Atypical Neurofibromatous Neoplasm with Uncertain Biologic Potential in the Posterior Mediastinum of a Young Patient with Neurofibromatosis Type 1: A Case Report

Abstract

Atypical neurofibromatous neoplasm with unknown biological potential (ANNUBP), proposed in a recent NIH consensus overview, is a rare precursor entity of malignant peripheral nerve sheath tumor (MPNST) in neurofibromatosis type 1 (NF1) patients. Only one report on imaging findings of ANNUBP is available. Herein, we present the case of a 19-year-old female, diagnosed with a mediastinal tumor by chance, who visited to our hospital. She had café-au-lait spots on her trunk and a past history of resected neurofibroma. Her family also had café-au-lait spots; therefore, an NF1-induced tumor was strongly suspected. MRI revealed a paravertebral mass of 7.5 cm in size consisting of an inner rim with low T2 signal intensity and an outer rim with high T2 intensity, which was similar to a target sign, adjacent to the pulmonary veins; the center of the tumor was well enhanced by gadolinium, and the peripheral region was myxoid and slightly enhanced. FDG-PET showed high FDG uptake, SUVmax of 8.5, although the peripheral region represented low FDG accumulation. CT-guided needle biopsy was repeated because of the suspicion of an MPNST, which resulted in the histopathological diagnosis of ANNUBP. Marginal tumor resection was performed, and the final post-resection histopathological diagnosis was ANNUBP transformed from neurofibroma; the region of ANNUBP lost p16 immunostaining, although it was retained in the peripheral region of the neurofibroma. There has been no recurrence or metastasis 1 year after treatment. In conclusion, ANNUBP could be represented as a well-enhanced homogeneous mass on MRI and a high FDG accumulated region on FDG PET/CT, as seen in MPNST, in NF1 patients.

Keywords: Atypical neurofibromatous neoplasm with uncertain biologic potential; Magnetic resonance imaging; Malignant peripheral nerve sheath tumor; Neurofibromatosis type 1; Positron emission tomography.

Fig. 1

a Chest X-ray shows a well-defined mass lesion in the right paravertebral region of the 6th to 9th thoracic vertebrae. b Contrast-enhanced computed tomography shows a right posterior mediastinal tumor with weak central enhancement in the early phase.

Fig. 2

Axial view of (a) T1-weighted and (b) T2-weighted images shows a mass consisting of an inner rim with low T2 signal intensity and an outer rim with high T2 intensity; coronal view of the (c) short TI recovery image and (d) T1-weighted gadolinium-enhanced image shows a mass that is continuous with the cranial and caudal region. Most of the tumor in the center shows slight high intensity on T2-weighted imaging and good enhancement by gadolinium on T1-weighted imaging. In contrast, the peripheral region of the tumor (arrowhead) depicts high-signal intensity on T2-weighted imaging and is slightly enhanced by gadolinium on T1-weighted imaging. e¹⁸F-FDG-PET/CT demonstrates FDG accumulation in the center of the tumor with an SUVmax of 8.6; the peripheral region of the tumor has low SUV accumulation.

Fig. 3

a Macroscopic appearance of the resected tumor shows a glossy peripheral region and a yellowish-white central region. b A loupe image of hematoxylin and eosin (HE) staining represents the central part of the tumor with high cellularity. c Microscopic image of H&E stain of the central region of the tumor shows tumor with nuclear atypia, high cellularity, loss of neurofibroma architecture, and (d) loss of p16 expression. e Microscopic image of H&E stain of the peripheral region of the tumor shows (d) interlacing bundles of elongated cells with thin wavy nuclei in the edematous matrix with interspersed collagen bundles and (f) tumor cells with retained p16 expression.