (c) of
the lower leg demonstrated a patent popliteal artery. Biopsy confirmed Non-Hodgkin's
lymphoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging findings:
•
•
•
•
Infiltrative mass - encases neurovascular bundle
Isointense to muscle on T1
Hyperinternse of T2
Contrast imaging does not add further to the tissue characterisation: both
homogenous and hetergenous enhancement post contrast has been
reported.
MUSCULAR/TENDON:
Giant cell tumour of tendon sheath (GCTTS):
•
•
•
•
Benign synovial proliferation within tendon sheath,
Same pathologic entity as pigmented villonodular synovitis (PVNS) but in a
different location
Lobulated soft tissue mass immediately adjacent to tendon
Hand > wrist > ankle/foot > knee > elbow > hip
Page 30 of 70
Fig. 36: Sagittal T1 MR (a) and axial STIR (b) demonstrates a soft tissue mass with
peripheral low signal rim on both sequences suggestive of haemosiderin. Biopsy
confirmed a giant cell tumour of the tendon sheath.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
Low T1 and T2 signal intensity soft tissue lobulated mass with intense
inhomogenous enhancement
Low signal hemosiderin rim "blooms" on gradient echo imaging
Leiomyosarcoma:
•
•
Vascular malignant neoplasm arising from smooth muscle
Arises in skin, soft tissue, blood vessel
Page 31 of 70
Fig. 37: AP radiograph (a) of the knee demonstrates abnormal increased soft tissue
density in the medial aspect of the keen (blue arrow). Axial (b) and sagittal (c) STIR
MR demonstrates a large soft tissue mass with multiple fluid-fluid levels (red arrows).
Biopsy confirmed a leiomyosarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
•
•
Nonspecific soft tissue mass +/- calcification, necrosis, hemorrhage, or
cystic change giving rise to fluid-fluid levels
Aggressive lesion invading adjacent bone Fig. 38 on page 62
BONE:
Osteosarcoma:
•
•
1.
2.
Osteosarcomas are malignant bone tumours and the second most common
primary bone tumour after multiple myeloma (murphey et al)
Majority occurs in lower extremity.
Primary: Typically young patients
Secondary: Typically elderly due to malignant degeneration of underlying
bone lesion eg Paget diseases, bone infarcts, osteochondroma.
Many sub-types with varying imaging findings including:
•
Intramedullary: ~80%
1.
2.
Conventional low and high-grade
Telangiectatic osteosarcoma
Page 32 of 70
•
Surface or juxtacortical: ~10-15%
1.
2.
3.
Intracortical osteosarcoma
Parosteal osteosarcoma
Periosteal osteosarcoma
•
Extraskeletal: ~5%
Conventional osteosarcoma:
Fig. 39: AP radiograph of the knee (a) demonstrates abnormal increased density of
the medial tibial plateau with an extra-osseous component (blue arrow). Sagittal T1
(b) and T2 FS (c) MR demonstrates an osseous lesion with cortical break through
and soft tissue extending into the popliteal fossa. Biopsy confirmed a conventional
osteosarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Parosteal osteosarcoma (differential diagnosis - heterotopic ossification/myositis
ossificans):
•
•
•
•
•
Low-grade osteosarcoma arising on surface of bone
65% of all lesions occur in the posterior cortex of the distal femoral
metadiaphysis
Fusiform mass along length of bone which tends to "wrap around" as it
enlarges
Initially associated with the surface and can present as an area of cortical
thickening, and therefore be misinterpreted as a healing stress fracture
As the mass grows, parts of it may be adjacent to cortex without being
contiguous to it, giving appearance of "cleft"
Page 33 of 70
•
Bone is mature at site of origin, less mature at periphery [opposite
ossification pattern of myositis ossificans (Figure 20)]
Fig. 40: AP and lateral radiograph of the knee demonstrating an ossified mass (blue
arrows) at the distal femur extending into the popliteal fossa and is intimately related
to the cortex. Differential diagnosis of this mass is heterotopic ossification (Figure 20).
