Abstract
Myoepithelial carcinoma with predominantly clear cell morphology is rare. A review of the literature identified 15 unequivocal cases, only two of which were of minor salivary gland origin. A case of minor salivary gland clear cell myoepithelial carcinoma of the retromolar region in a 70-year-old man is presented. It is important to recognize the clinicopathologic features of this unusual tumor, because of its histological similarity to several other primary and metastatic clear cell tumors and its aggressive behavior.
Myoepithelial carcinoma is a rare tumor, defined as a malignant neoplasm of the salivary glands in which tumor cells manifest almost exclusively myoepithelial differentiation . It arises predominantly from the major salivary glands, either de novo or as a carcinoma arising from a pre-existing pleomorphic adenoma or myoepithelioma. A wide variety of morphologic tumor cell types, including epithelioid, spindle, hyaline or clear cell, have been identified in myoepithelial carcinoma. Of these, the clear cell type is the least frequently encountered, either focally or, rarely, predominantly; the term clear cell myoepithelial carcinoma (CCMC) is applied for the latter circumstance . A review of the literature identified 15 unequivocal cases of CCMC , only two of which were of minor salivary gland origin . The authors present a new case of CCMC, which arose from the minor salivary gland, and discuss its clinicopathologic and immunohistochemical features. Awareness of the unique clinicopathologic features and immunohistochemical profile of CCMC is crucial for correct identification and treatment.
Case report
A 70-year-old man presented with a painful swelling in the left angle of mandible of 1 year duration. On examination, a 3×3 cm ulcerative lesion was noted in the left retromolar region, extending into the lateral wall of the pharynx. A panoramic radiograph ( Fig. 1 ) showed a mulitilocular, relatively well circumscribed radiolucency, extending from the distal of the first molar to the mandibular angle and ramus on the left side. There was no palpable cervical lymphadenopathy. General evaluation including a chest X-ray and abdominal sonography showed no evidence of distant metastasis. The treatment consisted of a wide surgical resection, in which the tumor mass, part of the mandible and surrounding soft tissues were removed. Intraoperative frozen section was used and it suggested low-grade malignancy and clear margins. The patient is being followed-up and shows no signs of local recurrence or lymph node or distant metastasis 10 months after surgery.
Histopathological findings
The resected tumor was solid, gray white and 6.5 x 5 x 4.5 cm in size with a nodular ill-defined outline. Macroscopically obvious areas of necrosis and hemorrhage were seen on a cut section. Microscopic examination revealed a carcinomatous lesion composed almost entirely of clear cells, which were arranged in nests, sheets and cords separated by fibrous stroma ( Fig. 2 a) . The clear cells were round to polygonal in shape with abundant cytoplasm, well-defined borders and vesicular nuclei ( Fig. 2 b). The cytoplasm of the clear cells was PAS positive and diastase soluble, indicating the presence of glycogen. Alcian blue-positive tumor cells were absent. Cells with abundant eosinophilic cytoplasm and spindle-shaped cells were occasionally interspersed between the nests and sheets of clear cells ( Fig. 2 c). In some areas, the neoplastic cell population consisted almost entirely of large polyhedral cells with prominent cytoplasmic clearing ( Fig. 2 d). Necroses were found focally and in larger areas. The number of mitoses was low (2 per 10 high power field). The tumor demonstrated extensive infiltration into adjacent bone and soft tissues, including mucosa, bone skeletal muscle and nerve ( Fig. 3 a and b ). There was no sign of a possible pre-existing pleomorphic adenoma or discrete ductal/tubular differentiation. Immunohistochemically, the tumor was diffuse positive for cytokeratin AE1/AE3, S-100, vimentin, p63 and maspin; weakly positive for calponin; negative for smooth muscle actin (SMA) ( Fig. 3 c and d). The Ki-67 labeling was approximately 15%. The association of the immunoprofile with the microscopic morphology confirmed the diagnosis of clear cell myoepithelial carcinoma.
Histopathological findings
The resected tumor was solid, gray white and 6.5 x 5 x 4.5 cm in size with a nodular ill-defined outline. Macroscopically obvious areas of necrosis and hemorrhage were seen on a cut section. Microscopic examination revealed a carcinomatous lesion composed almost entirely of clear cells, which were arranged in nests, sheets and cords separated by fibrous stroma ( Fig. 2 a) . The clear cells were round to polygonal in shape with abundant cytoplasm, well-defined borders and vesicular nuclei ( Fig. 2 b). The cytoplasm of the clear cells was PAS positive and diastase soluble, indicating the presence of glycogen. Alcian blue-positive tumor cells were absent. Cells with abundant eosinophilic cytoplasm and spindle-shaped cells were occasionally interspersed between the nests and sheets of clear cells ( Fig. 2 c). In some areas, the neoplastic cell population consisted almost entirely of large polyhedral cells with prominent cytoplasmic clearing ( Fig. 2 d). Necroses were found focally and in larger areas. The number of mitoses was low (2 per 10 high power field). The tumor demonstrated extensive infiltration into adjacent bone and soft tissues, including mucosa, bone skeletal muscle and nerve ( Fig. 3 a and b ). There was no sign of a possible pre-existing pleomorphic adenoma or discrete ductal/tubular differentiation. Immunohistochemically, the tumor was diffuse positive for cytokeratin AE1/AE3, S-100, vimentin, p63 and maspin; weakly positive for calponin; negative for smooth muscle actin (SMA) ( Fig. 3 c and d). The Ki-67 labeling was approximately 15%. The association of the immunoprofile with the microscopic morphology confirmed the diagnosis of clear cell myoepithelial carcinoma.
