Limitations and benefits of FDG‐PET/CT in NF1 patients with nerve sheath tumors: A cross‐sectional/longitudinal study

The purposes of this study were to re‐confirm the usefulness of PET/CT in the differentiation of benignity/malignancy of neurogenic tumors in NF1 patients, and to analyze the natural course of plexiform neurofibroma (pNF) and clarify whether PET/CT is also useful for detecting tumors other than neurogenic tumors. PET/CT was prospectively imaged in 36 NF1 patients. There were 14 malignant peripheral nerve sheath tumors (MPNSTs) in 14 patients, and 54 pNFs in 30 patients. Nine patients had both MPNST and pNF. Maximal standardized uptake value (SUVmax) was significantly higher in MPNST (median 7.6: range 4.1‐10.4) (P < .001) compared with that of pNF (median 3.7: range 1.6‐9.3). The cut‐off value of 5.8 resulted in a sensitivity of 78.6% and specificity of 88.9%. Median age was 29 y, and median maximum tumor diameter was 82 mm in 14 MPNST patients. The 5‐y overall survival rate was 46.8%. Three patients with low‐grade MPNST were alive without disease at the time of this report. In 9 patients in which pNF and MPNST co‐existed, 2 showed a higher SUVmax of pNF than that of MPNST. Natural history analysis of pNF (n = 43) revealed that no factors significantly correlated with increased tumor size. Nine lesions other than neurogenic tumors were detected by PET/CT including 5 thyroid lesions and 3 malignant neoplasms. This study revealed the usefulness and limitation of PET/CT for NF1 patients. In the future, it will be necessary to study how to detect over time the malignant transformation of pNF to MPNST, via an intermediate tumor.