Schwannoma (neurilemmoma) is a benign tumor arising from the peripheral nerve sheath (Schwann cells). It is the most common benign peripheral nerve sheath tumor and accounts for approximately 5% of soft tissue tumors. Most common between ages 20-50 with no gender predilection. Most frequently located along head-neck, flexor surfaces, and spinal nerve roots. Solitary schwannomas are sporadic; multiple schwannomas are associated with schwannomatosis or NF2 syndrome. The tumor is eccentrically located relative to the nerve and is encapsulated. Malignant transformation is extremely rare (<1%) and usually associated with NF2.
Age Range
20-60
Peak Age
40
Gender
Equal
Prevalence
Uncommon
Schwannoma arises from monoclonal proliferation of Schwann cells, with NF2 gene (merlin/schwannomin) loss or inactivation being fundamental to pathogenesis — loss of function of this tumor suppressor gene disrupts signaling pathways controlling cell proliferation. Histologically it contains two tissue types: Antoni A (compact, organized, palisading Schwann cells — Verocay bodies) and Antoni B (loose, myxoid, disorganized structure). This dual histologic composition explains the characteristic 'target sign' on MRI — the peripheral T2 hyperintense area corresponds to Antoni B (loose, water-rich myxoid tissue) and the central hypointense area to Antoni A (compact, cell-dense tissue). Schwannoma attaches eccentrically to the nerve and does not splay nerve fibers (unlike neurofibroma, nerve fibers do not pass through it); this feature allows nerve-sparing resection during surgery. The 'split-fat sign' — splitting of fat around the tumor — indicates neurogenic origin and growth along the nerve.
The combination of peripheral hyperintense ring (Antoni B — myxoid tissue) and central hypointense core (Antoni A — compact cellular tissue) pattern on T2-weighted sequences is pathognomonic for neurogenic tumor. This target pattern is more typical and prominent in schwannoma than neurofibroma.
On T2-weighted sequences, schwannoma shows the characteristic 'target sign': peripheral hyperintense ring (Antoni B — loose myxoid tissue) and central hypointense core (Antoni A — compact cellular tissue). This pattern is highly specific for neurogenic tumors and is more frequently seen in schwannoma than neurofibroma. In large or degenerative schwannomas, cystic change may disrupt the target pattern.
Report Sentence
On T2-weighted sequences, the mass demonstrates a peripheral hyperintense, central hypointense 'target sign' consistent with neurogenic tumor (schwannoma).
On T1-weighted sequences, schwannoma shows isointense or slightly hypointense signal relative to muscle. The mass usually has homogeneous T1 signal. T1 hyperintense areas may be seen in hemorrhagic or melanotic schwannoma. 'Split-fat sign' — thin strips of fat signal on T1 extend along the nerve at both ends of the mass and indicate neurogenic origin.
Report Sentence
On T1-weighted sequences, a mass isointense to muscle is seen along the nerve with perineural fat strips at both ends ('split-fat sign').
On STIR sequences, schwannoma appears generally hyperintense; after fat suppression, tumor tissue becomes conspicuous and is clearly separated from surrounding structures. The target sign may be preserved on STIR. Cystic degeneration areas show very bright signal, fibrous areas show lower signal. STIR is valuable for evaluating tumor extension along the nerve and compartment relationship.
Report Sentence
On STIR sequences, the mass appears hyperintense, and its extension along the nerve and compartment relationship have been evaluated.
Post-contrast, schwannoma shows homogeneous or heterogeneous enhancement. Small schwannomas (<3cm) usually show homogeneous, avid enhancement. Large schwannomas (>3cm) show heterogeneous enhancement due to cystic degeneration, hemorrhage, and fibrosis. Cystic areas do not enhance. Enhancement pattern is used in differentiation from malignant peripheral nerve sheath tumor (MPNST) but is not reliable alone.
Report Sentence
On post-contrast images, the mass shows homogeneous/heterogeneous enhancement with non-enhancing cystic degeneration areas.
On DWI, schwannoma shows variable diffusion characteristics. Antoni A-predominant solid areas may show mild diffusion restriction (intermediate ADC). Antoni B-predominant myxoid areas show high ADC (free diffusion). Cystic degeneration areas show high ADC. Marked diffusion restriction (very low ADC <0.9 × 10⁻³ mm²/s) raises suspicion for MPNST.
Report Sentence
On DWI, no marked diffusion restriction is identified in the mass, consistent with benign neurogenic tumor.
