Follicular carcinoma of the thyroid

Info from Burkitt pg 720

Follicular carcinoma is another well-differentiated thyroid malignancy.

Histologically, the neoplastic cells form a well-developed follicular pattern which is impossible to distinguish from benign adenomatous hyperplasia on FNA cytology; even on histology of the surgical specimen, it may also be difficult to diagnose malignancy unless there is evident capsular or vascular invasion.

Unlike papillary carcinoma, multicentricity is common.

The peak incidence of follicular carcinoma is between 40y and 50y, older than for papillary ca, but it is still three times more common in women. Generally, follicular carcinoma grows slowly and metastasises at a late stage.

In contrast to papillary carcinoma, metastasis tends to occur via the bloodstream to the lungs, bone and other remote sites rather than to local lymph nodes.

Since follicular carcinoma is rarely multicentric, management depends on the extent of local invasion. A tumour with only microinvasion of the capsule has a very good prognosis and only requires removal of the thyroid lobe containing the tumour. If, however, there is gross capsular invasion or vascular invasion (or known remote mets), total thyroidectomy is performed.

The purpose of total thyroidectomy is not to treat the local tumour but to enhance radioiodine uptake by metastatic lesions should this be required later for diagnosis, or any consequent Rx with high-dose radioiodine (as with papillary carcinoma).

Prognosis can be predicted by the degree of capsular and vascular invasion by the primary lesion and on histology. If there is no invasion, 10y survival is close to 100% but this falls to 30% if there is extensive local invasion.