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Overview of follicular thyroid cancer

Douglas S Ross, MD
Section Editor
Douglas S Ross, MD
Deputy Editor
Jean E Mulder, MD


Follicular thyroid cancer, a well-differentiated tumor of thyroid epithelium, is the second most common type of thyroid cancer after papillary thyroid cancer. In iodine-sufficient areas, up to 10 percent of all thyroid cancers are follicular cancers. An overview of follicular thyroid cancer will be provided here. Papillary thyroid cancer and the management of differentiated thyroid cancer are discussed separately. (See "Overview of papillary thyroid cancer" and "Differentiated thyroid cancer: Overview of management".)


Microscopically, the diagnosis of follicular cancer requires distinguishing adenoma from cancer, through identification of tumor extension through the tumor capsule and/or vascular invasion. The histology ranges from normal well-differentiated epithelium with follicular development and colloid (findings associated with a good prognosis) to poorly-differentiated with solid growth, absence of follicles, marked nuclear atypia, and extensive vascular and/or capsular invasion (characteristics that are associated with a worse prognosis) (picture 1) [1]. (See 'Prognostic features' below.)

Typically, the microfollicular architecture is uniform with an invariant collection of cuboidal cells lining the follicles. In addition, features consistent with papillary cancer, such as psammoma bodies and nuclear changes (ground-glass appearance, longitudinal grooves, nuclear overlapping, and inclusions), should be absent.

There are two patterns of growth that relate to capsular invasion; minimally invasive follicular cancer (MIFC) is limited to microscopic penetration of the tumor capsule without vascular invasion, while widely invasive follicular cancer (WIFC) extends beyond the tumor capsule into blood vessels and adjacent thyroid parenchyma (picture 2 and picture 3) [1,2]. Newer terminology also includes angioinvasive follicular cancer, but current prognosis scoring does not yet include this type of pathology. Angioinvasive cancer may be associated with distant metastases [3].

Less common pathologic features are associated with the variants of follicular thyroid cancer. Though considered by some to be a distinct type of tumor, Hürthle cell cancer is classified by the World Health Organization (WHO) as a variant of follicular thyroid cancer. It includes the presence of a cell population of "oncocytes," mostly eosinophilic oxyphilic cells with abundant cytoplasm, closely packed mitochondria, and round oval nuclei with prominent nucleoli (picture 4) [4]. Insular thyroid cancer (ITC), another rare subtype sometimes classified under follicular thyroid cancer, shows poor differentiation, an aggressive clinical course, and an unfavorable prognosis.


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Literature review current through: Mar 2017. | This topic last updated: May 03, 2016.
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