Teaching Case

Left lower limb pain.

A brainstem lesion was seen on the MRI scan of this patient, showing intense enhancement and increased metabolic activity on [18F]-Fluorodeoxyglucose Possitron Emission Tomography (FDG PET-TC) (Fig 1-c). Whole body FDG PET-CT scan was performed (Fig 2,3).

In this patient, multiple subcutaneous and plexiform neurofibromas (Fig 2,3), and a probable brainstem glioma with increased metabolic activity (Fig 1) were seen. Notice also the focal augmented metabolic activity in one lesion of the left thigh (Fig 3), what may be a sign of malignant transformation.
Neurofibromatosis (NF) is the most common of the neurocutaneous syndromes or phakomatoses. It is seen in two varieties: NF-1 and NF-2. NF-1 is seen in approximately in 1 in 3000 births. Unlike NF-2 which presents in adulthood, NF-1 commonly first presents in childhood or early adulthood.
Glial tumours predominate in NF1 while nerve sheath/meningeal tumours predominate in NF2. Optic pathway gliomas and brainstem gliomas are the most common intracranial neoplasms found in neurofibromatosis 1, although there also is an increased incidence of other brain tumours in this population. The majority of these intracranial neoplasms are benign pilocytic astrocytomas, which may behave in a less aggressive manner than histologically identical tumours in non-neurofibromatosis 1 patients. Owing to the indolent nature of these tumours, conservative management with close follow-up is recommended. When intervention is required, conventional treatment with surgery, radiation, or chemotherapy has been used with variable results.
A neurofibroma is generally considered a benign neural tumour which is composed of Schwann cells, fibroblasts and other neural elements. These components expand and diffusely infiltrate the nerve. A plexiform neurofibroma refers to a tortous interdigitating network of tumour occurring along a nerve. Plexiform neurofibromas are the most common extracranial lesion in neurofibromatois type 1.
CT typically demonstrates low-attenuation lobulated masses. No calcification or discernible capsules are usually identified. Plexiform neurofibromas usually have well-defined margins. In the retroperitoneal region, lesions are usually bilateral and symmetric which helps to distinguish plexiform neurofibromas from a more aggressive process.
MRI typically shows a well defined lobulated mass lesion which is slightly hyperintense to muscle or T1-weighted images and demonstrates very high signal intensity on T2-weighted images. Hypointense septations on T2-images can be seen coursing through the mass.
Sarcomatous degeneration of peripheral soft-tissue neurofibromas occurs in approximately 5-15% of patients with NF-1. Transformation of plexiform neurofibromas into malignant peripheral nerve sheath tumours (MPNSTs) is the main cause of mortality in patients with NF-1. Neither CT nor MRI can reliable distinguish malignant from benign lesions. However, pain, acute change in size, and destruction of normal adjacent structures can be clinical clues that a malignant transformation has occurred.
FDG-PET is a sensitive technique in the detection of MPNSTs in patients with NF-1. The addition of 11C methionine PET increases specificity in equivocal cases. PET may improve preoperative tumour staging by detecting metastases or second primary tumours, which often are present in patients with NF-1. Biopsy is needed for questionable lesions.
Gallium-67 scanning can also be helpful in screening benign and malignant lesions as malignant lesions are very gallium avid.
Surgical resection is the mainstay of treatment. Adjunctive chemotherapy and radiation therapy are of limited use.