Follicular Neoplasms of the Thyroid

Gerard M. Doherty, MD

In the normal thyroid gland, the basic functional unit is the follicle. Follicles are single-layer spheres of follicular cells surrounding a lake of viscous colloid that primarily stores thyroglobulin. Other cell types that are present in the thyroid gland are interposed between these follicular spheres. These cells include perifollicular cells, also called C cells, which secrete calcitonin, as well as some supportive fibrous tissue, vascular structures, and nerves. The most common tumors of the thyroid gland arise from the follicular cells, including both papillary tumors and follicular tumors. Papillary tumors are discussed elsewhere in this text; however, follicular cancers are probably best described in the ways in which they differ from papillary tumors. Papillary tumors consist of single layers of thyroid cells arranged around vascular stocks, forming papulations. A substantial minority of papillary tumors (-40%) also include laminated calcified spheres called psammoma bodies. Follicular tumors include neither of these characteristics. The tumor cells surround persistent spherical follicles that frequently contain a small amount of colloid compared with normal. The follicular cells are often bland in their cytologic appearance, although more aggressive lesions can have more abnormal-appearing cells. Notably, the tumors that are purely follicular do not have any of the papillations or psammoma bodies that are typical of papillary tumors. Tumors that have a mainly follicular appearance but also contain some papillations or psammoma bodies are classified as follicular variants of papillary cancer; their biologic behavior supports their inclusion as papillary cancers.

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