Subsequent biopsy confirmed parosteal osteosarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Page 34 of 70
Fig. 41: Axial CT and sagittal reformat of the ossified mass seen in Figure 40,
demonstrating mature bone centrally with peripheral soft tissue component. The mass
is seen contiguous with the underlying cortex of the distal femur. Biopsy confirmed
parosteal osteosarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Page 35 of 70
Fig. 42: Axial T1 (a) and sagittal T1 (c) and axial STIR (b) and sagittal STIR MR
images demonstrates the distal femoral mass extending posteriorly into the popliteal
fossa. The mass is isointense to skeletal muscle on T1 and heterogenous to high
signal on STIR with intralesional low signal intensities on both sequences representing
ossified tumour regions. Biopsy confirmed parosteal osteosarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Page 36 of 70
Imaging features:
•
•
•
Osseous component is low T1 signal with isointense to heterogeneous T1
signal to skeletal muscle of the soft tissue component
Heterogeneous high T2 weighted signal, with intralesional low signal of
tumor bone
High signal soft tissue seen at periphery of lesion
Chondrosarcoma:
•
•
•
•
malignant cartilaginous tumours that account for 20-27% of all primary
malignant bone tumours.
Predominantly in the long bones
Primary or secondary (osteochondroma, enchondroma)
Multiple subtypes:
1.
2.
3.
4.
conventional intramedullary chondrosarcoma (or central chondrosarcoma):
juxtacortical chondrosarcoma:
myxoid chondrosarcoma:
extraskeletal chondrosarcoma
Fig. 43: Lateral radiograph of the knee demonstrating areas of calcification within
the soft tissue posteriorly (blue arrow). MR Sagittal T1 (b), sagittal (c) and axial
(d) T2FS demonstrating high T2 signal mass. Biopsy confirmed an extraskeletal
chondrosarcoma.
References: Radiology, NHS Trust, University Hospitals of Leicester - Leicester/UK
Imaging features:
1.
2.
3.
Destructive lesion with intralesional calcification (chondroid matrix)
High T2 signal of chondroid matrix
Hetergenous contrast enhancement
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Images for this section:
Fig. 1: Axial PD MRI of the popliteal fossa.
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Fig. 2: Coronal PD MRI of the popliteal fossa.
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Fig. 3: MRI PD FS sagittal (a) and T1 axial (b) demonstrating a large posterior
intra-articular lobulated mass with villonodular proliferation of haemorrhagic synovium
demonstrated areas of low signal intensity representing haemosiderin (red arrows).
Typical imaging appearances of PVNS which was histologically confirmed.
Fig. 4: Typically appearances of PVNS. Radiograph of the knee (a) demonstrates
increased soft tissue density in the posterior aspect of the knee (red arrow). MR confirmed
a large intra-articular lobulated lesion with areas of nodular low signal intensity regions
(haemosiderin) seen on sagittal PD (b) which 'blooms' on axial T2* gradient echo (c).
Fig. 5: MRI PD sagittal (a) demonstrating a well-defined low signal lesion abutting the
posterior cruciate ligament which demonstrates intense inhomogenous enhancement
post contrast on sagittal (b) and axial (c) T1FS which is typical for PVNS. Contrast can
make small lesions more conspicuous but usually not required. Pre T1FS not shown.
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Fig. 6: Lateral radiograph of the knee (a) and sagittal gradient echo T2* (b) demonstrates
innumerable tiny ossified bodies both in the anterior and posterior joint space. Numerous
bodies can become conglomerate, appearing as a "mass" within the joint as in this case.
This was a case of biopsy proven synovial osteochondromatosis.
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Fig. 7: Transverse ultrasound of the left popliteal fossa in three different patients
demonstrating a fluid filled structure arising between the tendon of the medial head
gastrocnemius (yellow Gastroc) and the semimembranosus tendon (Sm) in keeping with
a Baker's cyst.
