Sigmoid Colon Schwannoma: A Case Report & Review of Literature

According to Bohlok et al. (2018), 95 cases of colorectal schwannomas have been reported distributed from the most common location to the least; cecum and right colon (30.5%), sigmoid (28.1%), rectum (21.1%), left colon (8.3%) the transverse colon (5.3%) and the appendix (1.15) with slight female predominance (58%) and a median age of 61.2 years.

The presentation of colorectal schwannoma varies on its presentation. It can be incidentally diagnosed in an asymptomatic patient during screening colonoscopy or with a wide range of symptomatic presentations, including abdominal pain, change in bowel habit - constipation, diarrhea or change in stool diameter, per rectal bleeding, tenesmus, decrease in appetite or weight loss (Zippi et al., 2013; Pansari et al., 2020). It can also present with a mass effect complication, e.g., intestinal obstruction or intussusception. Two reported cases where schwannoma was diagnosed during workup with a synchronous colorectal adenocarcinoma (world journal of surgical oncology page 97 & Page 77)

Due to its wide range of presentation, the workup of colorectal schwannoma follows a seemingly similar route to the workup of any colorectal mass. On colonoscopy, schwannoma varies in its gross appearance as a polypoidal lesion or a submucosal mass with normal overlying mucosa, which might mandate the use of endoscopic ultrasound and deeper biopsy or FNA to reach an accurate diagnosis (Wilde et al., 2010). However, the mucosal covering might be ulcerating with bleeding/sloughing/lumen compromisation mimicking malignancy.

The initial biopsy specimens were non-diagnostic, most likely due to the superficial nature of submitted tissue material and the submucosal nature of this neoplasm, like in any other mesenchymal tumors of the gastrointestinal tract where a deeper and targeted sampling approach results in a higher yield diagnostic neoplastic tissue. Schwannomas are characterized by biphasic growth patterns of hypercellular areas (Antoni A) of spindled tumors containing elongated and wavy nuclei admixed with loose/myxoid regions (Antoni B). However, some of the classic findings of Schwannoma described in soft tissue and central nervous system counterparts may be absent in the gastrointestinal tract tumors. The presence of lymphoid aggregates at the periphery of the tumor is a helpful diagnostic clue but not identified in our case. The gastrointestinal stromal tumors (GIST) are the most common mesenchymal gastrointestinal tumor and occasionally can share morphological similarities with Schwannoma; thus, the immunohistochemical staining panel should include GIST markers (CD117 and DOG1 stains). The diagnosis can be confirmed by diffuse positivity for S100 and SOX10 and after the exclusion of other more frequent mesenchymal tumors (such as GIST and smooth muscle neoplasms).

Radiological studies such as enhanced CT or MRI to characterize the colorectal mass with the use of contrast to delineate the tumor and lumen patency better are recommended as part of the workup, especially when malignancy is suspected with the need to rule out metastasis (Fotiadis, 2005; Cak et al., 2015). Findings on CT scans were mentioned differently among the reported cases, including a mural-originating colorectal mass with low attenuation with either smooth margins or suspicious edges, compromised lumens, thickened wall, fat stranding, and the presence of enlarged adjacent lymph nodes. All these variations add more challenges to the diagnosis of this pathology.

Moreover, a preoperative FDG-PET CT scan has been done for 5 reported cases, including our patient. Despite that, all of them were labeled as benign. Four of these cases were FDG avid lesions making it more difficult to rule out malignancy without a pathological diagnosis.

Whether colorectal schwannoma was diagnosed pre- or post-operatively, the standard treatment is radical colorectal resection depending on the tumor location, without any clear recommendations for neoadjuvant or adjuvant treatment (Kojima et al., 2020). Despite this, there have been several cases that were diagnosed preoperatively and found to have benign schwannoma treated more conservatively by endoscopic resection (page 12), cecal wedge resection (page 84), and trans-anal with no reported recurrence, yet small sample size is very limiting.

Postoperative histopathology is the final accurate way to diagnose schwannoma. Grossly it tends to be lobulated well-defined tumors, sometimes ulcerating into the mucosa with rich staining of S-100. Histologically, there are two recognized patterns, Antoni A, with a dense growth of fusiform cells compactly arranged in palisades to form Verocay bodies, and Antoni B, in which the fusiform cells are distributed more loosely with rounded and elongated nuclei, myxoid stroma and xanthomatous histiocytes (Nonose et al., 2009).

Miettinen et al. (2001), divided colorectal schwannoma into three clinicopathological types; spindle cell, epithelioid and plexiform. All three types are positive for S-100 protein and GFAP but lack CD117 (KIT), differentiating it from GIST tumors.

Despite these histological features and types, judging a colorectal schwannoma as benign or malignant depends pathologically on its mitosis rate, atypical mitotic figures, and nuclear hyperpigmentation, Ki-67 index with a value of more than 10% considered malignant. Clinically, long-term local recurrence or distant metastasis have also been used to judge the malignant state of a colorectal schwannoma, with only 3 cases reported in the literature.