Characteristic Appearances of Peripheral Schwannoma on High-Resolution Ultrasonography And MRI: Comparison Between Antoni A-Dominant and Antoni B-Dominant Types on High-Resolution Ultrasonography And

described Abstract Schwannomas or neurilemmomas are the most common tumors of the peripheral nerves. In this article, the characteristic appearances of schwannoma on high-resolution ultrasonography (US) and magnetic resonance imaging (MRI) were currently reviewed. The author previously mentioned that schwannomas were painful and vascularized superficial tumors of the acronym “ENGLAND” or ”LEND AN EGG”. From a current knowledge, the distinction between Antoni A-dominant and Antoni B-dominant types was suggested on high-resolution US. Antoni A-dominant schwannomas may tend to be a small size on gray-scale US and moderate vascular flow on color Doppler US. While, it is putative that the Antoni B-dominant schwannomas may represent relatively large tumor associated with anechoic lesion reflecting necrosis and/or hemorrhage on gray-scale US and a few vascularity on color Doppler US. It is strongly suggested that when target sign was recognized on MRI, the lesion may be an origin from nerve sheath and benign type.


Introduction
Schwannomas or neurilemmomas are the most common tumors of the peripheral nerves [1,2] It has been well known that schwannomas are one of a painful tumors of the skin [3]. The author has previously described the study on schwannomas as the painful cutaneous and subcutaneous tumors accompanied with vascularized appearances using high-resolution ultrasound in dermatology: the acronym "ENGLAND" or "LEND AN EGG" [4].
The author has also mentioned that it is important to differentiate between schwannoma and angioleiomyoma (ALM) [5,6]. It has been suggested that nerve-tumor associations such as nerve-tumor position and neve-tumor transition are significant features to differentiate schwannoma from neuroma [7,8]. Recently, Yang et al. [2] described that the target sign is a prominent feature on high-resolution US in peripheral schwannoma. In this article, the current knowledge of the characteristic appearances of schwannoma on high-resolution US and MRI will be reviewed.

ARTICLE INFO Abstract
Schwannomas or neurilemmomas are the most common tumors of the peripheral nerves. In this article, the characteristic appearances of schwannoma on high-resolution ultrasonography (US) and magnetic resonance imaging (MRI) were currently reviewed. The author previously mentioned that schwannomas were painful and vascularized superficial tumors of the acronym "ENGLAND" or "LEND AN EGG". From a current knowledge, the distinction between Antoni A-dominant and Antoni B-dominant types was suggested on high-resolution US. Antoni A-dominant schwannomas may tend to be a small size on gray-scale US and moderate vascular flow on color Doppler US. While, it is putative that the Antoni B-dominant schwannomas may represent relatively large tumor associated with anechoic lesion reflecting necrosis and/or hemorrhage on gray-scale US and a few vascularity on color Doppler US. It is strongly suggested that when target sign was recognized on MRI, the lesion may be an origin from nerve sheath and benign type.
Two major groups of the solitary benign peripheral nerve sheath tumors are regarded as schwannomas (neurilemmomas) and neurofibromas [23]. Schwannomas originate from the Schwann cells and usually well encapsulated without invading the nerves [24]. Clinically, schwannomas may be tender and firm and have a flash-to pale-pink color [3,4]. Pathologically, S-shaped nuclei and Verocay bodies are characteristic appearances. Both Antoni A and Antoni B tissue may be also observed [3,4].

Nerve-Tumor
Association: Differentiation between schwannomas and neurofibromas is known to be difficult based on the US features representing well-demarcated margin, a round to oval shape, a homogeneous hypoechoic pattern, through transmission and internal vascularity, the presence of the entering and exiting nerve, and the target sign [8]. Some studies have shown nerve-tumor connection in schwannoma [25][26][27][28]. Reynold et al. [25] have described that the presence of intrinsic blood flow signal on color Doppler US and nerve continuity features on grayscale suggest the peripheral nerve sheath tumor. Tsai et al. [7] described that a nerve eccentrically entering a mass was shown in schwannomas. With respect to the features of nerve-tumor position, Ryu et al. [8] proposed that schwannoma can show both eccentric and central positions, whereas an eccentric position can exclude the possibility of neurofibroma. Concerning the appearances of nerve-tumor transition, they mentioned that schwannoma can be clearly defined and neurofibroma shows infiltrative [8]. While, it has been described that the large schwannomas showed hyperechoic calcifications, internal degenerative cystic foci, and increased internal hierarchic vascular architecture [26,29].

MRI Appearances
Peripheral nerve sheath tumor (PNST) shows isointense signal intensity (SI) with muscle on T1-weighted image (TIWI) and heterogeneously hyperintense SI than muscle on T2-weighted image (T2WI), and heterogeneous enhancement pattern have been reported in patients with major-nerve PNST [30]. Five classical MRI features including the split-fat sign, target sign, fascicular pattern, entering and exiting nerve, and a thin hyperintense rim have been well known [30][31][32][33]. It has been suggested that the target sign was classified when the central hypointense focus comprised less than three-quarters of the lesion's diameter with a peripheral hyperintense rim on T2-weighted images [30,32]. On histological examination, schwannomas represent a target sign due to a hypercellular, hyporintense center or Antoni A area and a hypocellular periphery rich in marcrophages and collagen fibers or Antoni B area [32,34,35]. The target sign on MRI may suggest not only the tissue of origin from nerve sheath but also typical benign type [32,34]. The previous study indicated the qualitative presence and apparent increased conspicuity of the target sign on apparent diffusion coefficient (ADC) mapping in PNSTs [36]. Ahlawat et al. [34] have studied to evaluate the utility of target sign on diffusion weighted imaging (DWI) and ADC mapping for PNST findings.
They concluded that a "target sign" suggests a benign PNST and is more often visible on DWI and ADC maps compared with anatomic sequences [34].

1.
The author previously described schwannomas as a painful and vascularized superficial tumor. In addition, the

differences of vascular status between Antoni A-dominant and
Antoni B-dominant types may be suggested on color Doppler ultrasonography.

2.
Antoni A-dominant schwannomas may tend to be a small size on gray-scale ultrasonography and moderate vascular flow on color Doppler ultrasonography.

3.
It is putative that Antoni B-dominant schwannomas may represent relatively large tumor associated with anechoic lesion reflecting necrosis and/or hemorrhage on gray-scale US and a few vascularity on color Doppler ultrasonography.

4.
It is strongly suggested that when target sign was represented on magnetic resonance imaging, the lesion may be an origin from nerve sheath and benign type.