Retroperitoneal neurofibroma: CT and MR imaging findings

Clinical History

A 44-year-old man was referred for right upper quadrant abdominal pain. Sonography revealed the presence of a hypoechoic retroperitoneal mass. The patient had a history of neurofibromatosis type 1.

Imaging Findings

CT examination showed a sharply-defined, hypodense left paraaortic mass (Fig. 1). The lesion was relatively homogeneous, with a mean CT density of 25 HU and 35 HU, on unenhanced and contrast-enhanced images, respectively. A second low density mass, causing neural foraminal enlargement and pedicle erosion at the level of the TH10 lumbar segment was also detected (Fig. 2).
MR imaging of the abdomen was followed. The central part of the retroperitoneal mass was slightly hyperintense on T1-weighted images, heterogeneous, of intermediate signal intensity on T2-weighted images, mildly enhancing after gadolinium administration (Fig. 3). The peripheral part was markedly hyperintense on T2-weighted images. The second lesion had signal intensity similar to that of CSF, not contrast-enhancing (Fig. 4). None of these lesions had restricted diffusion (Fig. 5, 6). Based on imaging findings, multiplicity of the lesions and patient’s history the diagnosis of retroperitoneal neurofibroma, coexisting with a thoracic meningocele was made.

Discussion

Background
Neurofibromatosis type 1 (also known as von Recklinghausen disease) is inherited as an autosomal dominant disease and mainly characterised by cafe-au-lait spots and neurofibromas along the peripheral nerves [1, 2]. The classic neurogenic tumours seen outside the CNS include neurofibromas and plexiform neurofibromas [1, 2].
Primary retroperitoneal tumours represent an uncommon, diverse group of neoplasms that arise within the retroperitoneal space, but outside the major organs [3-5]. Imaging evaluation, including CT and MRI findings in combination with the history may narrow the differential diagnosis and even permit an accurate characterisation of a retroperitoneal mass, as in this case [3-5].
Imaging perspective
Imaging features of neurofibromas often correlate with the histopathologic findings [3-10]. In combination with clinical history and multiplicity of the lesions, as in this patient, accurate characterisation of these tumours is possible.
At CT, neurofibromas have a homogeneous density, round shape with smooth, distinct margins. The CT density is reported at 20-25 HU on plain images, mildly and homogeneously enhancing after contrast material administration, with a CT density of 30-35 HU, on contrast-enhanced images [3, 8, 9]. The above features were met in this patient. Lesion hypodensity on CT examination has been related to cystic degeneration, xanthomatous changes, areas of hypocellularity and lipid-rich Schwann cells. Bass et al reported the CT findings of 16 plexiform neurofibromas, detected as bilateral, symmetric parapsoas or presacral masses, homogeneously hypodense on CT [9].
At MR imaging, neurofibromas may show a target-like appearance, with different behaviour in the central part than in the periphery, as seen also in this case. On T1-weighted images, the central portion of the tumour is slightly hyperintense compared to the peripheral part, whereas on T2-weighted images the periphery of the mass appears hyperintense. The central part is of interemediate signal intensity on T2-weighted images, and enhances after gadolinium administration. Pathologically, the peripheral component of neurofibromas corresponds to gelatinous material due to myxoid degeneration and the central part to solid tumoral tissue, consisting of packed nerve sheath cells and fibrous tissue with xanthomatous changes [3-7]. A whorled appearance consisting of linear or curvilinear structures of low signal intensity on T2-weighted images within the central part has also been described, corresponding histologically to bundles of Schwann cells and collagen fibres within the mass [5].

Differential Diagnosis List

Final Diagnosis

Retroperitoneal neurofibroma

URL:	https://www.eurorad.org/case/9682
DOI:	10.1594/EURORAD/CASE.9682
ISSN:	1563-4086