Neurofibromas are benign tumors of the peripheral nerves that have a neuroectodermal origin. The essential cells in neurofibromas are of Schwann-cell origin4
. Diffuse neurofibroma is an uncommon, but distinctive, form of neurofibroma. It is variably sized, though often large. It is characterized by marked dermal and subcutaneous thickening that most often appears in the trunk or head and neck regions of adolescents or young adults4,5
. This lesion has also been termed "paraneurofibroma" to indicate the extension of the tumor beyond the confines of the perineurium4
. Intracranial extension of the extracranial variety of this tumor has rarely been reported6
. At least 10% of these tumors are associated with NF-1, although Megahed7
reported that 61% of his 13 patients with diffuse neurofibromas had NF-1. The lower estimate of the incidence of NF-1 is probably due to the fact that the patients with diffuse neurofibromas were young, and young age often precludes a reliable diagnosis of NF-1.
MRI is the investigative tool of choice for this diagnosis because it better defines the anatomic relationships between the neurofibroma and the adjacent tissues, including muscular, vascular, and neural structures5
. Most MR images of a diffuse neurofibroma reveal a focal, discrete lesion, which may have a typical "target" pattern1
. The neurofibroma may resemble an intramuscular hemangioma, with entrapped fat simulating the lace-like pattern of fat interdigitating between the vascular elements of the hemangioma. Previous neurofibroma studies concluded that these tumors may mimic fat-containing lesions due to the presence of adipocytes intermingling with neurofibroma cells, and due to the entrapment of perineural adipose tissue by diffuse neurofibromas8
. The MRI findings of our patient, who had a diffuse neurofibroma, supported all these previous observations.
On histopathological examination, the tumor is ill-defined, and it diffusely infiltrates the dermis and subcutaneous tissue. Despite its infiltrative nature, the tumor envelops the normal structures rather than destroying them. It is composed of elongated, spindle-shaped cells with round or fusiform nuclei and eosinophilic cytoplasm within a loose matrix of fine fibrillary collagen. Meissner bodies are characteristic, but are not always present. Neurofibromatous tissues show immunoreactivity with S-100 protein, a sensitive, but non-specific, marker of benign nerve sheath tumors9
Although malignant transformation of neurofibromas has been reported in patients with NF-1, it rarely occurs in solitary diffuse neurofibromas10
. Pain or enlargement of a neurofibroma may herald malignant transformation.
The treatment of large neurofibromas consists of partial or complete surgical excision. Even after complete excision, clinical recurrence may develop because of the infiltrative growth pattern of the tumor. Because of possible recurrence and the potential development of neurofibromatosis, yearly follow-up is recommended4-6
. In the present case, the imaging studies and clinical findings demonstrated an infiltrative pattern of growth, and complete excision of the diffuse neurofibroma was unlikely to achieve success without extensive and potentially mutilating surgery.
In summary, we reported a case of a diffuse neurofibroma occurring on the scalp of a young boy. This is a rare nerve tumor with a characteristic radiological appearance that correlates well with histopathological studies. The reticular appearance is due to a network of abnormal nerves set in a collagenous matrix. It is known to be best demonstrated on MRI, particularly with contrast-enhanced TI-W sequences.