Background: Papillary thyroid carcinomas (PTCs) are diagnosed mainly by the features of nuclei of the neoplastic cells. Theoretically, by similarity to other types of carcinoma, infiltrative tumor growth likely distinguishes the thyroid neoplastic lesions with potential of metastasis from those without this capability.
Methods: One hundred thirty-four cases of PTC, consisting of 41 encapsulated tumors and 93 nonencapsulated tumors, were examined for the presence of infiltrating PTC (IPTC), defined by tumor cells infiltrating a desmoplastic or sclerotic stroma.
Results: Encapsulated tumors with focal or extensive IPTC and nonencapsulated tumors with focal or extensive IPTC were associated with rates of lymph node metastases of 28%, 42%, 36%, and 65%, respectively. The tall cell variant of PTC was associated with a higher rate of metastasis. There was a small group of tumors without identifiable IPTC, designated as PTC without identifiable infiltrating carcinoma, which included six encapsulated tumors with areas of sclerosis in the capsule and three nonencapsulated tumors consisting of neoplastic cells with optically clear nuclei, nuclear grooves, and intranuclear inclusions in over two thirds of the cell population. Of these lesions, one case developed lymph node metastases. Papillary thyroid tumors without areas of IPTC and exhibiting the nuclear changes described, but in less than two thirds of the tumor cells, did not develop lymph node metastases.
Conclusions: Infiltrating tumor growth is an indicator of malignant behavior in most thyroid papillary neoplastic lesions. The extent of IPTC is likely proportional to the rate of lymph node metastases.