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Fig. 8: Axial PD MRI of the popliteal fossa demonstrating the characteristic appearance of
a Baker's cyst. A cystic structure with a 'talk bubble' configuration is seen arising between
the tendons of the medial head of the gastrocnemius and semimembranosus muscles.
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Fig. 9: Transverse ultrasound image of the right popliteal fossa demonstrating a Baker's
cyst. Image A depicts a normal popliteal fossa with the medial head gastrocnemius
marked by the green arrow and the adjacent semimembranosus tendon (yellow star).
Image B demonstrates a cystic structure arising between the medial head gastrocnemius
and semimembranosus tendon (yellow star) in a characteristic 'talk bubble' configuration
(yellow outline).
Fig. 10: A) Longitudinal ultrasound image, B) coronal T1 fat-saturated MR, C) coronal
T2 MR and D) coronal STIR MR demonstrating a ruptured Baker's cyst. Fluid is seen
tracking around the medial border of the medial head gastrocnemius.
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Fig. 11: Transverse ultrasound (a) demonstrates a large echogenic solid lesion in
the popliteal fossa with no increased doppler signal. T1 axial (b) and sagittal (c)
demonstrates a large T1 hyperintense collection arising between the medial head of
the gastrocnemius (blue arrow) and semimembranous tendon (red arrow) in keeping
with a large haemarrhagic Baker's cyst. Signal loss of the knee joint due to total knee
arthroplasty.
Fig. 12: MR STIR axial (a), coronal (b) and sagittal (c) of the knee demonstrating a
uniformly T2 hyperintense multilobulated soft tissue cystic mass arising from the posterior
knee joint capsule which was histologically proven as a ganglion.
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Fig. 13: Axial (a) and coronal (b) T1 and corresponding axial (c) and coronal (d) fat
suppressed MRI sequences demonstrating the fatty lesion seen in the radiograph of
Figure 10. The lesion is high signal on T1 in keeping with fat and demonstrates complete
suppression in keeping with a lipoma. Due to its size biopsy was performed confirming
a simple lipoma
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Fig. 14: AP and lateral radiographs of the knee demonstrating increased fat density
within the soft tissues of the posteromedial aspect of the knee (blue arrow). Note how this
is similar to the normal density of the suprapatellar pouch that contains fat (red arrow)
suggesting this mass is fatty in nature. Lipomas follow fat density/signal on all modalities.
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Fig. 15: Axial T1 (a), axial STIR (b) and corresponding T1 and STIR coronal MRI images
(c) and (d) respectively, demonstrate a soft tissue mass hypointense to skeletal muscle
(blue arrow) that was low signal on all sequences. There curvilinear areas of low signal
represent areas of mature collagen.
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Fig. 16: Transverse ultrasound of the popliteal fossa demonstrating a predominantly
hypoechoic mass with no increased doppler signal (images not shown). On its own,
appearances are non-specific and further characterisation with MR is needed.
Fig. 17: Sagittal T1 (a) demonstrates a soft tissue lesion anterior to the popliteal vessels
which is similar intensity to skeletal muscle on T1 and high signal intensity on axial
STIR (b). Pre (c) and post (d) contrast T1FS demonstrates predominantly peripheral
enhancement. Biopsy of the lesion confirmed proliferative fasciitis. MR appearances will
depend on the amount of collagen, myxoid and cellular component.
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Fig. 18: Axial MR images of the knee. Dermal soft tissue sesion seen in figure 15 is
low on T1 (a) and high on T2 (b) with internal low signal debris. Post contrast T1FS (c)
demonstrates only subtle peripheral enhancement. Biopsy proven epidermal inclusion
cyst.
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Fig. 19: Transverse ultrasound image of a superficial subepidermal lesion with internal
speckles and posterior acoustic enhancement. This lesion was histologically confirmed
as an epidermal inclusion cyst - though the classic 'pseudotestis' appearance is not seen.
