Obviously the presence of distant metastases is the most certain criterion. Most students of the disease agree that the ordinary criteria of malignancy have little prognostic value in thyroid tumors, except perhaps in the wildly growing anaplastic tumors. However, it may be noted that pathologists at the Mayo Clinic believe a histologic typing by their criteria provides significant information on prognosis. Examples of the histologic patterns of several of these tumors are given in Figure 18-9. The papillary adenocarcinoma typically shows tumor cells around a fibrovascular core and, not infrequently, areas of follicular differentiation.
Papillary lesions tend to be infiltrative, and encapsulation is rare.
Vascular invasion is rare. Psammoma bodies are often abundant. Multiple intraglandular foci are frequent, especially in children. Areas of lymphocyte infiltration, and even extensive lymphocytic thyroiditis, are common, especially in papillary tumors. Many tumors look much like follicular cancers, but have the characteristic nuclei of papillary cancers.