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Neurofibromatosis type 1 (NF1) is an autosomal dominant disease in which the most common tumor is neurofibroma. This benign tumor of the peripheral nerve sheath may present as a focal nodular cutaneous or subcutaneous lesion, an intraforaminal spinal nerve root tumor, or as plexiform neurofibroma (PNF). In addition, patients with NF1 are at high risk of developing malignant peripheral nerve sheath tumors (MPNST) [1–6]. Accordingly, differentiating between benign and malignant tumors in NF1 patients has important prognostic and therapeutic implications, but it can be difficult, especially in individuals with multiple benign tumors. MRI and CT can be used to determine the site and extent of the PNF, but these methods are not reliable in discriminating with high accuracy between benign PNF and tumors that have degenerated into MPNST [1–6].

Histology remains the gold standard for identifying malignant transformation within a PNF. However, it requires complete excision, which in many patients is not technically feasible. If a core biopsy is performed, the focus of malignant change, particularly within a large heterogeneous tumor, may be missed. Moreover, histopathology and tumor grading of MPNST do not strictly correlate with the prognosis [1–6].

Several studies have shown the potential role of whole-body FDG-PET and PET/CT in patients with NF1 for detecting malignant change in PNF, for predicting tumor progression in these patients, for predicting survival in patients with MPNST, and for surveillance in pediatric patients with NF1 (Figs. 31.1, 31.2, and 31.3). An overview of the literature on the role of FDG- PET and PET/CT in patients with NF1 has been recently provided [7]. Its conclusions can be summarized as follows: (a) FDG-PET and PET/CT are useful and highly sensitive noninvasive methods to identify malignant change in neurogenic tumors in patients with NF1; (b) FDG-PET and PET/CT allow the discrimination of MPNST from benign neurogenic lesions in NF1; nevertheless, an overlap between these two disease manifestations regarding their SUVs should be considered. Early and delayed imaging (at 4 h) and the use of a SUVmax cutoff of 3.5 in the latter allow accurate lesion characterization with maximal sensitivity; (c) FDG-PET and PET/CT may improve preoperative tumor staging, guide biopsy, and influence treatment, thereby reducing the number of surgical procedures for benign neurogenic lesions or allowing early intervention in NF1 patients whose tumors have a high probability of progression.

Fig. 31.1
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A 10-year-old girl with neurofibromatosis type 1. The plexiform neurofibroma involved the right cervical and axillary region. (a) Axial PET/CT study and (b) axial PET/CT control after 2 years (March 2010) show a mild nonhomogeneous 18F-FDG uptake (SUVmax 1.7) with a focal much intense radiotracer accumulation (SUVmax 3.9)

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Fig. 31.2
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A 9-year-old girl with neurofibromatosis type I and multiple neurofibromas extending from the mediastinum to the cardias. In abdomen the neurofibromas enclose the celiac trunk reaching the porta hepatis. (a) Axial PET/CT and (b) CT images show a mass surrounding the celiac trunk (SUVmax 1.8)

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Fig. 31.3
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A 7-year-old boy with neurofibromatosis type I. (a) Coronal CT, (b) PET, (c) PET/CT fusion, and (d) sagittal maximum intensity projection PET images show increased FDG uptake corresponding to a paravertebral mass in the left posterior mediastinum (orange arrow in c and yellow arrow in d) between seventh and tenth dorsal vertebrae (SUVmax 9.5). Histology demonstrated the presence of a malignant nerve sheath tumor

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FormalPara Teaching Point

FDG-PET and PET/CT are useful methods to identify malignant change in neurogenic tumors in NF1 and to discriminate malignant from benign neurogenic lesions. Both FDG-PET and PET/CT may improve preoperative tumor staging, guide biopsy, and influence treatment planning [7].