Plexiform neurofibroma is a subcutaneous or dermal tumor of the peripheral nerve sheath surrounding one or more nerve fascicles. It can occur in isolation or in patients with neurofibromatosis type 1. This tumor is usually benign although a small minority can undergo a malignant transformation. Diagnosis relies on ultrasound, computed tomography, magnetic resonance imaging, and histopathological evaluation.
The name plexiform neurofibroma (PN) is derived from its appearance in the form of a network or a plexus surrounding branches of a peripheral nerve. It is typically a benign tumor and is composed of Schwann cells, perineurial components, fibroblasts, and hematopoietic cells surrounded by collagenous or myxoid stroma . Although it can occur anywhere on the body, the head and neck region are the commonest sites .
Clinically, patients present with an irregular, tortuous subcutaneous or dermal mass resembling a "bag of worms" . Other symptoms and signs depend on the mass effect of this swelling and it is essential to exclude extrinsic compression on neighboring structures or bone abnormalities. Tissue damage can lead to pain and sensorimotor deficits even in benign PN .
Although a plexiform neurofibroma is not pathognomonic of neurofibromatosis type 1 (NF -1), it is suggestive of the condition as it is found in up to 30% patients with NF-1. So the physician should look for features of NF-1 in patients with a plexiform neurofibroma.
Approximately 5% of PN can undergo malignant transformation. These patients present with pain, neurological deficits, and a rapid tumor growth, especially during puberty and pregnancy .
Beside history and physical examination, the workup of PN depends on radiological studies and histopathological evaluation. Ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI) are helpful in the diagnosis.
Ultrasonographic appearance is a target shaped mass with a hyperechoic center and resembling a cyst. However, the mass demonstrates a continuous peripheral nerve and this finding is confirmatory of a nerve sheath tumor . Ultrasound cannot differentiate between a benign and malignant lesion.
MRI is the gold standard study as it is able to delineate the extent of the tumor as well as its origin from the nerve sheath . Plexiform neurofibroma appears as a lesion in the form of a target with hyperintense T2 and a hypointense central signal . MRI also helps to exclude sarcoma, schwannoma, lymphatic and venous malformations. In the case of a malignant transformation, the MRI will reveal an edematous zone around the lesion with cysts within the tumor while a tumor diameter exceeds five centimeters .
Positron emission tomography (PET) scan and CT scan can be performed if a biopsy is planned  .