Desmoplastic Small Round Cell Tumor of the Submandibular Gland: A Case Report and Literature Review

Desmoplastic small round cell tumor (DSRCT) is a rare and aggressively malignant tumor mostly occurring in the abdominal and pelvic cavity of young patients. However, few cases had been reported concerning DSRCT occurring in the head and neck region. We presented a rare case of DSRCT of the right submandibular in a 25-year-old man. MRI revealed a 3 × 2-cm solid nodule located in the right submandibular, and physical examination showed no other occupying lesion elsewhere. Histologically, the tumor was composed of various-sized small round cell nests, embedded in an abundant desmoplastic stroma. Immunohistochemically, the tumor cells were typically positive for epithelial (CK and EMA), mesenchymal (vimentin and desmin), and neuroendocrine (CD56, NSE, Syn, and CgA) markers, but negative for WT1. Fluorescence in situ hybridization revealed the presence of a break apart involving the Ewing sarcoma (EWS) gene. The patient received chemotherapy and radiotherapy and relapsed after 19 months of follow-up. DSRCT of the submandibular gland is rare, and the diagnosis of this tumor in an uncommon location relies on the histomorphology, immunophenotype, and EWS gene translocation detection. Differential diagnosis including primary salivary gland tumors and the other small round cell tumors needs to be excluded.


Introduction
Desmoplastic small round cell tumor (DSRCT) is a rare and highly malignant tumor which involves the abdominal or pelvis organs in young men with a mean age of 22 years [1]. Extra-abdominal DSRCT was reported occurring in the lung [2], brain [3], nasal cavity and paranasal sinuses [4], soft tissues, and bone [5]. However, few cases of DSRCT had been reported in the submandibular gland [6][7][8][9]. Herein, we present a case of a primary submandibular gland DSRCT in a 25-year-old man who survived for 21 months and relapsed for the first time after 19 months and summarize the clinical and pathological features of DSRCT in this uncommon location. We present the following case in accordance with the CARE reporting.

Case Presentation
A 25-year-old man noticed a mass in the right submandibular region with no specific medical history in the past. Eight months after discovery, the mass grew rapidly, characterized by a hard texture, poor mobility, adhesion to surrounding tissues, and no obvious tenderness. MRI revealed a 3.0 × 2.0-cm nodule in the right submandibular gland (Fig. 1a). Physical examination and preoperative CT scan showed no tumors elsewhere, followed by complete resection of the nodule.
Grossly, the mass showed a 3.0 × 2.0 × 1.8 cm in size with a gray appearance, and there was no evident macroscopic hemorrhage or necrosis of tumor on the cut surface. Histologically, the tumor was composed of small round tumor cell nests with different sizes and shapes, surrounded by abundant hyperplastic fibrous tissue (Fig. 2ac). The tumor infiltrated into the submandibular gland, and a residual normal gland could be identified. Necrosis could be seen in the center of large tumor cell nests which arranged in a palisade pattern (Fig. 2d). The tumor cells were small to medium sized, with sparse cytoplasm, unclear borders, round or oval hyperchromatic nuclei, and inconspicuous nucleoli, and nerve invasion readily seen (Fig. 2e, f). Immunohistochemistry results indicated that the tumor cells had epithelial, neuroendocrine, and myogenic multiple differentiation and expression patterns. The first antibodies are summarized in Table 1. The tumor cells were immunoreactive for epithelial markers CK (Fig. 3a) and EMA (Fig. 3b) with a diffuse cytoplasmic pattern, vimentin (Fig. 3c) and desmin with dot-like paranuclear cytoplasmic positivity (Fig. 3d), and strong membrane and cytoplasm immunoreactivity for CD56 (Fig. 3e), CgA (Fig. 3f), Syn, and NSE (Fig. 3g). In addition, the tumor cells were also immunoreactive for E-cadherin, Fli-1, and CD99 (Fig. 3h), but negative for WT1, Myogenin, S-100, CK5/6, p63, Melan-A, actin, and TTF-1. The Ki-67 labeling index was almost 50%. Fluorescence in situ hybridization (FISH) findings revealed EWSR1 split signals that were detected in 86% of The tumor comprised well-defined and variable-sized nests in a desmoplastic stroma. c The tumor cells were small to medium size with inconspicuous nucleoli. d Central necrosis of the tumor nest was occasionally seen. Tumor nest invaded into salivary gland tissue (e), and nerve invasion was readily seen (f).
tumor cells (>15% were diagnostic). The final diagnosis of DSRCT depended on the postoperative pathological examination with immunohistochemistry and FISH results. The patient received adjuvant chemotherapy (ifosfamide and etoposide) and radiotherapy (PGTV tb : 66Gy/30f; PTV1: 60Gy/30f) after surgery and exhibited recurrence from the CT scan after 19-month follow-up.

