Vascular invasion: way to follicular thyroid carcinoma

There are 4 types of thyroid carcinoma: papillary, follicular, medullary, and anaplastic carcinoma. One of these types, follicular thyroid carcinoma, can look very much like a benign thyroid adenoma. Both follicular carcinoma and thyroid adenoma are composed of follicles (resembling normal thyroid follicles).

The only way to tell apart follicular thyroid carcinoma (which is malignant) from thyroid adenoma (which is benign) is to take out the entire nodule and examine the entire thing very carefully. If you see tumor cells invading the capsule, or if you see them within vessels (as in the photo above), that means it’s follicular carcinoma. Malignant tumor cells invade; benign ones do not.