The most common form of thyroid cancer
So with that, let’s talk a little bit about thyroid cancer itself. A variety of different types. Differentiated cancers arising from thyroid follicular cells are of course papillary or follicular cancers. The less differentiated is the anaplastic, which fortunately we see very rarely. Arising from an embryologically distinct population of C cells is medullary thyroid cancer. I’ll talk very briefly about it towards the end. I don’t know if the speakers, when they were talking about some of the other islet cell neoplasms, discussed this at all or not. Finally, there are sort of non-thyroid thyroid things. Non-Hodgkin’s lymphoma can present as a goiter or a thyroid nodule occasionally. Metastatic disease and some other less common lesions. But we are going to focus on the differentiated follicular-derived thyroid cancers. About 17,000 cases are diagnosed yearly in the U.S. and the incidence of diagnosed cases has been increasing. It’s not clear to me if this represents some true alteration in the incidence of the disease, or whether this is in part a reporting issue of previously undiagnosed cases, since thyroid cancer often will be fairly indolent. The disease is more common in women than in men.
The most common form of thyroid cancer that we see in this country, and in general in areas of iodine sufficiency, is papillary thyroid cancer. Probably accounts for 80-85% of the thyroid cancers that we see in the United States. Papillary thyroid cancer can be noted as an incidental finding. Occasionally one will undergo a thyroidectomy and a microscopic focus or maybe several microscopic foci of papillary thyroid cancer will be found almost as an incidental finding. It would appear that that form of microscopic papillary thyroid cancer is probably of little clinical concern, since the finding is that these things are found on autopsy series in unsuspected patients much more frequently than they present clinically. So there are probably several people in this room who are harboring microscopic foci of papillary cancer and very likely you will never know about it. Papillary cancer can be multifocal throughout the thyroid gland, or bilateral. The general spread of thyroid cancer is local invasion spread to cervical lymphatics and then distant metastases, particularly to the lung.
Risk factors for papillary cancer; thyroid irradiation. Papillary cancer is seen with increased frequency as a second malignancy in individuals with a history of childhood cancer. This in part may be related to the irradiation issue. It may be in part related to general genetic factors with regard to tumor predisposition. There is some argument as to whether or not Grave’s disease, because of its sort of the endogenous thyroid stimulation is a risk factor. Dietary iodine increases the relative risk of papillary thyroid cancer but decreases the risk of follicular cancer. If you look at areas of the world that are iodine deficient you tend to see more follicular thyroid cancer relative to the United States.