[Rare malignant tumors of the thyroid]

The incidence of rare malignant tumors of the thyroid is about 4 cases/100,000 people and represent only 1.8% of all the thyroid cancers. When we talk about "rare" tumors, obviously, we do not refer to the most frequent cancers (papillary, follicular), or less frequent tumors (medullary, anaplastic), but to some types of thyroid tumors that have been almost always sporadically observed. Mucoepidermoid carcinoma and squamous carcinoma have been described in the literature. They present occasional papillary formation so that, according to some authors, could be considered as variants of the papillary carcinoma. Teratoma is another rare tumor which in the paediatric age is benign, but its prognosis could be unfavourable because it causes an important respiratory distress, while in the adult it presents a very aggressive clinical course like the anaplastic carcinoma. Lymphoma is the most frequent of the "rare" tumors of the thyroid (1-5% of all the thyroid cancers). It arises often in a setting of a long history of goitre and Hashimoto thyroiditis. Fine-needle aspirate is important to make diagnosis and to start a correct treatment which allows a 5-year survival up to 85% in the favourable cases.