Introduction
Myoepithelial tumors are mostly seen in soft tissues like skin. Therefore, these tumors are mostly seen in body extremities like limb girdles, lower limbs, upper limbs, head and neck, and trunk region. They occur both in males and females in equal ratios. 20 percent of myoepithelial tumors are also seen even in children. Most pediatric myoepithelial tumors are malignant. Myoepithelial tumors are recognized to occur primarily in soft tissues. Myoepithelial carcinomas are aggressive and show metastases, whereas benign tumors show low-grade transformation into metastases and exhibit low risk. However, there is always 20 percent of local recurrence, even in benign tumors.
What Is the Morphology of Myoepithelial Tumors?
These myoepithelial tumors show different cytologic and architectural features within and between tumors. Sometimes they are well-circumscribed, but at times both benign and malignant tumors are unencapsulated and have infiltrative borders. Benign myoepithelial tumors are well-circumscribed and encapsulated, composed of uniform myoepithelial cells, absent nuclear atypia, lack invasion into the surrounding tissues, and have low mitotic activity. Myoepithelial carcinoma has a variable appearance which can range from well-circumscribed to infiltrative growth patterns, have increased infiltration into the surrounding tissues, and have increased mitotic activity. Mixed tumors contain both myoepithelial and epithelial cells.
What Are the Typical Characteristics of These Myoepithelial Tumors?
Growth is either lobular or multinodular, and the cells are oval, spindled, and epithelioid. They are arranged as reticular, trabecular, solid, and nested. Tumors arising in soft tissues like skin lack normal cellular components. They show a range of cytologic and architectural variations within and between different tumors. Both benign and malignant tumors are well-circumscribed, unencapsulated, and have infiltrative margins.
What Are the Types of Myoepithelial Tumors?
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Mixed tumors or chondroid syringoma.
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Myoepithelial carcinoma.
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Myoepithelium.
Mixed tumors or chondroid syringoma are similar to salivary gland tumors showing tuboductal differentiation. Myoepitheliomas consist mainly of spindle or oval myoepithelial cells with little nuclei, fine chromatin, and small nucleoli. In skin and soft tissues, myoepithelial malignancies are differentiated by cell abnormality. In contrast, in salivary gland tumors, malignancies are diagnosed by architectural features like invasive growth, which is frequently associated with benign growth.
Myoepithelial carcinomas show mild to moderate abnormality of cells with vesicular nuclei and nucleoli. On the other hand, cell behavior, like high mitotic rates and necrosis, is the main reason for the aggressive behavior of soft tissue tumors. Hence myoepithelial carcinomas are graded on cell abnormality, mitoses, and necrosis.
What Is Cutaneous Syncytial Myoepithelioma?
Cutaneous syncytial myoepithelioma is a rare benign tumor that occurs from myoepithelial cells in the skin. Typical features include they occur in the dermis or subcutaneous tissue and affect more commonly the head and neck region, they have syncytial growth pattern (the tumor cells form multinucleated aggregates), and the tumor cells show positive myoepithelial marker staining such as S-100 protein, smooth muscle actin (SMA), calponin, and p63. Cutaneous syncytial myoepithelioma are benign tumors that have a good prognosis. Complete surgical excision is the treatment, and the recurrence rate is very rare.
The Behaviour of Myoepithelial Tumors:
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Mixed tumors and myoepithelioma show a benign course. 18 percent of myoepithelioma show recurrence, and the risk increase with incomplete removal of the tumor.
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Myoepithelial carcinomas are aggressive and recurrent and show distant metastasis. Sites for distant metastases include lung, bone, lymph nodes, and soft tissues.
Myoepithelial Tumours of Salivary Glands:
Myoepithelial tumors of salivary glands are rare and seen mostly in middle-aged and older people. They mainly affect minor and major salivary glands. In addition, the parotid gland, submandibular gland, and palatal glands are also affected.
Myoepithelial tumors of salivary glands include both:
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Benign myoepitheliomas (ME): Consists of neoplastic myoepithelial cells with well-circumscribed borders. The cells are epitheloid, plasmacytoid, clear cell, spindle, or mixed morphology.
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Malignant myoepithelial carcinomas (MECA): Consists entirely of neoplastic myoepithelial cells with infiltrative borders. They are multi-nodular, well-defined tumors within the capsule in the case of pleomorphic adenoma (PA).
Difference between ME and MECA includes cell abnormality, infiltrative growth, frequent mitosis, and cell necrosis. Salivary glands contain myoepithelial cells in their acini and intercalated ducts. These myoepithelial tumors arise in myoepithelial cells in acini and intercalated ducts.
Many salivary gland tumors follow a benign course; however, when left untreated or in recurrent cases, they transform into malignant tumors. They are mostly seen in the parotid gland. The percentage of occurrence is parotid gland (50 %), sublingual gland (33 %), and submandibular gland (13 %). Other sites include the breast, soft tissue, nasopharynx, lungs, and larynx.
Treatment: The treatment of myoepithelial tumors includes surgical excision and removal of the gland along with free margins with or without nodular dissection. In patients with MECA, the chances for aggressiveness and increased risk of recurrence are more when compared to benign myoepitheliomas. Early diagnosis and diagnosis reduce the risk of recurrence and metastases. In addition, palliative therapy, chemotherapy, or adjuvant radiotherapy is useful to the patient, thus improving the life span.
What Is the Role of Genes in Myoepithelial Tumors of Soft Tissue?
It is an area of ongoing research. Myoepithelial tumors of the skin, soft tissue, and bone show an EWSR1 gene rearrangement. However, even without EWSR1 gene rearrangement, the SMARCB1 gene is known to cause myoepithelial carcinoma in Fusion partners, including POU5F1, PBX1, ZNF444, ATF1, and PBX3. PLAG1 gene arrangement is seen in mixed tumors or chondroid. Various chromosomal abnormalities have been found in myoepithelial tumors, such as copy number alterations and chromosomal rearrangements. Fusion genes resulting from chromosomal rearrangements have been identified in specific subtypes of myoepithelial tumors. Inactivation or loss of function of certain tumor suppressor genes may be involved in the development of myoepithelial tumors. Dysregulation of various signaling pathways implicated in tumor development and progression, such as the MAPK and PI3K/AKT pathways, has been observed in myoepithelial tumors.
Conclusion
Myoepithelial tumors of soft tissue are similar to salivary gland tumors. EWSR1 translocation is seen in myoepithelial carcinoma and myoepitheliomas. Mixed tumors or chondroid syringoma show PLAG1gene rearrangement and ductal differentiation are closely related to the salivary gland. Myoepithelial tumors are increasingly being recognized, like cutaneous syncytial myoepithelioma harboring EWSR1-POU5F1 fusion. In addition, myoepithelial soft tissue tumors are supported by immunohistochemical studies like cytokeratin, EMA, S-100, and GFAP, staining for p63 and INI1.
