Other Types of Nerve Sheath Tumors

Slide 1 · Other Types of Nerve Sheath Tumors

Peripheral nerve sheath pathology

MMNST • Granular cell tumor • Perineurioma

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Relate nerve-sheath anatomy to Schwann-cell and perineurial differentiation.

Recognize the defining morphologic, immunophenotypic and molecular findings of three uncommon tumors.

Resolve key diagnostic differentials using integrated pathology rather than a single stain.

Link tumor identification to biologic behavior and potential syndromic evaluation.

Slide 3 · Nerve sheath contains distinct cellular compartments

Perineurium

Original editable schematic — not a specimen

Schwann cells ensheath peripheral axons and form myelin around many axons.

Perineurial cells encircle nerve fascicles and contribute to the blood–nerve barrier.

Histologically different tumors can arise from these related peripheral nerve compartments.

Slide 4 · Three unusual tumors: Not one biologic group

Diagnostic feature

Malignant melanotic

Granular cell

Perineurioma

Lineage

Schwannian / melanocytic

Schwannian

Perineurial

Defining clue

Melanin ± psammoma bodies

Lysosomal granules

Perineurial architecture

Typical behavior

Aggressive potential

Usually benign

Benign

Slide 5 · Conventional tumors supply the diagnostic baseline

Schwannoma: Encapsulated, often Antoni A/B and Verocay bodies.

Neurofibroma: Heterogeneous Schwannian and stromal cells; Unencapsulated.

MPNST: Usually infiltrative, atypical spindle-cell sarcoma with variable Schwannian phenotype.

A rare nerve-sheath tumor may be confused with any of these.

Slide 6 · How to approach an unfamiliar nerve-associated mass

Location / architecture

Cell morphology

Lineage immunostains

Clinicogenetic correlation

Start with anatomical compartment and growth pattern.

Schematic mechanism / diagnostic sequence — concept simplified for teaching

Slide 7 · Morphology directs immunohistochemistry

Initial morphologic pattern

Pigmented epithelioid or spindle lesion → Schwannian plus melanocytic markers.

Polygonal cells with granular eosinophilic cytoplasm → Schwannian and lysosomal stains.

Whorled spindle cells or concentric intraneural layers → perineurial marker panel.

Schematic decision tree — interpret with site and histology

Slide 8 · MMNST is separate from conventional schwannoma

Former name: Melanotic schwannoma.

Renamed to reflect malignant clinical behavior and distinct genetics.

Key alteration: Inactivation of PRKAR1A; Not the typical NF2-driven schwannoma pathway.

Slide 9 · Typical MMNST clinical setting

Spinal

Axis

Paraspinal nerve-root relation

Original editable schematic — not a specimen

Often located near spinal nerve roots or autonomic nerves.

May present with a mass, pain, or neurologic compression symptoms.

Occurs sporadically or with Carney complex; Syndromic context must be assessed.

Slide 10 · Gross appearance of MMNST

Original morphology schematic — not a histologic specimen

Typically a well-demarcated mass with conspicuous dark pigment.

Dark appearance results from intracytoplasmic melanin.

Macroscopic appearance alone cannot reliably establish its biological behavior.

Slide 11 · MMNST shows dual differentiation

Schwannian

Phenotype

Melanocytic

Phenotype

MMNST

Original editable schematic — not a specimen

Schwannian features support nerve-sheath lineage.

Melanocytic differentiation explains melanin production and melanocytic marker expression.

Dual lineage is a diagnostic clue, not by itself proof against melanoma.

Slide 12 · MMNST histologic architecture

Original morphology schematic — not a histologic specimen

Plump spindle and epithelioid tumor cells may be arranged in nests, sheets, or short fascicles.

Cytoplasmic melanin ranges from focal to heavy pigmentation.

Nuclear grooves, intranuclear pseudoinclusions, or conspicuous nucleoli may occur.

Slide 13 · Psammoma bodies are an important clue

Original morphology schematic — not a histologic specimen

Concentric lamellated calcifications may appear in MMNST.

Their presence supports a psammomatous phenotype, not a separate benign tumor.

Absence of psammoma bodies does not exclude MMNST.

