Nerve Sheath Tumors

KEY FACTS

Terminology

• Tumor arising from nerve; histologically classified as either schwannoma or neurofibroma
• 3 types of neurofibroma: Localized, diffuse, and plexiform

Imaging

• Any peripheral nerve can be affected

Intermuscular > subcutaneous > intramuscular
• Well-defined, largely homogeneous, hypoechoic mass
• Typically fusiform or oblong-shaped and oriented along long axis of nerve
• Nerve sheath tumors align centrally or eccentrically along course of nerve; arise from neurovascular bundle
• Nerve entering or exiting from tumor seen in majority (90%)
• Posterior enhancement common (70%)
• Nerve sheath tumors, especially schwannoma, often contain cystic areas representing myxoid degeneration, hemorrhage, or necrosis
• Vascular pattern varied; usually moderate intrinsic vascularity is present
• US appearances of schwannoma and neurofibroma overlap considerably
• US appearances of benign and malignant nerve sheath tumors overlap considerably
• Accurate diagnosis of nerve sheath tumor is usually feasible based on US findings alone

MR better at assessing extent of diffuse neurofibroma or plexiform neurofibroma

Diagnostic Checklist

• Consider nerve sheath tumor even if parent nerve is not seen, provided other US features are compatible
• Nerve sheath tumors of medium-sized or large nerves can nearly all be diagnosed with confidence based on US appearances alone

Specificity for diagnosis of nerve sheath tumors arising from smaller nerves is not as high

TERMINOLOGY

Definitions

• Tumor arising from peripheral nerve and histologically classified as schwannoma and neurofibroma

Associated Syndromes

• Neurofibromatosis type 1 (NF1), von Recklinghausen disease

IMAGING

General Features

• Best diagnostic clue

Well-defined, largely hypoechoic mass arising from nerve
• Location

Any peripheral nerve

– Intermuscular > subcutaneous > intramuscular

Head and neck > upper limbs > lower limbs > > trunk
• Size

Usually < 5 cm

– Larger tumors from large nerves (up to 20 cm)

Nerve tumors usually grow slowly until critical mass is reached and then stop growing
• Morphology

Soft tissue mass along neurovascular bundle

MR Findings

• T1WI

Soft tissue mass isointense to muscle; split-fat sign (intermuscular fat splits to encompass tumor)
• T2WI

Well-defined fusiform mass hyperintense to muscle; target sign hypointense center due to collagen and condensed Schwann cells; neural tail sign
• T1WI C+

Variable enhancement: May see mainly peripheral enhancement or reverse target sign (enhancement of central fibrous component but no enhancement of peripheral myxoid component)
• MR appearances of schwannoma and neurofibroma overlap considerably

Ultrasonographic Findings

• Well-defined, largely homogeneous, hypoechoic mass; occasionally heterogeneous

Heterogeneity due to myxoid component, hemorrhage, fibrosis, or calcification

– Target sign: Central hyperechogenicity representing fibrocollagenous component surrounded by peripheral hypoechogenicity representing myxoid component; more commonly seen in neurofibroma

Intratumoral hemorrhage may lead to sudden ↑ in tumor size
• Arise from nerve

Typically fusiform or oblong in shape and oriented along long axis of nerve

– Nerve sheath tumors align centrally or eccentrically along course of nerve

Nerve entering or exiting from tumor is seen in majority (90%)

– Nerve often shows slight enlargement just proximal and distal to tumor

– Entering/exiting nerve may not be seen when nerve sheath tumor arises from small peripheral nerves

– Compressed fascial planes may mimic nerve on longitudinal plane; always confirm on transverse plane
• Nerve sheath tumors often contain cystic areas

Degenerative ancient schwannomas are predominantly cystic

Cystic areas can be due to myxoid accumulation, previous hemorrhage, or necrosis

Cystic areas are more common to schwannoma but also commonly seen in neurofibroma
• Occasionally tumor has echogenic rim due to fibrous capsule, fibrous pseudocapsule, or compression of surrounding fat
• Vascular pattern varied

Usually moderately vascular with central irregular vascular pattern

– Some display predominantly peripheral vascularity

– Some do not have any demonstrable vascularity on color Doppler imaging
• Diffuse neurofibroma: Thickening of skin and subcutis with retinacular hypoechoic pattern