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Fig. 20: AP and lateral radiographs of the knee demonstrates ossification of the soft
tissues at the posterior aspect of the knee. Importantly, no connection to the femur
is seen and periosteal reaction. Axial CT (b) and MR STIR (d) demonstrates bone
density within the soft tissues posterior to the medial femoral condyle with no aggressive
features. Differentials include heterotopic ossification but a parosteal osteosarcoma
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(Figure 40) cannot be entirely excluded although less likely. Biopsy confirmed heterotopic
ossification.
Fig. 21: Axial STIR (a) demonstrates a complex soft tissue lesion superficial to
the deep fascia. Pre (b) and post (c) contrast T1Fs demonstrates heterogenous
enhancement. Soft tissue sarcomas are tumors without specific imaging findings beyond
solid enhancing, soft tissue mass and needs biopsy for histological diagnosis. Biopsy
confirmed undifferentiated pleomorphic sarcoma.
Fig. 22: Axial T2FS (a) and sagittal T1 (b) MRI demonstrates a high T2/low T1 signal
mass in the posterior aspect of the knee with internal areas of intermediate and low signal
which bloom on sagittal gradient echo T2* (c) in keeping with calcification. This lesion
was histologically proven as undifferentiated pleomorphic sarcoma.
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Fig. 23: Transverse ultrasound of the knee demonstrates a fatty lesion with increased
doppler signal (a). Panoramic ultrasound view (b) of the lesion demonstrates a large
fatty lesion. Axial STIR MR (c) and T2 sagittal (d) demonstrates mixed signal intensity
mass with heterogeneous enhancement (e) post contrast (pre contrast T1FS not shown).
Biopsy confirmed a myxoid liposarcoma.
Fig. 24: Transverse ultrasound (a) demonstrates a soft tissue lesion with increased
doppler signal. Axial T1 MR (b) and sagittal T2FS (c) demonstrates a juxtaarticular low
T1/high T2 soft tissue mass which enhanced post contrast (images not shown). Lesion
proved to be a synovial sarcoma.
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Fig. 25: Longitudinal panoramic view of the popliteal artery demonstrating a focal fusiform
aneurysm with mural thrombus.
Fig. 26: CT axial (a) and coronal reformat (b) demonstrating a right popliteal aneurysm
(yellow arrow) which is completely occluded with thrombus.
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Fig. 27: MR axial (a) and coronal T1 (b), coronal STIR (c) demonstrating the occluded
right popliteal artery which contains hyperintense T1 mural thrombus with central low
signal material in keeping with chronic blood products. Image (d) is a zoomed up STIR
coronal image demonstrate layers of differing blood products within the aneurysm.
Fig. 28: MR Axial T1 (a), T2FS (b) and post contrast T1FS (c) demonstrating a low T1/
High T2 soft tissue mass with intense enhancement. Intralesional low signal areas are
suggestive of flow voids. Biopsy confirmed a haemangioma.
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Fig. 29: Longitudinal ultrasound image (a) demonstrating a vascular soft tissue mass.
Axial MR T2 (b) and sagittal T1 (c) demonstrates a predominantly hyperintense lesion
with internal branching flow-voids (red arrows) in keeping with vessels. Biopsy proved a
haemangiopericytoma.
Fig. 30: MR axial STIR (a) and pre (b) and post (c) axial T1FS demonstrates
an intermuscular maass more conspicuous post contrast. Biopsy confrmed a
myopericytoma.
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Fig. 31: MR sagittal gradient echo T2 (a) and T2FS (b) demonstrates a hyperintense
fusiform mass oriented along long axis of nerve which is arising from the common
peroneal nerve. The mass can be clearly seen arising from the nerve (blue arrow) with
the nerve entering the mass. Biopsy confirmed a peripheral nerve sheath tumour.
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Fig. 32: Coronal (a) and axial (b) MR T2FS demonstrating the classical fusiform 'target'
lesion with the nerve seen entering and exiting from the lesion in keeping with a peripheral
nerve sheath tumour (PNST).