Discussion
DSRCT is a highly aggressive and rare mesenchymal tumor first described as a separate identity by Gerald and Rosai [10]. DSRCT characteristically arose in the abdominal or pelvis cavity in young men, and only 6% of DSRCT have occurred in sites outside the abdomen [11]. DSRCT of the submandibular gland as in the case presented here had similar morphologic and immunophenotypic features to the previously reported cases. Histologically, DSRCT was characterized by well-defined and variablesized nests comprising small to medium tumor cells with apparent collagen background. Immunohistochemically, DSRCT had a polyphenotypic immunoprofile with neoplastic cells expressing epithelial, mesenchymal, and neuroendocrine markers. WT1 was considered as a useful antibody for the diagnosis of DSRCT and differentiating it from other tumors with small blue cell morphology. Frequent strong nuclear expression of WT1 had been reported in DSRCT which was attributed to EWSR1-WT1 gene fusion [12]. However, WT1, Clone EP122, showed negative expression in our study. Hatanaka et al. [13] demonstrated that the Clone of N-WT1 and 6F-H2 antibodies resulted in negative or abnormal expression of the cytoplasm. And, their research recommended using C-WT1 antibody [13]. In addition, Zhang et al. [12] report showed that a small number of cases were indeed C-WT1 (Clone: C19) negative. Therefore, it was recommended to perform molecular detection so as to avoid diagnostic pitfall. We then performed FISH assay in order to determine whether EWSR1 gene fracture was presented. Not surprisingly, EWSR1 split signals were detected in 86% of the tumor cells, which conclusively confirmed the diagnosis of DSRCT.
DSRCT needed to be differentiated from primary salivary gland tumors. Poorly differentiated myoepithelial carcinoma, the solid variant of basal cell adenocarcinoma (BCA) and adenoid cystic carcinoma (ACC), also had similar morphological areas which were composed of a large solid tumor island. Lack of myoepithelial differentiation (actin and S-100 negative) and expression of neuroendocrine markers (CD56, NSE, Syn, and CgA) would  be helpful in excluding BCA and ACC. Moreover, the tumor islands of DSRCT were lacking peripheral palisade structures in BCA or the focal presence of cribriform structures and hyaline globules in ACC. CD117, frequently used for the diagnosis of ACC, had been demonstrated to be negative or focal positive expression in DSRCT [12].
Although it is also rare, the other small round cell tumors, such as Ewing sarcoma, small cell carcinoma, rhabdomyosarcoma, and malignant melanoma, still need to be excluded. It was reported that CD99, a marker frequently used for the diagnosis of Ewing's sarcomas/PNET, had been shown to be positive in 57% of DSRCT [12]. Fli-1 was also found to show positive expression in the current study. Given this finding, CD99 and Fli-1 had a limited role in distinguishing DSRCT from Ewing's sarcomas and primitive neuroectodermal tumors. Primary small cell carcinoma in the salivary glands is rare and mostly occurs in patients older than 50 years of age [14]. Positive for CD56 and neuroendocrine marker may increase the risk of misdiagnosis as small cell carcinoma. Abundant desmoplastic stroma and collagenous background may be an important clue for DSRCT. Rhabdomyosarcoma in young patients as a differential diagnosis should also be included. Unlike rhabdomyosarcoma, DSRCT was negative for Myogenin and myoD1. In general, tumor cells of DSRCT had immunohistochemical differentiation toward epithelial, neuroendocrine, and muscular cells. Therefore, a set of immunostaining markers, including epithelial (CK and EMA), mesenchymal (vimentin and desmin), neuroendocrine (CD56, NSE, Syn, and CgA), CD99, WT1, Fli-1, CD117, Melan-A, and Myogenin, are essential for diagnosis and differential diagnosis.
So far, the origin of DSRCT remains unclear. Although the common site of DSRCT, peritoneum-lined surfaces, suggested that it may be derived from the mesothelium, immunohistochemical expression (CK5/6 and calretinin negative) and ultrastructural level did not support tumor cells with mesothelium differentiation [7]. Furthermore, there was no mesodermal distribution in the submandibular area. It seemed reasonable to explain that the tumor cells may originate from multipotential differentiated primitive mesenchymal cells or neuroectodermal tissue. ly in the English language literature. All 5 cases, including the present one, occurred in males, and the age ranged from 24 to 41 years (mean age, 30 years). All 5 tumors reported were <5 cm in size. According to the previous reports, the prognosis of intra-abdominal DSRCT was particularly poor, median survival ranged from 17 to 25 months, and 5-year survival rate was around 15% [15]. Lymph node metastases had occurred in most cases at the time of diagnosis. Moreover, tumor occurring in the abdominal cavity was relatively hidden, and the tumor appeared to be progressive when it was discovered. However, the prognosis of DSRCT in this uncommon location remained unknown. Of the 5 patients presented in Tables 2, 3 were alive at the time of publication, one died with systemic metastasis at 25 months, and another one who had a history of diabetes mellitus and end-stage renal failure died of disease at 5 weeks. It was worthy of note that both of them had nodal metastasis. It was reasonable to speculate that lymph node metastasis was a risk factor for poor prognosis. On the other hand, the early occurrence of a local mass in the submandibular gland provided a higher chance of complete surgical resection. In our case, the patient showed recurrence after 19 months of follow-up.

Conclusions
We report a rare primary DSRCT arising from the submandibular gland of a young man. DSRCT is diagnosed mainly based on the radiological findings, histological features, and immunohistochemical and molecular examination. Prognostic factors related to DSRCT in this uncommon location remain to be further studied.

Statement of Ethics
The research was conducted ethically in accordance with the World Medical Association Declaration of Helsinki. Written informed consent was obtained from the patient for publication of this case report. The present study was approved by the Ethics Committee of the Xiangya Hospital of Central South University (Approval No. 202010139).