Answer: Follicular carcinoma

Histology: Within the thyroid there appears to be a solitary mass which is surrounded by a dense fibrous capsule. The proliferation within the mass is extremely cellular compared to the surrounding thyroid tissue. The proliferation within the mass grows in a solid sheet-like pattern; the cells have pale pink cytoplasm and relatively round appearing nuclei. The nuclei lack significant grooves and do not appear to be overlapping, which are characteristics seen in follicular variant of papillary thyroid cancer. An occasional mitotic figure is observed, but they are relatively rare. The growth within the nodule does show evidence of thyroglobulin production. The most striking abnormality within the capsule is the evidence of vascular invasion by the tumor. These areas do indeed appear to be definite vascular invasion as the foci are lined by endothelial cells surrounding the vessel. In addition to the easily identified areas of vascular invasion, the tumor also shows focal capsular penetration as well.

Discussion: The major differential diagnosis in this case is between a follicular adenoma versus a minimally invasive follicular carcinoma. This lesion does appear to have a dense fibrous capsule which would not be seen in the usual hyperplastic nodule. The fact that the tumor is much more cellular than the surrounding thyroid tissue and also surrounded by a dense fibrous capsule brings one to the major differential between follicular adenoma versus follicular carcinoma. Another entity which might come into consideration might be the follicular variant of papillary carcinoma, however, this lesion lacks the nuclear features typical of papillary carcinoma. In making the distinction between follicular carcinoma and follicular adenoma it is extremely important to carefully examine the capsule surrounding the tumor mass. The most important observation in making the diagnosis of malignancy is the demonstration of blood vessel and/or capsular invasion. When one observes vascular invasion it is important to see clusters of tumor cells attached to the wall of the vessel which may or may not be associated with thrombus material. Sometimes immunohistochemical stains to identify endothelial cells may be helpful in identifying vascular invasion within the vessels of the capsule. When using capsular invasion as a criteria for malignancy, interruption of the capsule should be obvious and must be distinguished from peculiar cuts through the capsule which may actually mimic capsular invasion. Sometimes one sees the tumor extending through the capsule in a mushroom-like fashion. It is also important to remember that some of these lesions have had needle biopsies prior to excision and fine needle aspiration may result in areas of capsular rupture. The latter type of capsular rupture may be suspected when there are areas of recent hemorrhage, stromal fibrosis, and hemosiderin associated with the area suspicious for capsular invasion. As one of the primary criteria for establishing the diagnosis of follicular carcinoma is vascular invasion. It is not surprising that the mode of metastasis for this tumor is usually blood-borne, particularly to the lung and bones in contrast to the nodal metastases which are characteristically seen in papillary thyroid carcinoma. The prognosis of follicular carcinoma is usually related to the degree of encapsulation and extent of invasiveness of the tumor.