Myoepithelioma of the nasal septum: A case report with radiological and pathological insights

INTRODUCTION

Myoepithelioma is a rare salivary gland tumor, accounting for <1% of all salivary gland neoplasms.^[1] It most commonly arises in the major salivary glands, particularly the parotid. In contrast, cases originating from the minor salivary glands are relatively uncommon, with reported sites including the palate, lips, buccal mucosa, nasal cavity and sinuses, orbit, and external auditory canal, indicating a broad distribution across the maxillofacial region.

The tumor exhibits marked morphological diversity in its cellular composition and may display varying degrees of biological aggressiveness. Some cases have shown recurrence or potential for malignant transformation.^[2] Immunohistochemical analysis, particularly cellular phenotyping, plays a critical role in confirming the diagnosis and assessing malignancy. This report presents a case of benign myoepithelioma of the nasal septum, with a focus on diagnostic insights from radiological and pathological perspectives.

CASE REPORT

A 68-year-old man presented with progressive right-sided nasal obstruction that began 6 months ago, accompanied by intermittent dizziness. Four years prior, he underwent surgery for cerebral infarction. He denied any history of smoking, allergies, or exposure to industrial toxins, dust, or radiation. There was no known family history of genetic disorders. On physical examination, a noticeable bulge was observed over the external nose. A pinkish mass was found obstructing the right nasal cavity, with mild deviation of the nasal septum to the left. There was no tenderness over the paranasal sinus region.

Magnetic resonance imaging (MRI) revealed an expanded right nasal cavity. An irregular, lobulated soft tissue mass measuring ~4.9 cm × 2.1 cm × 3.5 cm was located primarily in the anterior and inferior portions of the right nasal cavity. The lesion exhibited slightly hypointense signal intensity on T1-weighted images [Figure 1a] and predominantly hyperintense signals on fat-suppressed T2-weighted images [Figure 1b and c]. Diffusion-weighted images (DWI) showed an isointense signal that was similar to the apparent diffusion coefficient map derived from DWI [Figure 1d and e]. Following intravenous administration of gadopentetate dimeglumine, the lesion demonstrated homogeneous and moderate enhancement, similar in intensity to adjacent muscle [Figure 1f]. The margins of the mass were well defined, with apparent attachment to the bony and cartilaginous nasal septum medially. No clear evidence of invasion into adjacent bony structures or paranasal sinuses was observed.

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Figure 1: A 68-year-old man, with progressive right-sided nasal obstruction. (a) The lesion was located in the right nasal cavity (arrows), which appeared slightly hypointense signal on T1-weighted images (T1WI), (b and c) Predominantly hyperintense on fat-suppressed T2-weighted images, (d) Isointense signal on apparent diffusion coefficient map, (e) And diffusion-weighted images, and (f) Post-contrast T1WI demonstrated uniform and moderate enhancement.

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Imaging studies raised suspicion of a neoplastic lesion but did not provide a definitive diagnosis. At the patient’s request, the mass was completely excised under nasal endoscopy. Intraoperatively, a neoplasm was observed in the common nasal meatus, with its base attached to the nasal septal mucosa. The excised tissue was submitted for histopathological examination. Morphologically, the tumor was composed of relatively uniform cells that were oval or polygonal in shape, with abundant cytoplasm. The cells were arranged in sheets and embedded in a myxoid stroma [Figure 2a and b]. Immunohistochemical analysis revealed that the tumor cells were positive for S-100 [Figure 2c], Cytokeratin 5/6 (CK5/6) [Figure 2d], Cytokeratin 7 (CK7) [Figure 2e], P63 [Figure 2f], Pan-Cytokeratin [Figure 2g], B-cell lymphoma 2 [Figure 2h], vimentin [Figure 2i], with partial positivity for smooth muscle actin (SMA) [Figure 2j]. Only occasional positive cells were observed, indicating a very low Ki-67 proliferation index [Figure 2k]. Based on these findings, a final diagnosis of myoepithelioma was established.