Fig. 33: Axial STIR MR of a neurofibroma which demonstrates the typical 'target'
appearance of a nerve sheath tumour. Central (red arrow) fibrocollagenous component
(low T2) surrounded by peripheral (blue arrow) myxoid component (high T2).
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Fig. 34: MR sagittal T1 (a) and axial STIR (b) demonstrates a large fusiform T2
hyperintense mass in the region of the common peroneal nerve. Biopsy confirmed a
malignant peripheral nerve sheath tumour.
Fig. 35: Axial MR T1 (a) and STIR (b) demonstrates an ill-defined infiltrative soft tissue
mass encasing the popliteal artery - typical of lymphoma. Axial CT angiogram (c) of
the lower leg demonstrated a patent popliteal artery. Biopsy confirmed Non-Hodgkin's
lymphoma.
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Fig. 36: Sagittal T1 MR (a) and axial STIR (b) demonstrates a soft tissue mass
with peripheral low signal rim on both sequences suggestive of haemosiderin. Biopsy
confirmed a giant cell tumour of the tendon sheath.
Fig. 37: AP radiograph (a) of the knee demonstrates abnormal increased soft tissue
density in the medial aspect of the keen (blue arrow). Axial (b) and sagittal (c) STIR MR
demonstrates a large soft tissue mass with multiple fluid-fluid levels (red arrows). Biopsy
confirmed a leiomyosarcoma.
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Fig. 38: Axial CT demonstrates another example of a leiomyosarcoma causing adjacent
bone destruction of the tibia.
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Fig. 39: AP radiograph of the knee (a) demonstrates abnormal increased density of the
medial tibial plateau with an extra-osseous component (blue arrow). Sagittal T1 (b) and
T2 FS (c) MR demonstrates an osseous lesion with cortical break through and soft tissue
extending into the popliteal fossa. Biopsy confirmed a conventional osteosarcoma.
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Fig. 40: AP and lateral radiograph of the knee demonstrating an ossified mass (blue
arrows) at the distal femur extending into the popliteal fossa and is intimately related
to the cortex. Differential diagnosis of this mass is heterotopic ossification (Figure 20).
Subsequent biopsy confirmed parosteal osteosarcoma.
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Fig. 41: Axial CT and sagittal reformat of the ossified mass seen in Figure 40,
demonstrating mature bone centrally with peripheral soft tissue component. The mass is
seen contiguous with the underlying cortex of the distal femur. Biopsy confirmed parosteal
osteosarcoma.
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Fig. 42: Axial T1 (a) and sagittal T1 (c) and axial STIR (b) and sagittal STIR MR images
demonstrates the distal femoral mass extending posteriorly into the popliteal fossa. The
mass is isointense to skeletal muscle on T1 and heterogenous to high signal on STIR
with intralesional low signal intensities on both sequences representing ossified tumour
regions. Biopsy confirmed parosteal osteosarcoma.
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Fig. 43: Lateral radiograph of the knee demonstrating areas of calcification within the
soft tissue posteriorly (blue arrow). MR Sagittal T1 (b), sagittal (c) and axial (d) T2FS
demonstrating high T2 signal mass. Biopsy confirmed an extraskeletal chondrosarcoma.
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Conclusion
•
•
•
Numerous pathologies can occur in the popliteal fossa and using a surgical
seive, more appropriate differentials can be formulated to avoid delaying
patient management.
It can be difficult to completely characterise a lesion in the popliteal fossa
and often histology is required for definitive diagnosis as many malignant
lesions in the popliteal fossa can have non-specific imaging features.
We have presented both benign and malignant entities that can occur in the
popliteal fossa.
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Personal Information
Dr Amit Shah, Department of Radiology, Royal Orthopaedic Hospital, Birmingham B21
3AP, UK
Dr Rajesh Botchu, Department of Radiology, Royal Orthopaedic Hospital, Birmingham
B21 3AP, UK
Dr Steve L James, Department of Radiology, Royal Orthopaedic Hospital, Birmingham
B21 3AP, UK
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