Slide 14 · Psammomatous does not mean indolent

Psammomatous lesions may be associated with Carney complex.

Both psammomatous and nonpsammomatous tumors can behave aggressively.

Do not use calcification to assign benign versus malignant potential.

Slide 15 · MMNST immunophenotype: Two lineages

Diagnostic feature

Schwannian panel

Melanocytic panel

Examples

S100; SOX10

HMB45; Melan-A; tyrosinase

Interpretation

Neural-crest differentiation

Melanin synthesis pathway

Limitation

Also positive in melanoma

Also positive in melanoma

Slide 16 · PRKAR1A is the molecular anchor

PRKAR1A inactivation

Disordered PKA regulation

Tumor cell growth

PRKAR1A encodes a regulatory subunit of cAMP-dependent protein kinase A.

Schematic mechanism / diagnostic sequence — concept simplified for teaching

Slide 17 · PRKAR1A immunostaining has limitations

Loss of PRKAR1A protein expression is supportive where present.

Retained protein staining does not independently exclude MMNST.

Interpret immunostaining alongside morphology, site, and molecular analysis.

Slide 18 · Carney complex changes the clinical question

Pigmented

Macules

Cardiac

Myxomas

Endocrine

Lesions

Carney complex: Clinical features (not exhaustive)

Original editable schematic — not a specimen

Autosomal-dominant tumor-predisposition syndrome associated with PRKAR1A.

Includes spotty skin pigmentation, cardiac myxomas, and endocrine neoplasia.

An MMNST diagnosis may prompt focused clinical and genetic evaluation.

Slide 19 · Syndromic association requires context

Not every MMNST occurs in a patient with Carney complex.

A psammomatous pattern is suggestive but not diagnostic of the syndrome.

Personal and family history plus expert genetic assessment are more informative than a single morphologic feature.

Slide 20 · MMNST should not be graded as a routine schwannoma

Diagnostic feature

Conventional schwannoma

MMNST

Behavior

Usually benign

Malignant potential

Molecular theme

NF2 inactivation common

PRKAR1A inactivation common

Management implication

Local control often curative

Long-term follow-up relevant

Slide 21 · Histology alone may not predict MMNST outcome

Low mitotic activity can coexist with later dissemination.

Neither psammoma bodies nor absence of necrosis reliably establishes low risk.

Avoid making definitive behavior predictions from one small sample.

Slide 22 · Pigmented spindle-cell lesion: Differential diagnosis

Metastatic melanoma: Frequently overlaps in S100, SOX10 and melanocytic stains.

Pigmented epithelioid melanocytoma: Different clinicopathologic setting.

Other pigmented neural-crest tumors and pigmented schwannoma-like lesions require integrated assessment.

Slide 23 · MMNST versus melanoma: What contributes?

Evidence domain

Interpretation

Anatomy

Nerve-root relationship favors MMNST

Architecture

Psammoma bodies can support MMNST

IHC

Substantial overlap; not decisive alone

Genetics

PRKAR1A adds support; not infallible

Slide 24 · MMNST diagnostic reporting: Preserve uncertainty

Use the recognized entity name rather than “benign melanotic schwannoma.”

Record the anatomic site and relationship to a nerve when established.

Document relevant histologic features and limitations of sampling.

Recommend multidisciplinary correlation for prognosis and syndromic features.

Slide 25 · MMNST case: Anatomy and phenotype

Interpretive case / question

A pigmented paraspinal mass displays spindle/epithelioid cells and psammoma bodies.

Tumor cells are positive for SOX10 and HMB45.

Question: Why can these markers not completely exclude melanoma?

Slide 26 · MMNST case: Unexpected low proliferation

Interpretive case / question

A nerve-root mass has pigment, bland nuclei, and very few mitoses.

Question: Is “benign melanotic schwannoma” an appropriate final diagnosis?

No—low proliferative activity does not neutralize the entity’s metastatic potential.

Slide 27 · Granular cell tumor: A lysosome-rich neoplasm

Nucleus

Original editable schematic — not a specimen

Typically a small superficial, subcutaneous, or mucosal mass.