No discrete mass, hyperemic

– Can mimic soft tissue infection or vascular malformation
• Plexiform neurofibroma: Diffuse nodular enlargement of nerves with “bag of worms”

Discrete lobulated mass present with lobulated neural enlargement
• US diagnosis of nerve sheath tumor is based on consortium of features rather than single feature

US features usually typical enough to make diagnosis without need for biopsy

– Biopsy may be very painful
• US appearances of schwannoma and neurofibroma overlap considerably

Do not focus on making this histological distinction on imaging

Instead report on centric/eccentric location of tumor to parent nerve as this affects surgical outcome
• US appearances of benign and malignant nerve sheath tumors overlap considerably

No single feature can reliably differentiate between benign and malignant peripheral nerve sheath tumor

Clinical indicators are relatively more reliable: Large size (> 5 cm), rapid growth, and change in neurology

Irregular margins, inhomogeneity, internal necrosis, and peritumoral inflammation are also more common in malignant peripheral nerve sheath tumors

NF1 is most important risk factor for malignant peripheral nerve sheath tumor
• PET imaging is more accurate in predicting malignancy in plexiform neurofibromas

Imaging Recommendations

• Best imaging tool

Diagnosis of nerve sheath tumor is generally feasible based on US findings alone

MR better at assessing extent of diffuse neurofibroma or plexiform neurofibroma

DIFFERENTIAL DIAGNOSIS

Malignant Lymph Node

• Malignant or lymphomatous node with loss of distinction cortex and hilum may simulate nerve sheath tumor

Does not have entering or exiting nerve

Occurs at different locations than nerve sheath tumors

Ganglion

• May simulate ancient schwannoma
• Does not have entering or exiting nerve

Vascular Leiomyoma

• Does not have entering or exiting nerve
• May be painful
• Typically has eccentric vascular confluence
• Can mimic small subcutaneous nerve sheath tumors in which entering or exiting nerve not apparent

Glomus Tumor

• Glomus tumor arising in site other than subungual region may simulate nerve sheath tumor

Does not have entering or exiting nerve

PATHOLOGY

General Features

• Genetics

NF1, von Recklinghausen disease

– Affects 1 in 3,000 people

– Autosomal dominant; 50% sporadic

– Genetic mutation on NF1 gene chromosome 17

Staging, Grading, & Classification

• Schwannoma

Usually solitary; multiplicity associated with NF1

Malignant change extremely rare
• Neurofibroma

3 types

– Localized: Most common type of neurofibroma (90%)

Most are solitary and not associated with NF1

– Diffuse: Most are solitary; 10% associated with NF1

Infiltrative retinacular pattern in subcutaneous tissue or plaque-like thickening of skin and subdermal tissues

– Plexiform: Pathognomonic of NF1

Multinodular confluent neural mass with mass effect on bones, lungs, or gastrointestinal tract; “bag of worms”

Malignant change occurs in 15% of NF1 patients

– Common with plexiform neurofibromas (50% risk of malignant transformation), less common with diffuse neurofibroma, and rare in localized neurofibroma

Gross Pathologic & Surgical Features

• Schwannoma

Mesenchymal tumors with cellular differentiation to Schwann cells; myelin-producing neuroglial cells covering peripheral axons

True capsule composed of epineurium

Tumors lie eccentric to nerve with nerve fascicles displaced to periphery of tumor mass

Tend to be more prone to myxoid degeneration, hemorrhage, and calcification

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Apr 9, 2020 | Posted by admin in MUSCULOSKELETAL IMAGING | Comments Off

KEY FACTS

Terminology

TERMINOLOGY

Definitions

IMAGING

General Features

DIFFERENTIAL DIAGNOSIS

Malignant Lymph Node

PATHOLOGY

General Features

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Nerve Sheath Tumors

Nerve Sheath Tumors

Imaging

Diagnostic Checklist

Associated Syndromes

MR Findings

Ultrasonographic Findings

Imaging Recommendations

Ganglion

Vascular Leiomyoma

Glomus Tumor

Staging, Grading, & Classification

Gross Pathologic & Surgical Features

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Nerve Sheath Tumors

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