Answer: Follicular carcinoma, microinvasive (encapsulated) type

Histology: Grossly and microscopically the lesion is a solitary, circumscribed and encapsulated. Although the lesion demonstrates a predominant follicular pattern of growth, in some areas papillae are present. These papillae are broad-based and lack complex tertiary branching. The follicular epithelium lining the papillae and follicles are uniformly arranged. Their nuclei are not crowded and do not demonstrate significant atypia (e.g. optic clearing, inclusions, grooves, etc…).

The follicular proliferation is surrounded by a rather thick fibrous capsule. Upon thorough histologic investigation of this tumor/capsule interface, focal areas are identified where the tumor invades into the capsule and penetrates blood vessels within the fibrous capsule.

Discussion: Two categories of follicular thyroid carcinoma are recognized from a morphologic and prognostic standpoint – the minimally invasive type and the widely invasive type. Unlike its widely invasive counterpart, minimally invasive follicular carcinoma is surrounded by a relatively intact capsule. Malignancy, of course, is defined by tumor invasion through the capsule or into blood vessels within the capsule, but this is usually a focal and microscopic finding in the minimally invasive type. In the present case, focal areas of vascular invasion confirm the malignant nature of this tumor and exclude the diagnosis of some benign process such as adenoma or Grave’s disease.

The diagnosis of follicular carcinoma can be made only if features that define some other tumor type are not present. If, for example, the nuclear atypia of papillary thyroid carcinoma (PTC) were present and well developed, the tumor would be classified as a follicular variant of PTC. In the present case, these nuclear features are not present. One might argue that the tumor does exhibit some papillary formations, but the presence of papillae is not specific for PTC. Indeed, the relative simplicity of the papillae, their centripetal pattern of growth (i.e. the tips of the papillae all point to the center of the follicles) and the absence of nuclear atypia in the cells lining these papillae indicate that these are not the malignant papillae of PTC.

The highly unusual features of this case are the age of the patient and the functional status of the tumor. One should be cautious about diagnosing follicular carcinomas in children and adolescents since – unlike PTC – follicular carcinoma is a tumor that is almost always encountered in adults. The fact that this follicular carcinoma was hyper functional is also exceptionally unusual; and it defies the adage that follicular carcinomas are always “cold” on scintigraphic examination.