Minimally invasive (encapsulated) follicular carcinoma of the thyroid gland is the low-risk counterpart of widely invasive follicular carcinoma but not of insular carcinoma.

Minimally invasive (encapsulated) follicular carcinoma (MIFC), widely invasive follicular carcinoma (WIFC), and insular carcinoma (IC) are three distinct types of thyroid carcinomas composed of follicular cells, IC representing the poorly differentiated counterpart. Our aim was to test whether there is a time-dependent relationship among these three entities, in particular whether MIFC is the low-risk form of the entire group. We used overall survival analysis and multiple correspondence analysis (MCA), the latter exploring the clinicopathological characteristics at onset (sex, age, TNM/pTNM classification). The study included 18 MIFCs, 29 WIFCs, and 27 ICs. On the basis of both overall survival and MCA, WIFC emerged as the high-risk counterpart of MIFC, occurring in older age, with a larger size, a widespread growth into the thyroid parenchyma, and presenting with distant metastases. In both types of follicular carcinoma distant metastases involved only lung and bone, and there was no tendency for local spread beyond the thyroid capsule or nodal metastases. By contrast, IC confirmed to be a distinct high-risk lesion, showing both follicular and papillary-related biological behavior characteristics.