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Figure 2: (a and b) Hematoxylin and eosin staining showed that the plasmacytoid myoepithelial cells were arranged in a reticular pattern, against a background of myxoid stromal degeneration, some exhibiting a clear-cell appearance. Immunohistochemical staining revealed that tumor cells are positive for (c) S-100, (d) Cytokeratin 5/6, (e) Cytokeratin 7, (f) P63, (g) pan cytokeratin, (h) B-cell lymphoma 2, (i) Vimentin, (j) Smooth muscle actin. (k) Ki-67 Immunostaining demonstrated minimal proliferative activity. Scale bar: 100 μm, ×20 magnification; 50 μm, ×40 magnification.

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DISCUSSION

Nasal cavity myoepithelioma was first reported by Louis in 1991.^[1] Due to its low incidence and possible misdiagnosis, only a few cases have been documented. Most tumors arise from minor mucous or serous glands on the lateral nasal wall. Their origin may be from ectopic embryonic epithelial cells or remnants of the vomeronasal organ.^[3] The main symptoms include chronically progressive nasal obstruction, recurrent rhinorrhea, or epistaxis. A small subset of patients may experience hyposmia or pain. The clinical presentation lacks specificity compared to other nasal or sinonasal tumors. Four previously reported cases published within the last decade were reviewed [Table 1].^[4-7]

Although rare, nasal myoepitheliomas can closely mimic other neoplastic lesions in radiological appearance, making preoperative differentiation challenging. However, imaging examination plays a critical role in localization and qualitative diagnosis. Computed tomography (CT) is particularly useful for detecting bony erosion or invasion into the paranasal sinuses, while MRI offers superior soft tissue contrast, aiding in the differentiation between benign and malignant tumors, precise delineation of lesion extent, assessment of proximity to the skull base, and guidance for endoscopic surgical planning.

In some cases shown in Table 1, signs of internal hemorrhage and remodeling of the adjacent paranasal sinus bone were observed, with no evidence of invasion of the skull base structures. In our case, although the MRI findings lacked highly specific features, the mass showed well-defined margins and homogeneous signal intensity; no diffusion restriction was observed on the DWI sequence, which is consistent with the characteristics of a benign lesion. Moderate and uniform enhancement after contrast administration may be attributed to the tumor’s abundant cellular components and scattered myxoid stroma.

Consistent with salivary gland myoepithelioma, myoepithelioma cells in the nasal cavity or paranasal sinuses show marked cellular diversity, including spindle cell, plasmacytoid, and clear cell types. The tumor cells are typically arranged in solid sheets or reticular cord-like structures, and the stroma often exhibits myxoid or hyaline-like changes.^[1-3] Microscopically, the four cases respectively showed a mixed pattern of spindle cells and plasmacytoid cells, a plasmacytoid type, a spindle cell type, and a spindle cell type with accompanying myxoid stroma. In our case, the tumor presented as a mixed type, composed predominantly of plasmacytoid and clear cells. As a salivary gland-type epithelial tumor, myoepithelioma is considered to share histological features with pleomorphic adenoma. The tumor consists of epithelial cellular arrangements within a mesenchyme-like stroma. Small ductal structures may occasionally be present, though such ductal differentiation is significantly less common than in pleomorphic adenoma (<10%).^[2,3]

Immunohistochemical staining demonstrated that tumor cells in all cases consistently expressed S100 and cytokeratin. In some cases, α-SMA, calponin, glial fibrillary acidic protein (GFAP), epithelial membrane antigen (EMA), vimentin, p40, and other markers were positive. In our case, CK5/6, CK7, and pan-cytokeratin staining were all positive, supporting the epithelial differentiation of the tumor. Although focal expression of these epithelial markers may be compatible with limited ductal or glandular epithelial differentiation, no definite ductal structures were identified histologically. Notably, EMA immunoreactivity has demonstrated substantial variability among previously reported myoepitheliomas of the nasal cavity and paranasal sinuses, with some cases showing positive staining while others exhibited a complete lack of EMA expression. In plasmacytoid myoepithelioma cases reported by Gourh, λ light chain and CD138 showed strong and diffuse positivity, overlapping with the immunophenotype of nasal plasmacytoma, whereas such validation was not performed in our case.

Given that the histomorphological features are dominated by cellular diversity and myxoid stroma, we constructed an immunohistochemical staining profile to differentiate nasal cavity or paranasal sinus tumors with similar characteristics, including pleomorphic adenoma, chondromyxoid fibroma, synovial sarcoma, melanoma, and schwannoma [Table 2]. Compared with myoepithelioma, pleomorphic adenoma is characterized by a chondroid matrix and prominent ductal differentiation; however, the two entities share considerable immunophenotypic overlap, making histomorphological features of critical diagnostic value in this context.

