18-fluorodeoxyglucose-positron emission tomography (FDG-PET) evaluation of nodular lesions in patients with Neurofibromatosis type 1 and plexiform neurofibromas (PN) or malignant peripheral nerve sheath tumors (MPNST)

Abstract

Background: Individuals with Neurofibromatosis type 1 (NF1) are at risk for developing malignant peripheral nerve sheath tumors (MPNST), which frequently arise in preexisting plexiform neurofibromas (PN). Magnetic resonance imaging (MRI) with volumetric analysis and 18-fluorodeoxyglucose-positron emission tomography (FDG-PET) were utilized to monitor symptomatic nodular lesions.

Procedure: Patients with NF1 and PN on a NCI natural history trial were monitored for total body tumor volume (TTV) using volumetric MRI. FDG-PET was performed in individuals with a nodular well-demarcated lesion ≥3 cm if they were growing, painful, or there was a prior history of MPNST (target lesions). Asymptomatic nodular lesions were evaluated as non-target lesions.

Results: Fifteen patients (8m, 7f) median age of 18.3 years (range, 10-45 years) had a single target and non-target (n = 46) nodular lesions identified on MRI. Target lesions arose within (n = 8) or outside (n = 3) a PN, and all but 1 had increased FDG uptake. FDG uptake was increased in non-target lesions but to a lesser degree. FDG uptake in the surrounding PN was low, similar to background activity. Pathologic evaluation performed in 11 patients demonstrated neurofibroma (n = 6), atypical neurofibroma (n = 2) and malignancy (n = 3).

Conclusions: Nodular target lesions identified on MRI in individuals with NF1 and PN demonstrate increased FDG uptake similar to MPNST, but may be benign on biopsy. Nodular target lesions may be at greater risk for malignant transformation, but their biologic and clinical behavior has not been well studied. Careful longitudinal evaluation will be required to better understand the malignant potential of these lesions.

Fig. 1.

Coronal (A) and axial (B) STIR MRI images of two nodular lesions (arrows) within a neck PN (A) and outside of a pelvic (B) PN. These lesions are well demarcated and lack the central dot sign characteristic of PN.

Fig. 2.

Patient 6 with a nodular target and non-target lesions within a PN on composite MRI (A) and FDG uptake on corresponding PET scan (B). FDG uptake of the PN surrounding the nodular lesion was similar to normal tissue.

Fig. 3.

SUVmax (g/ml) values based on histology for patients with target nodular, FDG-PET avid lesions of clinical concern as compared to uptake in non-target nodular lesions and surrounding PN.

Fig. 4.

Patient 1 with a history of MPNST and a growing nodular lesion within a PN on axial and coronal MRI scan and increasing FDG uptake on corresponding PET scan. Pathology of this lesion was consistent with recurrent MPNST.