Most lesions are benign and show Schwannian differentiation.

Abundant intracytoplasmic lysosomes produce their characteristic granularity.

Slide 28 · Common presentation of granular cell tumor

May occur in the skin and mucosal surfaces, including the tongue.

Usually presents as a slowly growing nodule rather than a neuropathy.

Anatomic site may influence differential diagnosis and biopsy technique.

Slide 29 · Granular cell tumor: Growth pattern

Original morphology schematic — not a histologic specimen

Usually unencapsulated; Nests and sheets may extend between native tissue structures.

This infiltrative-looking margin does not independently indicate malignancy.

Site-specific epithelial changes may complicate superficial biopsies.

Slide 30 · Granular cytoplasm has a subcellular explanation

Nucleus

Original editable schematic — not a specimen

Tumor cells are plump or polygonal with abundant eosinophilic granular cytoplasm.

The granules correspond to lysosomes rather than melanin or mucin.

This observation leads to lysosome-related staining and molecular investigation.

Slide 31 · Granular-cell nuclear features

Original morphology schematic — not a histologic specimen

Usually small, relatively uniform, centrally placed nuclei.

The cytoplasm commonly occupies much more of the cell than the nucleus.

Marked atypia, necrosis, or increased mitoses require additional assessment.

Slide 32 · Granules can be highlighted histochemically

Cytoplasmic granules often show PAS positivity with diastase resistance.

PAS reflects the granular lysosomal contents—not a specific genetic event.

Interpret special stains with H&E and immunohistochemistry.

Slide 33 · Granular cell tumor: Practical immunopanel

Diagnostic feature

S100 / SOX10

CD68

Pan-cytokeratin

Typical role

Schwannian phenotype

Lysosome-associated staining

Epithelial exclusion

Caution

Not completely specific

Not evidence of histiocytic origin

Needs control/context

Slide 34 · V-ATPase biology explains a key molecular link

ATP6AP1 / ATP6AP2 loss

Altered V-ATPase function

Lysosomal phenotype

Genes ATP6AP1 and ATP6AP2 encode accessory components of the vacuolar H+-ATPase complex.

Schematic mechanism / diagnostic sequence — concept simplified for teaching

Slide 35 · Granularity is a phenotype, not a diagnosis

Numerous neoplasms and histiocytic lesions may contain granular cytoplasm.

Confirm lineage and pattern rather than diagnosing on eosinophilic granules alone.

Site, nuclear morphology, and immunophenotype remain decisive.

Slide 36 · Superficial granular cell tumor can mimic SCC

Epithelial thickening

Underlying granular-cell proliferation

Original editable schematic — not a specimen

Reactive pseudoepitheliomatous hyperplasia may overlie mucosal or cutaneous GCT.

In a shallow biopsy, the thickened squamous epithelium can dominate the section.

Look beneath epithelium for the underlying granular-cell neoplasm.

Slide 37 · Pseudoepitheliomatous hyperplasia is not invasion

Diagnostic feature

Overlying reactive epithelium

Squamous carcinoma

Driver

Response to underlying GCT

Autonomous epithelial malignancy

Key question

Is there an underlying GCT?

Is there malignant epithelial invasion?

Pitfall

Overcall in superficial biopsy

Miss carcinoma in inadequate sample

Slide 38 · When should granular cell tumor be considered atypical?

Concerning findings include necrosis, spindling, prominent nucleoli, and high mitotic activity.

High nuclear-to-cytoplasmic ratio and significant pleomorphism are also assessed.

These features should be interpreted systematically, not individually.

Slide 39 · Fanburg-Smith criteria: Historical framework

Diagnostic feature

Architecture / injury

Nuclear / proliferation

Features

Spindling; necrosis

Prominent nucleoli; increased mitoses

Additional

High N:C ratio; pleomorphism

Slide 40 · Granular cell tumor: Behavior is usually benign

Most lesions remain localized after adequate removal.

Rare malignant examples show recurrence or metastasis.

The word “granular” itself makes no assertion of benign or malignant behavior.

Slide 41 · Granular cell tumor versus other granular lesions

Initial morphologic pattern

Histiocytic lesions may be CD68 positive but lack typical diffuse Schwannian phenotype.

