Retroperitoneal neurofibroma is a benign peripheral nerve sheath tumor composed of Schwann cells, perineural cells, fibroblasts, and collagen fibers. It may be sporadic or associated with NF1 (neurofibromatosis type 1). NF1 patients develop multiple and plexiform neurofibromas with risk of malignant transformation (8-13%) — this risk makes close imaging follow-up critically important in NF1 patients. Usually seen in young adults. The 'target sign' on MRI (central low signal surrounded by high signal ring on T2) is characteristic and pathognomonic — this is the most valuable MRI finding for differentiation from schwannoma. Surgical resection is diagnostic and therapeutic but carries risk of neurological deficit due to nerve origin. MEK inhibitor (selumetinib) is FDA-approved for plexiform neurofibroma.
Age Range
15-50
Peak Age
30
Gender
Equal
Prevalence
Rare
Neurofibroma develops from biallelic inactivation of the NF1 gene (17q11.2). NF1 gene encodes neurofibromin — a GTPase-activating protein (GAP) that inhibits the RAS signaling pathway, and its loss leads to RAS/MAPK hyperactivation and cell proliferation. The tumor contains Schwann cells, fibroblasts, and collagen matrix, and this mixed cellular structure forms the pathoanatomic basis of the target sign. In the target sign, a central fibrous zone (collagen-dense → short T2 → low signal) is surrounded by a myxoid peripheral zone (water-rich → long T2 → high signal). Plexiform neurofibroma extends along the nerve creating a 'bag of worms' appearance — each fascicle shows independent tumoral expansion. Malignant transformation (to MPNST) occurs in 8-13% of NF1 patients and requires additional genetic aberrations (TP53, CDKN2A loss, PRC2 inactivation). Rapid growth, pain, and increased heterogeneity are clinical and radiological warning signs of malignant transformation.
Central low signal (fibrosis/collagen) surrounded by hyperintense ring (myxoid periphery) on T2 — pathognomonic for neurofibroma and the most valuable MRI finding for differentiation from schwannoma. Preservation of target sign suggests benign lesion, while its loss strongly suggests malignant transformation to MPNST — therefore evaluating target sign presence is critically important in MRI follow-up of NF1 patients.
On T2-weighted MRI, neurofibroma shows the classic 'target sign': central low signal (fibrous/collagen-dense zone) surrounded by hyperintense ring (myxoid periphery). This finding is pathognomonic for neurofibroma and the most valuable finding for differentiation from schwannoma. Target sign may be less prominent in large or plexiform neurofibromas. Preservation of target sign suggests benign lesion while its loss strongly suggests malignant transformation (MPNST).
Report Sentence
The mass shows classic target sign on T2 with central hypointense and peripheral hyperintense zone; consistent with neurofibroma.
On contrast-enhanced MRI, neurofibroma shows heterogeneous enhancement. Peripheral myxoid zone shows moderate to intense enhancement while central fibrous zone enhances less — described as 'reverse target sign'. The enhancement pattern is the inverse of the target sign and consistent with benign character.
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The mass shows peripheral enhancement with central low enhancement on contrast-enhanced MRI, demonstrating reverse target sign consistent with neurofibroma.
On non-contrast CT, neurofibroma appears as a low-density (20-30 HU), well-defined, fusiform or round mass. Low density reflects myxoid component and low cellular density. Calcification is very rare. In plexiform type, multiple low-density masses extending along nerve course may be seen — 'bag of worms' appearance can also be visualized on CT.
Report Sentence
A low-density, well-defined fusiform mass is seen in the retroperitoneal/paravertebral area; neurofibroma should be considered.
On T1-weighted images, neurofibroma shows isointense to slightly hypointense signal relative to muscle. Hyperintense foci (hemorrhage, fat) are not expected. Homogeneous low-to-intermediate signal is typical T1 finding for neurogenic tumors and has limited diagnostic value alone for benign/malignant differentiation.
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The mass shows homogeneous isointense signal to muscle on T1.
In NF1 patients, plexiform neurofibroma shows 'bag of worms' appearance with multiple fusiform masses extending along nerve plexus. Each mass may show target sign. This pattern is pathognomonic for NF1 and retroperitoneal plexus (lumbar, sacral) involvement may be seen. Surgical resection of plexiform neurofibroma is challenging due to nerve preservation.
Report Sentence
Multiple fusiform masses extending along nerve plexus create bag of worms appearance consistent with NF1-associated plexiform neurofibroma.
On DWI, benign neurofibroma does not show diffusion restriction — ADC values are high (1.5-2.5 × 10⁻³ mm²/s). This finding is the most valuable quantitative MR parameter for differentiation from MPNST (ADC 0.6-1.0). ADC monitoring in NF1 patients can be used for early detection of malignant transformation — stable ADC suggests benign lesion, decreasing ADC suggests malignant transformation.
Report Sentence
No diffusion restriction is seen in the mass; high ADC values (___ × 10⁻³ mm²/s) consistent with benign neurofibroma.
Criteria
Single lesion, may be sporadic or NF1-associated, young adult
Distinct Features
Well-defined fusiform mass, target sign, low malignant transformation risk (<1%), recurrence rare after surgery
Criteria
Extending along nerve plexus, involving multiple segments, pathognomonic for NF1, bag of worms
Distinct Features
Bag of worms appearance, high MPNST risk (8-13%), surgery difficult (nerve preservation), MEK inhibitor (selumetinib) FDA-approved treatment
Criteria
Infiltrative spread into skin and subcutaneous tissue, plaque-like, presents at young age
Distinct Features
Usually trunk or head-neck, retroperitoneal rare, surgical margins indeterminate, low MPNST risk
Distinguishing Feature
Ganglioneuroma homogeneous T2 hyperintense without target sign; neurofibroma shows target sign and more heterogeneous T2 — ganglioneuroma delayed enhancement, neurofibroma reverse target sign enhancement
Distinguishing Feature
Target sign lost in MPNST, necrosis, heterogeneous enhancement, rapid growth and diffusion restriction (ADC <1.0); target sign preserved in neurofibroma with ADC >1.5
Distinguishing Feature
SFT low T2 signal (collagen) and intense early enhancement (hypervascular); neurofibroma target sign T2 and mild enhancement — exactly opposite MR profile
Urgency
routineManagement
conservativeBiopsy
Not NeededFollow-up
12-monthPrognosis is excellent in sporadic solitary neurofibroma with malignant transformation risk less than 1%. Close follow-up is required in NF1 patients — MPNST risk in plexiform neurofibromas is 8-13% and this risk persists lifelong. Rapid growth, new pain, and increased FDG avidity (SUVmax >3.5) are warning signs for malignant transformation and create urgent biopsy indication. Surgical resection is recommended for symptomatic or growing lesions — neurological deficit risk exists due to nerve origin and patients should be informed. MEK inhibitor (selumetinib) is FDA-approved for plexiform neurofibroma and reduces tumor volume in inoperable, symptomatic plexiform neurofibromas (>50% volume reduction). Annual whole-body MRI screening is recommended in NF1 patients — detecting new lesions and size changes in existing lesions enables early diagnosis of malignant transformation.
Neurofibroma is benign but carries risk of malignant transformation in NF1 patients (8-13% develop MPNST). Rapid growth, increasing pain, and increased FDG-PET avidity should raise concern for malignant transformation. Surgical resection is curative but carries risk of nerve damage.