Tumors are composed of variably sized cysts as well as sheets, lobules, and nests of cells. Convoluted nests of squamoid cells with cystic alteration are embedded in a fibrous stroma in this image. The neoplastic cells are polygonal, with slightly eosinophilic cytoplasm, round nuclei, and small visible nucleoli.
The mucinous cells resemble goblet cells. They are large and polygonal, with copious intracellular blue mucin that may be extruded into the lumen.
A predominantly cystic and hence low grade appearance is seen in this particular case.
Focal areas of stromal sclerosis with scattered mucin cells can be appreciated. In some cases, areas may resemble clear cell carcinoma, but a mucicarmine stain will highlight the mucin.
There are no high grade cytological features in our particular case, and in the majority of reported cases.
Thymic carcinomas have a variety of characteristic morphologic variants, and among these, mucoepidermoid carcinoma is one of the rarest. Only 20 or so have been reported in the literature (Moran).
Histologically, it is identical to its counterpart in the salivary glands. It is characterized by nests of epidermoid cells within which scattered mucin-producing cells and variably sized lumina containing mucin are present. Secondary cystic change is common (Moran, Fletcher).
In a review of 10 cases, patients presented with respiratory symptoms, such as shortness of breath and dyspnea on exertion with retrosternal pain, and less commonly weight loss and pericardial effusion (Nonaka).
Thymic mucoepidermoid carcinomas were thought to be predominantly low-grade tumors, however, aggressive tumors leading to death have been reported. Those with a poor prognosis showed high-grade histology and high-stage disease.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 1333.
Moran CA, Suster S. Thymic carcinoma: current concepts and histologic features. Hematol Oncol Clin North Am. 2008 Jun;22(3):393-407.
Nonaka D, Klimstra D, Rosai J. Thymic mucoepidermoid carcinomas: a clinicopathologic study of 10 cases and review of the literature. Am J Surg Pathol. 2004 Nov;28(11):1526-31.