On US, schwannoma typically appears as an oval, well-defined, homogeneously hypoechoic mass. Posterior acoustic enhancement is common and may mimic a cystic lesion. In large schwannomas, internal heterogeneity (cystic change, hemorrhage) is difficult to demonstrate by US. 'Tail sign' — visualization of nerve structure at proximal and distal ends of the mass is valuable as a US finding of neurogenic origin.
Report Sentence
On US, a well-defined hypoechoic mass is seen along the nerve with posterior acoustic enhancement.
On CT, schwannoma appears as a well-defined, soft tissue density, oval mass. It shows homogeneous or heterogeneous enhancement post-contrast. Cystic areas show low density. CT's advantage over MRI is in evaluation of bony foramina and erosion (intervertebral foramen widening in spinal schwannoma, internal auditory canal widening in intracranial schwannoma). Calcification is rare (2-5%) and may be seen in the ancient schwannoma variant.
Report Sentence
On CT, an enhancing soft tissue mass is seen along the nerve foramen with associated foramen widening.
Criteria
Mixture of Antoni A and Antoni B areas, encapsulated, eccentric to nerve, target sign prominent
Distinct Features
Most common type (80%). Good prognosis, curable with surgery. Nerve-sparing resection possible (nerve fibers adherent but not passing through). Recurrence rare (5%). Classic target sign and split-fat sign on MRI.
Criteria
Long-standing degenerative changes (cystic degeneration, hemorrhage, fibrosis, calcification, nuclear atypia), large size, heterogeneous appearance
Distinct Features
Heterogeneous MRI signal due to degenerative changes; target sign may be disrupted. Cystic areas, hemorrhage, calcification. Nuclear atypia on histology may mimic malignancy but is benign. More commonly encountered in retroperitoneal or posterior mediastinal schwannomas.
Criteria
Predominantly Antoni A histology, high cellularity, low mitotic activity, S100 positive, benign behavior
Distinct Features
High cellularity may be confused with MPNST. Target sign may be absent on T2 (Antoni B minimal). Mild diffusion restriction on DWI. Despite benign behavior, local recurrence rate slightly higher than conventional schwannoma. Immunohistochemistry (S100+, SOX10+, H3K27me3 retained) distinguishes from MPNST.
Criteria
Multiple nodular masses along nerve plexus, 'bag of worms' appearance, may be associated with NF2 or schwannomatosis
Distinct Features
May be confused with plexiform neurofibroma (NF1 associated). Plexiform schwannoma associated with NF2/schwannomatosis. Each nodule shows schwannoma histology. Multiple well-defined nodules along nerve on MRI. Unlike plexiform neurofibroma, malignant transformation is very rare.
Distinguishing Feature
Soft tissue sarcoma (UPS) not nerve-associated, no target sign or split-fat sign, more heterogeneous with marked necrosis, irregular borders; schwannoma eccentric to nerve, encapsulated, target sign positive
Distinguishing Feature
Lipoma follows fat signal on all sequences, complete suppression on fat suppression; schwannoma intermediate-to-low T1 signal, target sign on T2, no fat signal content, shows enhancement
Distinguishing Feature
Desmoid tumor musculoaponeurotic location, no nerve association, variable T2 depending on collagen content (low signal areas), no target sign; schwannoma along nerve, target sign positive
Distinguishing Feature
Liposarcoma contains fat signal and non-adipose components, heterogeneous suppression on STIR; schwannoma no fat signal, homogeneous hyperintensity on STIR, nerve association present
Urgency
non-urgentManagement
surgical excision with nerve preservation; observation for small asymptomatic lesions; NF2/schwannomatosis screening if multipleBiopsy
Not NeededFollow-up
MRI follow-up for observation cases; post-surgical follow-up to monitor recurrence; NF2 screening (audiometry, brain MRI) if multiple schwannomasSchwannoma is a benign tumor with negligible risk of malignant transformation. Nerve-sparing surgical excision is curative for symptomatic lesions; recurrence rate is below 5%. Small, asymptomatic lesions may be followed with clinical/MRI surveillance. Classic target sign and split-fat sign on MRI carry strong diagnostic specificity and biopsy is usually unnecessary. Biopsy is mandatory when MPNST is suspected (rapid growth, pain, NF1 history, marked DWI restriction, increased FDG uptake on PET). Multiple schwannomas require screening for schwannomatosis (SMARCB1/LZTR1 mutation) or NF2 (bilateral vestibular schwannoma).
Schwannomas are benign with very low malignant transformation risk. Surgical excision is curative and the nerve can usually be preserved (schwannoma grows pushing the nerve aside, unlike neurofibroma). Multiple schwannomas should raise suspicion for schwannomatosis or NF2. Asymptomatic small lesions can be followed.