Interpretation of the overall immunophenotypic profile, rather than reliance on a single marker, is important in the differential diagnosis of myoepithelial tumors. This includes the evaluation of co-expression patterns among multiple markers and their distribution across tumor cells. In myoepithelial tumors, schwannomas, and melanomas, S100 protein expression is positive or diffusely strong; these entities can be further distinguished by evaluating whether S100 is co-expressed with CK, p63, SMA, or GFAP. In contrast, positive staining in chondromyxoid fibroma and synovial sarcoma is typically patchy or focal.

Notably, co-expression of S100, CK, EMA, and vimentin, while narrowing the differential diagnosis to tumors with biphasic epithelial and mesenchymal differentiation, raises synovial sarcoma as an important consideration. Its genetic mechanism involves the specific SS18-SSX fusion gene, which aberrantly activates epithelial gene expression programs and drives a biphasic differentiation phenotype.^[8] However, myoepithelial tumors commonly demonstrate p63 nuclear positivity and may show SMA expression, whereas these markers are usually not expressed in synovial sarcoma. In addition, SRY-box transcription factor 10 (SOX10) is a transcription factor indicative of neural crest-derived cells and is expressed in melanocytes, neural cells, and myoepithelial cells, providing supportive diagnostic value in differential diagnosis.^[9] The final diagnosis should integrate histomorphology, immunophenotype, clinical presentation, and imaging findings. In cases where immunophenotypic overlap is extensive, or in the presence of suspected anaplastic or dedifferentiated regions with potential loss of specific marker expression, molecular testing is recommended when necessary.

Surgical resection remains the first-line treatment, performed via endoscopic or open approaches. In this case, the tumor was resected endoscopically under general anesthesia. In general, recurrence is uncommon after complete excision of benign tumors, and no recurrence was observed in previously reported cases with at least 1 year of follow-up. In the present case, only short-term follow-up was available, which represents a limitation. Although nasal ventilation was restored after surgery, this does not necessarily indicate complete tumor eradication, and symptom improvement may lead to reduced surveillance adherence. Postoperative reparative changes, including mucosal edema and granulation tissue within the surgical cavity, may be difficult to distinguish from residual or recurrent tumor on endoscopy and imaging. Moreover, most benign nasal tumors grow slowly, and the absence of short-term recurrence does not exclude late recurrence. Therefore, long-term clinical and endoscopic follow-up is recommended. When suspicious lesions are detected, biopsy should be performed promptly, and contrast-enhanced MRI may assist in further evaluation.

Genetic testing may assist in diagnosis and treatment. Pleomorphic adenoma gene 1 (PLAG1) rearrangements are common in salivary myoepithelial carcinomas and have been associated with aggressive biological behavior, whereas Ewing sarcoma breakpoint region 1 (EWSR1) rearrangements are identified in ~50% of soft tissue myoepithelial neoplasms.^[10] These recurrent genetic alterations are associated with distinct molecular subsets of myoepithelial tumors and may contribute to a more refined molecular classification beyond conventional histopathological assessment. Considering multiple factors, including the imaging, pathology, and multidisciplinary clinical diagnostic opinions, as well as the patient’s preference, molecular pathological testing was not performed in this case. Nevertheless, molecular techniques such as fluorescence in situ hybridization and next-generation sequencing may provide additional diagnostic information in rare, complex, or histologically ambiguous cases. As the molecular landscape of myoepithelial neoplasms continues to be elucidated, these approaches may also facilitate the future development of molecularly informed therapeutic strategies.

SUMMARY

The tumor in our case showed low cytological atypia, rare mitotic figures, no evidence of invasive growth, and a low Ki-67 proliferation index, all of which are consistent with a diagnosis of benign myoepithelioma. Definitive diagnosis relies on histopathology and immunohistochemistry, while early complete resection and regular follow-up are essential. As the postoperative follow-up period was ~3 months, regular monitoring with endoscopic examinations is strongly recommended to detect any potential recurrence or malignant transformation.