Plain radiograph lateral view
A 31-year-old woman with a palpable knee mass reports a 2-year history of progressive limitation in knee flexion. This mass is present since infancy but has progressively developed in the preceding 2 years. She denies a history of trauma.
The neurologic exam was normal, without sensory deficits.
In the plain radiograph, a soft tissue opacity in the posterior knee was identified. (Fig. 1)
Ultrasonography (high-frequency linear array 6-15 Mhz) revealed a well-defined, oval, echo-poor mass, mixed but predominantly solid, with enhanced through transmission.
There were no signs of vascularization in colour Doppler (Fig.2).
Magnetic resonance imaging (MRI) depicted a well-defined extra-articular lesion.
It was limited anteriorly by sartorius and gracilis tendons and posteriorly by semimembranosus and semitendinosus tendons.
The lesion depicted homogeneous hypointensity in T1-weighted intense (WI), heterogeneous and hyperintense signal in Proton-Density Fat-saturated (DP-FS) image (Fig 3.), with heterogeneous enhancement after contrast, mainly in the central part. (Fig. 4) There was no perilesional oedema. The fat plane around the lesion was preserved.
The pathological exam was compatible with neurofibroma, as it marked positive for CD 34.
Neurofibromas are peripheral nerve sheath tumours (PNSTs) related to Schwann cell’s proliferation.  Classically divided into 3 categories: solitary, diffuse, and plexiform. Diffuse neurofibromas and plexiform neurofibromas are closely associated with neurofibromatosis. Sporadic and diffuse neurofibromas only rarely progress to malignancy.  The vast majority of neurofibromas (>90%) are solitary.
Ultrasound is now considered the first-choice modality concerning the evaluation of nerves. 
However, the deep location of this lesion may be a limitation to ultrasound imaging.
MRI can accurately localize and determine the extent of the lesion. Several MRI findings were described as useful in the diagnosis of PNSTs, including the split fat sign, fascicular pattern, target sign, thin hyperintense rim, and identification of entering and exiting nerve.  Localized neurofibroma lesions usually show nonspecific signal intensity and variable contrast enhancement.  The classic target sign appearance was not present in our cause, which is characterized on T2-WI by peripheral high-signal-intensity due to myxoid material and a relatively low-signal-intensity fibrous component centrally. 
Localized neurofibromas often affect superficial cutaneous nerves, although involvement of larger nerves also occurs. 
Neurofibromas are intimately associated with the parent nerve, growing in a longitudinal fusiform manner with the nerve entering and exiting from the lesion. On the other hand, schwannomas are fusiform masses eccentrically located and adjacent to the involved nerve. Moreover, both the schwannoma and the affected nerve are contained within a true capsule, the epineurium. 
Demonstration of the nerve in contiguity with mass, with variable tubular or fusiform enlargement, is the key diagnostic feature and confirms the neural origin, but it was not evident in our case, even at MRI.
As opposed to schwannomas, most neurofibromas are solid tumours macroscopically; areas of cystic degeneration, hypocellularity and xanthomatous material are uncommon. 
MRI is useful for the differentiation of neurofibromas and schwannomas. However, no single or combination of findings allows a definitive differentiation between schwannoma and neurofibroma. 
Recently, Snoj et al. suggested that high-field-strength MR microscopy allows a better differentiation of fascicles anatomy than high-frequency US. This will potentially increase the confidence in suggesting a neurogenic origin in the future. The diagnostic value of ultrasound elastography is still debatable in literature. [3, 11]
Written informed patient consent for publication has been obtained.
 Joe YS, Chen SH, et al. (2010) Solitary neurofibroma at the popliteal fossa in a patient with leg numbness – A case report. J Med Ultrasound. 18 (3):136-140.
 Meyer A, Billing S. (2019) What’s new in nerve sheath tumors. Virchows Arch. Jan; 476 (1):65-80. (PMID: 31707590)
 Sconfienza LM, Albano D, Allen G, Bazzocchi A, et al. (2018). Clinical indications for musculoskeletal ultrasound updated in 2017 by European Society of Musculoskeletal Radiology (ESSR) consensus. Eur Radiol. ;28(12):5338-5351. (PMID: 29876703)
 Chee D, et al. (2011). Pictorial Essay: imaging of peripheral nerve sheath tumours. Canadian Association of Radiologists Journal 62 (176-182). (PMID: 20510574)
 Patel NB, Stacy GS (2012) Musculoskeletal manifestation of neurofibromatosis type I. AJR Am J Roentgenol. Jul;199(1): W99-106. (PMID: 22733937)
 Lim R, Jaramillo Diego, et al. (2005) Superficial Neurofibroma: a lesion with unique MRI characteristics in patients with neurofibromatosis type I. 184 (3):962-8. (PMID: 15728625)
 Vanhoenacker, F; Parizel, Paul; Gielon, Jan. (2017) Imaging of soft tissue tumors. (Fourth edition). Springer. DOI 10.1007/978-3-319-46679-8
 Lee, S; Kim, Jee-Young, et al. (2020). Intramuscular peripheral nerve sheath tumors: schwannoma, ancient schwannoma, and neurofibroma. Skeletal Radiology. (PMID: 31932870)
 Tagliafico Alberto Isaac, Amanda, et al. (2019). Nerve tumors: what the MSK Radiologist should know. Semin Musculoskelet Radiol 2019; 23:76–84. (PMID: 30699454)
 Snoj Ž, Serša I, Matičič U, Cvetko E, Omejec G. (2020). Nerve Fascicle Depiction at MR Microscopy and High-Frequency US with Anatomic Verification. Radiology. 297(3):672-674. (PMID: 33048035)
 Snoj Ž, Wu CH, Taljanovic MS, et al. (2020). Ultrasound Elastography in Musculoskeletal Radiology: Past, Present, and Future. Semin Musculoskelet Radiol.;24(2):156-166. (PMID: 32438441)
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