Adrenal schwannoma is a rare benign tumor arising from Schwann cells of the peripheral nerve sheath. Less than 1% of retroperitoneal schwannomas localize to the adrenal gland — the vast majority occur in spinal nerve roots, extremities, or head and neck region. Adrenal schwannoma presumably originates from Schwann cells of nerve fibers innervating the adrenal medulla. It is a well-circumscribed, encapsulated, slowly growing mass, typically detected incidentally. Cystic degeneration (Antoni B areas), myxoid changes, and hyaline degeneration are common; therefore, it appears heterogeneous and partially cystic on imaging. It is non-functional (hormonally inactive) with normal catecholamine levels. There is a slight female predominance with a mean age at diagnosis of 40-60 years. Malignant transformation is extremely rare, and unlike neurofibroma, NF1 association is not prominent. Treatment is surgical resection with low recurrence rate.
Age Range
20-70
Peak Age
45
Gender
Equal
Prevalence
Rare
Adrenal schwannoma originates from the Schwann cell sheath of sympathetic and parasympathetic nerve fibers innervating the adrenal medulla. Schwann cells form the myelin sheath surrounding peripheral nervous system axons; neoplastic proliferation of these cells produces an encapsulated tumor. Histologically, it contains two distinct components: Antoni A areas (compact, spindle cells showing palisading arrangement — Verocay bodies) and Antoni B areas (loose, myxoid stroma, irregular cell distribution). These two histological patterns form the basis of the heterogeneous appearance on imaging — Antoni A areas appear as solid enhancing components due to denser cellularity, while Antoni B areas appear as myxoid/cystic regions. T2 hyperintensity on MRI results from the long T2 relaxation time of the high-water-content myxoid stroma in Antoni B areas. Cystic degeneration develops from expansion and coalescence of Antoni B areas as the tumor grows. Heterogeneous enhancement on CT reflects the mixture of Antoni A (vascular, enhancing) and Antoni B (avascular, non-enhancing) areas. The target sign is the combination of peripheral hyperintense myxoid area and central hypointense fibrous/cellular area on MRI T2 — a characteristic MRI finding of schwannoma. The capsule presence reflects the tumor's growth pattern pushing (not invading) surrounding nerve fibers and facilitates surgical enucleation.
On T2-weighted MRI, the combination of peripherally markedly hyperintense ring (myxoid Antoni B stroma) and centrally lower signal core (cellular Antoni A tissue) creates a target-like appearance. This pattern is pathognomonic for schwannoma and is the most reliable MRI finding for differentiation from other adrenal masses (pheochromocytoma, adenoma, carcinoma).
Characteristic target sign on T2-weighted images: peripherally markedly hyperintense area (myxoid Antoni B stroma) and centrally lower signal area (cellular Antoni A tissue and fibrous component). This pattern is highly characteristic of schwannoma and is rarely seen in other adrenal lesions. In large lesions, cystic degeneration areas may be seen as additional hyperintense foci. T2 hyperintensity generally shows a more heterogeneous and less uniform pattern than the 'light bulb sign' of pheochromocytoma.
Report Sentence
Peripheral hyperintense and central hypointense pattern (target sign) in the adrenal mass on T2-weighted images, characteristic of schwannoma.
Well-circumscribed, encapsulated, low to intermediate density (20-40 HU) homogeneous or mildly heterogeneous mass on non-contrast CT. Cystic degeneration areas show lower density (0-20 HU). Calcification is rare (5-10%). Pre-contrast density is generally higher than the low density of lipid-rich adenoma (<10 HU) but does not meet adenoma washout criteria.
Report Sentence
Well-circumscribed, encapsulated, low to intermediate density mass in the adrenal gland with cystic degeneration areas; adrenal schwannoma should be considered.
Characteristic late progressive enhancement pattern on contrast-enhanced CT. Minimal enhancement in arterial phase, with progressively increasing enhancement in portal venous and especially delayed phases (3-5 minutes). This pattern reflects slow contrast accumulation in the fibrous and cellular structure of schwannoma. Cystic degeneration areas do not enhance in any phase. Late progressive enhancement pattern differs from pheochromocytoma (intense arterial enhancement) and adenoma (washout).
Report Sentence
Progressive increasing enhancement pattern from minimal arterial phase to delayed phase in the adrenal mass, consistent with schwannoma or fibrous-rich tumor.
