SMECE is a recently described tumor that bears no resemblance to any of the usual epithelial thyroid neoplasms. Histologically this tumor is composed of small nests or strands of squamous cells with rare mucous cells and characterized by extensive sclerosis, squamous and glandular differentiation, a concomitant inflammatory infiltrate rich in eosinophils and a background of Hashimotos thyroiditis in the involved thyroid tissue. The neoplastic cells are immunoreactive for cytokeratin, but not for thyroglobulin or calcitonin. The causes of eosinophilic infiltration is not known, but secretion of eosinophil chemotactic factors by the tumor cells may be responsible [11
In a review of all 24 SMECE cases (Table 1), including our own, patients ranged in age 32-74 years (mean, 56.5 years). Woman were predominantly affected (female:male ratio, 23:1). Most of the patients with SMECE had a relatively indolent clinical course and were alive with or without evidence of disease for periods ranging up to 12 years. Rare cases of lung, bone and other distant metastasis have been reported [3,4,7,8]. Our case had a more aggressive clinical course than any other case of SMECE previously reported. It is difficult to exclude definitely that in this patient SMECE was associated with an anaplastic carcinoma, not sampled by the biopsy, although numerous incisional biopsies were performed. We hypothesized that our patient developed an anaplastic carcinoma that led to the death of the patient. The presence of anaplastic carcinoma, well reported as a complication of several low-grade thyroid carcinomas, would better explain the aggressive clinical course in our patient .
Table 1:Survey of sclerosing mucoepidermoid carcinoma with eosinophilia of the thyroid by literature data
SMECE should be differentiated from other primary thyroid tumors that can show foci of squamous differentiation, especially papillary carcinoma or medullary carcinoma; primary or metastatic squamous cell carcinoma, and conventional mucoepidermoid carcinoma (MEC) of the thyroid gland [9,13,14]. Papillary carcinoma can be distinguished from SMECE by its characteristic nuclear morphology and its positivity for thyroglobulin [13,14]. Lack of lymphocytic thyroiditis and positive calcitonin staining differentiate medullary carcinoma from SMECE [13,14]. Primary squamous cell carcinoma of the thyroid is extremely rare and is classified as anaplastic carcinoma. It is distinguished from SMECE by its histologic features including diffuse growth sheets and large islands, marked nuclear atypia, numerous mitoses, and frequent necrosis .
Histologically MEC exhibits larger, more confluent sheets and nests of tumor cells when compared with the small cell nests and strands of SMECE and no eosinophilic infiltration. Vascular invasion with luminal obliteration of the medium-size vessels is observed in only SMECE. SMECE shows the more classical histological features of Hashimotos thyroiditis in the uninvolved thyroid parenchyma. However, in MEC the remaining thyroid usually shows lymphocytic thyroiditis in about half of the MEC cases. By immunohistochemistry, MEC is frequently thyroglobulin positive, whereas SMECE is negative for thyroglobulin [16,17].
The histogenesis of SMECE of the thyroid remains unclear and controversial. Based on its constant association with Hashimotos thyroiditis, it has been suggested that SMECE originates in the benign squamous nests that are often found in Hashimotos thyroiditis [1,3,4]. Many authors have suggested that this tumor develops from ultimobranchial body vestiges or solid cell nest [9,14].
In conclusion, although originally described as a relatively indolent neoplasm, SMECE may display aggressive behavior manifested by distant metastasis. It occurs predominantly in older women with Hashimotos thyroiditis. It is necessary to add SMECE to the list of differential diagnosis of thyroid tumors, although it is an extremely