Supraclavicular Solitary Hybrid Schwannoma/Neurofibroma: A Case Report

Peripheral nerve sheath tumors (PNSTs) are benign lesions arising from the connective tissue sheath surrounding the neurons and are labeled schwannoma, perineurioma, or neurofibroma according to their histopathological characteristics. Lesions with a mixture of two or more of the aforementioned tumors are known as hybrid peripheral nerve sheath tumors (HPNSTs). These hybrid tumors have been described as rare entities. In this report, we present a case of a solitary hybrid schwannoma/neurofibroma in an unusual location.


Introduction
Peripheral nerve sheath tumors (PNST) are benign lesions arising from the connective tissue sheath surrounding the neurons. Most of these tumors present as focal soft tissue swelling with symptoms attributable to mass effect to adjacent structures. Diagnostic classifications based on the histopathology of PNST have been published previously [1]. The most common of these are schwannoma, perineurioma, and neurofibroma. A lesion with histopathological evidence of a nerve sheath tumor not specific to one of the aforementioned tumors is classified as a hybrid peripheral nerve sheath tumor (HPNST). These tumors are a benign combination with shared characteristics of the previously mentioned tumors. HPNSTs were officially introduced in the fourth edition of the World Health Organization (WHO) Classification of Tumors of Soft Tissue and Bone published in 2013, with the revised edition being released in 2016 [2,3]. Combinations occur either sporadically, accounting for schwannoma/perineurioma, or with associated syndromes such as neurofibromatosis (NF), which includes schwannoma/neurofibroma [2,4,5].

Case Presentation
A 35-year-old male, a known case of type I autosomal dominant cerebellar ataxia, was referred to our otolaryngology clinic for a slowly progressing neck mass for three years. Upon further questioning, the patient had no associated symptoms or significant family history. Examination of the swelling yielded a palpable, non-tender mobile mass in the left supraclavicular region. The left upper limb had no sensory or motor deficits. The rest of the examination was insignificant. The patient showed no signs of stigmata of NF or schwannomatosis.
The patient was sent for a neck MRI, which showed a left lower supraclavicular, wellcircumscribed, heterogeneous hyperintense enhancing mass measuring 30 x 26 mm ( Figure 1). The lesion was located posteromedial to the sternocleidomastoid muscle and abutting the C6  Core biopsy of the mass was taken, and histopathology was well consistent with schwannoma.
The lesion was excised surgically through an anterior supraclavicular approach. The brachial plexus was identified intraoperatively and preserved ( Figure 2). On gross inspection, the tumor was pale, encapsulated, and measured 30 mm in its greatest dimension.  Postoperatively, the patient was not able to abduct his left shoulder beyond 90 degrees. Nerve conduction study (electromyography) suggested mild hyperacute upper trunk brachial plexopathy. However, at the two-month follow-up visit, the patient was able to fully abduct his shoulder. At the six-month follow-up, the patient showed no evidence of tumor recurrence.

Discussion
PNSTs were classically divided into three categories: schwannoma, neurofibroma, and perineurioma [6]. In recent years, a new entity of soft tissue tumor known as hybrid peripheral sheath tumor was introduced by the World Health Organization, in which two discrete histology are evident within the same mass [3]. Histopathologically, schwannoma is consistent with hypercellular areas with spindle cells arranged in fascicles (Antoni A) and myxoid areas with low cellularity (Antoni B), whereas neurofibroma shows axons, Schwann, perineural, and inflammatory cells on a background of collagen [7]. In these hybrid tumors, we see a mixture of schwannian nodules in a background of neurofibromatous component.
Unfortunately, solitary hybrid schwannoma/neurofibroma tumors have been rarely reported in the literature. Such cases were initially described by Feany et al. in a series of nine cases. Three out of the nine reported cases failed to show signs or features of NF, suggesting that hybrid tumors are not confined to NF alone [5]. Only one of the cases had a recurrence.
Year Age at presentation Sex Site Size (mm) Follow-up  The development of schwannian differentiation within a neurofibroma remains unclear. It has been suggested to be due to clonal genetic alteration in the microenvironment [5]. Nevertheless, no malignant transformation of hybrid schwannoma/neurofibroma has been reported in the literature. Harder et al. has reported the presence of hybrid schwannoma/neurofibroma in 23 patients, which were found in association with NF type1, type 2, and schwannomatosis [12]. Unlike our present case, the majority of them were plexiform.

Conclusions
This report described a rare entity of PNST that might be diagnostically challenging or can even go underdiagnosed. The absence of NF or schwannomatosis stigmata makes the diagnosis even more difficult. However, advances in molecular testing have shown an increase in the number of reported cases. To date, the exact etiological factors remain unclear. In conclusion, schwannoma/neurofibroma tumors resemble benign peripheral nerve sheath pathology with no reported malignant transformation reported in the literature so far.

Additional Information Disclosures
Human subjects: Consent was obtained by all participants in this study.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.