Plexiform schwannoma is a rare benign peripheral nerve sheath tumor first described in 1978 by Harkin et al. that macroscopically grows in a multinodular or plexiform pattern [4, 5]. Plexiform schwannoma usually develops in the skin or superficial soft tissue and rarely in deep-seated nerves [2, 3]. Cases of this tumor originating in uncommon sites, such as the deep soft tissue and gastrointestinal tract, have been documented but are scarce [6,7,8,9]. We herein report the first case of plexiform schwannoma originating from the peripancreatic nerve plexus.
According to a review of plexiform schwannoma by Iida et al., the age of the patients can range from 2 to 80 years (mean, 30 years), with no pronounced gender predominance [8]. Most plexiform schwannomas present as single, soft to rubbery, movable, nontender, and sometimes painful nodules less than 2.5 cm in diameter [10]. Neurofibromatoses, including neurofibromatosis type 1, neurofibromatosis type 2, and schwannomatosis, are genetic neurogenetic disorders characterized by the development of multiple nerve sheath tumors [11]. Plexiform schwannoma mostly occurs sporadically, as in our case, occasionally in patients with neurofibromatosis type 2 or schwannomatosis and rarely in patients with neurofibromatosis type 1 [3]. Our patient had a solitary tumor and no clinical findings or familial history of neurofibromatoses.
Histologically, most plexiform schwannomas have the essential features of conventional schwannoma except for a multinodular growth pattern. These include composition solely of spindle-shaped Schwann cells, a fibrous capsule, hyaline vessels, cellular (Antoni A) and loose-textured (Antoni B) areas, and Verocay bodies (opposing rows of spindle nuclei separated by anucleate rows of eosinophilic processes). In plexiform schwannoma, Antoni A areas generally predominate, while Antoni B areas are less frequent. Degenerative changes such as necrosis, cyst formation, and hemorrhage are uncommon [2, 6, 12]. Similar to all schwannomas, the tumor cells of plexiform schwannoma are uniformly positive for S-100 on immunohistochemistry, while they are negative for c-kit, smooth muscle actin, and desmin [6, 8]. These features are consistent with the findings in our case.
Plexiform schwannoma shows complex multinodular growth and often involves multiple nerve fascicles, which differs from the single nerve fascicle involvement of conventional schwannoma [13, 14]. Although plexiform schwannoma also shows a chronic course that reflects its histopathologically benign nature as well as conventional schwannoma, plexiform schwannomas arising from major peripheral nerves often cause motor deficits with poor functional prognosis due to the difficulty of their total resection [2, 14]. Preoperative magnetic resonance imaging is useful for distinguishing between plexiform and conventional schwannomas. Plexiform schwannoma has the morphological features suggestive of multinodular configuration in contrast to conventional schwannoma with generally globular shape. In addition, conventional schwannoma demonstrates marked enhancement after gadolinium administration, while plexiform schwannoma shows somewhat less predictable enhancement [14].
It is crucial to differentiate plexiform schwannoma from plexiform neurofibroma because the latter carries a risk of malignant transformation (2–5%), in contrast to the former [15, 16]. The multinodular growth pattern of plexiform schwannoma can mimic that of plexiform neurofibroma, although plexiform neurofibroma is essentially pathognomonic for neurofibromatosis type 1 and usually occurs in early childhood. Since plexiform schwannoma and plexiform neurofibroma exhibit similar findings on CT and MRI, the final distinction between them depends on the histopathological examination [17, 18]. S-100 immunostaining is helpful since plexiform schwannoma shows diffuse and strong positivity with S-100. On the other hand, plexiform neurofibroma lacks Antoni A and B areas of schwannoma and shows weak, patchy S-100 positivity [19].
Plexiform schwannoma is a benign, noninfiltrating tumor, so the prognosis is favorable when complete excision is achieved, although recurrence has been reported in cases of incomplete resection [2, 20]. Kuo et al. reported 17 cases of peripancreatic schwannoma, although the subtypes of schwannoma were not mentioned [21]. In that report, 14 patients were treated by tumor excision with no recurrence, while concomitant pancreatectomy was performed in only 3 patients. In our case, tumor excision without pancreatectomy was considered to be a sufficient treatment as a result. However, concomitant pancreatectomy seems to be unavoidable when it is difficult to distinguish peripancreatic schwannoma from pancreatic tumors pre- or intraoperatively, as in our patient and other case reports [22, 23].
The managements of peripancreatic and pancreatic schwannomas are guided by not only anatomical locations, but also histological results. Accurate preoperative diagnoses of peripancreatic and pancreatic schwannoma are difficult due to the nonspecific and variable radiographic appearances of schwannomas, even with the use of multiple imaging modalities. They may mimic other, more common pancreatic lesions, such as pancreatic neuroendocrine tumors, mucinous cystic neoplasms, solid pseudopapillary neoplasms, mucinous cystadenocarcinomas, serous cystic neoplasms, acinar cell carcinomas, and pancreatic pseudocysts [24]. EUS-guided fine needle biopsy is useful to establish precise preoperative diagnosis and avoid unnecessary extensive radical resection [25, 26], although its high false-negative rate and the difficulty of performing biopsy for cystic or small lesions are problems [24, 27, 28]. In our case, EUS-guided fine needle biopsy provided an accurate diagnosis, resulting in a clinically reasonable treatment.
