Up to half of all cases of malignant peripheral nerve sheath tumors are associated with neurofibromatosis type 1, and deep plexiform neurofibromas have an even higher risk of malignant transformation that superficial or solitary neurofibromas.
It can be difficult to differentiate malignant peripheral nerve sheath tumors from neurofibromas, a task that is even more difficult in patients with neurofibromatosis type 1.
Imaging features can help. A recent study by Wasa et al reviewed 41 cases of malignant peripheral nerve sheath tumor and 20 cases of neurofibroma, about half of whom had neurofibromatosis type 1. They found four statistically significant features that were useful in differentiating malignant peripheral nerve sheath tumors from neurofibromas, and suggest that a tumor with two or more of these features can be considered highly suspicious for malignancy. They also found an additional feature that was useful in patients with neurofibromatosis type 1.
The four (plus 1) features in their study were:
- Large long-axis dimension: Malignant lesions tend to be larger than about 5 cm in largest dimension.
- Peripheral enhancement: Focal central enhancement is usually seen in benign neurogenic tumors. Although neurofibromas can also show a peripheral enhancement pattern, this is more commonly seen in malignant peripheral nerve sheath tumors.
- A perilesional edema-like zone:
- Intratumoral cystic areas: Can be commonly seen in schwannomas and malignant peripheral nerve sheath tumors, but rarely in neurofibromas. Often is seen in association with peripheral enhancement
- Heterogeneity on T1-weighted images (in neurofibromatosis type 1): Can be helpful in differentiating malignant peripheral nerve sheath tumors from neurofibromas in patients with neurofibromatosis 1, but doesn't seem helpful in the general population.