Neurogenic tumor: Teaching Points

 Case Report

39 yr male with biopsy from swelling in posterior aspect of left knee suggesting neurogenic tumor presents for MRI left knee with contrast which shows –

Moderate size (25 mm approx.) regular intense homogeneously enhancing rounded SOL posterior to knee joint in soft tissue separate from popliteal vessels ( indenting them) with –

-          T1 hypointense / T2 heterogeneous hyperintense / few dots of possible vessels in GRE.

-          Split fat sign positive.

-          Fascicular sign positive.

-          Target sign negative.

-          No denervation of muscles in vicinity.

-          No perilesional edema like zone.

-          No intratumoral cyst formation.

-          No fluid levels.

-Consistent with history suggesting peripheral nerve sheath tumor like schwannoma with no definite MR features of malignant transformation.

A well defined subcortical, complex, subtly & heterogeneously enhancing posterior metadiaphyseal femoral lesion &  an ill defined subchondral similar enhancing posterolateral tibial condylar lesion with no cortical breach / expansion / soft tissue involvement / bleed / intra articular extension -  not specific to etiology, however may represent nonossifying fibroma of femur & nonspecific subchondral tibial lesion. DDs- hemangioma/ infarctions / unusual intraosseous neurogenic components.

Case Submitted by Dr MGK Murthy & Dr GA Prasad

Discussion –

Peripheral nerve sheath tumors consist of neurofibromas and schwannomas in the benign category & malignant peripheral nerve sheath tumor (MPNST) in malignant category.

Imaging features and signs that help to identify and characterize a nerve sheath tumor are, distribution of the tumor along a major nerve, an entering or exiting nerve sign, target sign, a fascicular sign and a split-fat sign.

-          Distribution of the lesion along the course of the nerve is most important feature. Benign lesions may be located along the cutaneous or deep nerves, however, their malignant counterparts are seen most commonly along the major nerve trunk. Neurofibromas are seen most commonly in the head and neck regions, while schwannomas are more common in the lower extremity. The dumbbell shape is characteristically seen in paraspinal neurofibromas, in which intraspinal extension of the tumor causes widening of the neural canal.

-          Entering or exiting nerve sign -  Neurofibromas are characterized by fusiform enlargement of the nerve, with the tapered ends of the lesion toward the parent nerve. Schwannomas are usually located eccentrically in relation to the nerve. The plexiform variant of the neurofibroma causes a diffuse expansion and enlargement of the parent nerve resulting in a ‘bag of worms’ appearance on imaging as well as on gross pathological examination.

-          Target sign – in T2W images as a central area of hypointensity with a peripheral hyperintensity,most often seen with neurofibromas with postcontrast study showing  central enhancement with hypointense rim due to central fibrocollagenous core and a surrounding myxomatous tissue. However, the target sign may be seen in schwannomas   due to central distribution of the more cellular Antoni type A cells, with a surrounding rim of hypocellular Antoni type B cells. The presence of this sign in a malignant peripheral nerve sheath tumor is indicative of benign tissue within the lesion, however, the absence of this sign in a lesion is indicative of a malignant transformation of the primary benign lesion.

-          Fascicular sign – seen as  multiple ring-like structures, which appear as hypointense foci within the hyperintense area on T2W images, possibly reflecting the fascicular bundles seen histologically and is often seen in benign nerve sheath tumors and can occasionally be seen in tumors with focal malignant transformation.

-          Split fat sign – presence of fat at the upper and lower poles of a lesion on T1W images suggestive of the intermuscular location of the lesion, although it is not specific to neurogenic tumors, can be seen in other soft tissue tumors arising in the intermuscular location. It is a feature of benignity, as malignant lesions tend to be more infiltrative in nature, resulting in the obliteration of fat at the ends of the lesion.

-          Muscular atrophy - The presence of a lesion with associated regional muscular atrophy in the neural distribution is indicative of a nerve sheath tumor. On imaging, it is best seen on T1W images as hyperintense areas, secondary to fatty replacement, interspersed within normal-appearing muscles.

-          T2 hyperintense rim - seen commonly in cases of schwannomas, as compared to neurofibromas.

-          Intra-tumoral cysts - more common feature of schwannomas, as compared to neurofibromas. 

Signs of malignant peripheral nerve sheath tumors -

-          Benign lesions are usually smaller than 5 cm, and an increase in size of these lesions is highly suggestive of a malignant transformation that requires histopathological confirmation.

-          Malignant lesions tend to be more heterogeneous on T1W and T2W images due to the necrotic and hemorrhagic areas within, however, a similar heterogeneity can be seen in cases of long-standing cases of schwannomas that have undergone degeneration.

-          Perilesional edema seen as an area of increased signal intensity on T2W images, adjacent to the primary lesion, suggestive of perilesional infiltration or edema.

-          Malignant tumors may show solid or peripheral enhancement, which differentiates them from neurofibromas that show focal central enhancement (Target sign), however, enhancement patterns similar to malignant peripheral nerve tumors can be seen in schwannomas, due to the areas of degeneration in them.

-          Bony involvement in the form of destruction favors malignant lesion.

-          FDG PET/CT can possibly differentiation between benign and malignant lesions based on the difference in the fluorodeoxygenase (FDG) uptake by the benign and malignant lesions with malignant lesions showing significantly higher specific uptake values (SUVs) as compared to the neurofibromas.  The SUVs in cases of schwannomas are higher as compared to the neurofibromas and may sometimes have an overlapping pattern, as in malignant peripheral nerve sheath tumor.