Granular variants of non-neural tumors retain their original lineage markers.

A broad panel is preferable when morphology is atypical or site is unusual.

Schematic decision tree — interpret with site and histology

Slide 42 · Granular cell tumor case: Tongue biopsy

Interpretive case / question

A tongue nodule contains sheets of bland polygonal cells with abundant granular cytoplasm.

An overlying squamous proliferation extends into the lamina propria.

Question: Which two components must be interpreted independently?

Slide 43 · Granular cell tumor case: CD68 positivity

Interpretive case / question

A bland granular-cell neoplasm is S100 positive and CD68 positive.

Question: Does CD68 establish histiocytic lineage?

No. Granular lysosomes can produce CD68 positivity in Schwannian granular cell tumors.

Slide 44 · Perineurioma originates in a different sheath layer

Perineurium

Original editable schematic — not a specimen

Perineurial cells normally surround nerve fascicles.

Perineurioma demonstrates perineurial rather than predominantly Schwann-cell differentiation.

Classic perineuriomas are benign lesions.

Slide 45 · Two principal anatomic forms of perineurioma

Diagnostic feature

Intraneural

Soft tissue

Location

Within involved nerve

Outside major nerve

Pattern

Pseudo-onion bulbs

Storiform / whorled spindle cells

Presentation

Progressive neuropathic deficit

Localized soft tissue mass

Slide 46 · Intraneural perineurioma: Lesion centered on fascicles

Original morphology schematic — not a histologic specimen

Typically presents as fusiform or segmental enlargement of a nerve.

Neoplastic perineurial cells are arranged around intraneural fibers.

Affected patients may develop slowly progressive motor or sensory impairment.

Slide 47 · The pseudo-onion-bulb architecture

Original morphology schematic — not a histologic specimen

Concentric perineurial-cell processes wrap a central axon-associated compartment.

These layered structures are a hallmark of intraneural perineurioma.

True reactive onion bulbs in chronic neuropathies have different cellular origins.

Slide 48 · Central axons help establish intraneural disease

Perineurium

Original editable schematic — not a specimen

Neurofilament immunostaining can highlight residual central axons.

S100 labels associated Schwann cells but not the predominant perineurial tumor cells.

An axon-centered pattern helps distinguish intraneural from extraneural masses.

Slide 49 · Soft-tissue perineurioma: Cell shape and pattern

Original morphology schematic — not a histologic specimen

Bland spindle cells with long delicate bipolar processes.

Whorled, storiform, reticular, or fascicular growth in variable collagenous/myxoid stroma.

Usually a localized soft-tissue lesion with relatively limited atypia.

Slide 50 · Sclerosing perineurioma emphasizes matrix

Original morphology schematic — not a histologic specimen

A subset has conspicuous collagen deposition and lower cellularity.

Dense sclerotic stroma can obscure delicate neoplastic cell processes.

Site, bland morphology, and perineurial stains are diagnostic anchors.

Slide 51 · Perineurial IHC is a panel interpretation

Diagnostic feature

Perineurial cell markers

Schwann-cell markers

Typical pattern

EMA; claudin-1; GLUT1

S100; SOX10

Perineurioma

Often positive

Usually negative in tumor cells

Interpretive caution

Also present in native perineurium

Resident Schwann cells may be positive

Slide 52 · Why EMA positivity alone is insufficient

EMA can stain nonneoplastic perineurium around other nerve tumors.

Other spindle-cell neoplasms may express EMA.

The correct question is whether the tumor cells have the expected pattern and architecture.