Homogeneously hypointense to isointense encapsulated mass on T1-weighted images. Cystic degeneration areas appear more prominently hypointense on T1. Hemorrhagic areas (rare) may be seen as hyperintense foci on T1. The capsule may be visible as a thin hypointense linear structure on T1. Late progressive enhancement on contrast-enhanced T1 — minimal arterial, increasing delayed enhancement.
Report Sentence
Hypointense, encapsulated adrenal mass on T1-weighted images with late progressive enhancement pattern on contrast-enhanced series.
Mild diffusion restriction may be seen in solid components on DWI; no diffusion restriction in cystic/myxoid areas. ADC values are generally intermediate (1.0-1.5 x 10⁻³ mm²/s) — higher than the low ADC values of malignant tumors (<0.8). DWI does not significantly contribute to definitive schwannoma diagnosis but is supportive for differentiation from malignant peripheral nerve sheath tumor (MPNST) — MPNST shows more pronounced diffusion restriction (low ADC).
Report Sentence
Mild diffusion restriction in solid components with intermediate ADC values; these findings are consistent with benign schwannoma and not in favor of malignant peripheral nerve sheath tumor.
No uptake on MIBG scintigraphy. Schwannoma originates from Schwann cells; these cells do not express norepinephrine transporter system (NET), therefore MIBG uptake is not expected. This finding is critically important for differentiation from pheochromocytoma — both tumors can show cystic degeneration and appear similar on imaging. Schwannoma generally shows low to moderate FDG uptake on FDG PET-CT; high FDG uptake should raise MPNST suspicion.
Report Sentence
No MIBG uptake in the adrenal mass, excluding pheochromocytoma and consistent with nerve sheath tumor (schwannoma).
Criteria
Mixture of Antoni A and Antoni B areas, Verocay bodies present, capsule intact, mitoses rare
Distinct Features
Most common type; typical target sign and late progressive enhancement pattern; variable cystic degeneration; prognosis excellent, no recurrence after surgery
Criteria
Long-standing schwannoma with extensive degenerative changes: cystic degeneration, calcification, hyalinization, nuclear atypia (degenerative)
Distinct Features
More heterogeneous imaging — extensive cystic areas, calcifications, hemorrhage; target sign may be lost; nuclear atypia may be confused with malignancy on histology but absence of mitoses supports benignity; surgical resection curative
Criteria
Predominant Antoni A pattern (>80%), minimal Antoni B areas, mitotic activity may be increased but no atypical mitoses
Distinct Features
More solid appearance, less cystic degeneration; T2 hyperintensity less prominent; more homogeneous enhancement; target sign may not be seen; differentiation from MPNST may be difficult — low recurrence risk
Distinguishing Feature
Pheochromocytoma shows uniform 'light bulb sign' T2 hyperintensity (different from schwannoma's target sign); intense arterial enhancement (different from schwannoma's late progressive pattern); MIBG positive; catecholamines elevated; hypertensive attacks
Distinguishing Feature
Ganglioneuroma also shows late progressive enhancement and may be T2 hyperintense; but ganglioneuroma is generally more homogeneous without target sign; calcification more common in ganglioneuroma; ganglioneuroma in pediatric/young adult age group, schwannoma more common in middle age
Distinguishing Feature
Lipid-poor adenoma usually <4 cm, homogeneous, cystic degeneration rare; washout analysis meets adenoma criteria; schwannoma larger, cystic degeneration common, target sign present, washout analysis inconsistent
Distinguishing Feature
Simple cyst is thin-walled, water density, no enhancement, no calcification; schwannoma has thick capsule, solid+cystic structure, late progressive wall/solid component enhancement, target sign on T2; simple cyst shows uniform T2 hyperintensity
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralAdrenal schwannoma is a benign tumor but definitive preoperative diagnosis is usually not possible. Biochemical evaluation is mandatory in all cases — catecholamines and metanephrines (pheochromocytoma exclusion) and cortisol/aldosterone (functional adenoma exclusion). Presence of target sign and MIBG negativity strongly support the diagnosis but definitive diagnosis requires histopathological examination. Surgical resection (adrenalectomy — laparoscopic or open) is the standard treatment; follow-up may be considered for small (<4 cm) lesions showing all typical features. Prognosis is excellent — post-surgical recurrence is extremely rare. MPNST, while very rare, should be excluded in cases showing rapid growth, irregular margins, or high FDG uptake.
Adrenal schwannoma is benign and surgical resection is curative. Preoperative diagnosis is usually difficult and it is detected as an adrenal incidentaloma. NF2 patients should be monitored.