Slide 53 · Perineurioma molecular findings depend on location

Diagnostic feature

Intraneural

Soft tissue

Reported genetics

TRAF7, occasionally NF2

NF2/22q alterations and others

Biology

Benign perineurial proliferation

Benign perineurial proliferation

Main diagnostic tool

Architecture + IHC

Morphology + IHC

Slide 54 · Perineurioma is not a demyelinating onion-bulb neuropathy

Diagnostic feature

Intraneural perineurioma

Reactive onion-bulb neuropathy

Cell population

Perineurial neoplastic layers

Schwann cell / reactive layers

EMA

Layers often positive

Does not mimic a full perineurial tumor pattern

Clinical context

Focal nerve enlargement

Diffuse / multifocal neuropathy context

Slide 55 · Soft-tissue perineurioma versus neurofibroma

Diagnostic feature

Perineurioma

Neurofibroma

Predominant lineage

Perineurial

Schwannian plus stromal cells

EMA

Tumor cells often positive

Often highlights admixed perineurial cells

S100/SOX10

Tumor cells largely negative

Schwannian tumor subset positive

Slide 56 · Perineurioma versus low-grade fibromyxoid sarcoma

Both may show bland spindle cells and myxoid/collagenous areas.

MUC4 expression or relevant gene fusion can support low-grade fibromyxoid sarcoma in a difficult case.

EMA alone may not resolve the differential.

Slide 57 · Perineurioma reporting should prioritize anatomy

Specify intraneural versus soft-tissue localization when established.

Describe the perineurial growth pattern and the tumor-cell immunophenotype.

Correlate neural involvement with functional deficits and surgical planning.

Slide 58 · Intraneural perineurioma case: Progressive weakness

Interpretive case / question

A young patient has slowly progressive focal weakness and segmental nerve enlargement.

Biopsy reveals axon-centered concentric cellular layers.

Question: Which stains best assign the layers and central axons?

Slide 59 · Soft-tissue perineurioma case: Bland spindle mass

Interpretive case / question

A superficial mass contains slender spindle cells in a whorled fibromyxoid pattern.

Tumor cells show EMA and claudin-1, with little/no SOX10 in lesional cells.

Question: Which cell lineage is supported?

Slide 60 · Three tumors: A high-yield integrated comparison

Diagnostic feature

MMNST

Granular cell tumor

Perineurioma

Cytoplasm

Melanin

Lysosomal granules

Slender processes

Main markers

S100 / SOX10 + HMB45

S100 / SOX10; CD68

EMA / claudin-1 / GLUT1

Molecular clue

PRKAR1A

ATP6AP1 / ATP6AP2

TRAF7 or NF2

Behavior

Aggressive potential

Mostly benign

Benign

Slide 61 · Use stains to answer a morphologic question

S100/SOX10: Support Schwannian lineage but not a unique diagnosis.

HMB45/Melan-A: Establish melanocytic phenotype, not necessarily melanoma.

CD68: May reflect lysosomes rather than macrophage lineage.

EMA/claudin-1/GLUT1: Assess perineurial tumor cells, not just capsule.

Slide 62 · Molecular findings are supportive—not solitary diagnoses

Morphology

Immunophenotype

Molecular support

Integrated diagnosis

PRKAR1A points toward MMNST in the right clinicopathologic context.

Schematic mechanism / diagnostic sequence — concept simplified for teaching

Slide 63 · Three distinct diagnostic hazards

Tumor

Pitfall

Corrective strategy

MMNST

False reassurance

Recognize aggressive entity

Granular cell

Squamous mimic

Inspect underlying GCT

Perineurioma

Lineage confusion

Cell-specific marker panel

Slide 64 · Integrated practice vignette: Choose the diagnosis

Finding cluster

Leading consideration

Pigment + nerve root + dual markers

MMNST

Mucosal nodule + cytoplasmic granules

Granular cell tumor

Nerve enlargement + pseudo-onion bulbs

Intraneural perineurioma

Slide 65 · Essential messages

The microscopic compartment and phenotype dictate the diagnostic workup.

MMNST differs fundamentally from benign conventional schwannoma.

Lysosomal granularity and perineurial layering are distinctive clues—not standalone diagnoses.

Integrated evaluation prevents both underestimation of malignancy and overtreatment of benign lesions.

Slide 66 · Selected references

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. Ch. 27.

Magro et al. Diagnostics. 2022;12:1463.

Meyer & Billings. Virchows Arch. 2020;476:65–80.

Pareja et al. Nat Commun. 2018. doi:10.1038/s41467-018-05886-y.

Fanburg-Smith et al. Am J Surg Pathol. 1998. PMID:9669341.