Most studies that have examined minimally invasive, encapsulated, follicular carcinoma (FC) or Hurthle cell carcinomas (HCs) have contained only a few metastatic neoplasms. We studied 34 patients with a single, minimally invasive, metastatic FC or HC and compared them with 38 patients with similar, nonmetastatic FCs or HCs. The numbers of incomplete capsular penetration (neoplasm into but not through the capsule), complete capsular penetration (neoplasm through the capsule), and vascular invasion foci were quantified. The median number (three), range, and distribution of complete capsular penetration and vascular invasion foci were similar in the nonmetastatic and metastatic carcinomas. All of the metastatic FCs and HCs had at least one vascular invasion or complete capsular penetration focus. Sixty-two percent of the metastatic carcinomas had two to four complete capsular penetration foci, and 60% had two to four vascular invasion foci. Two metastatic neoplasms had incomplete capsular penetration but had one and two vascular invasion foci, respectively. One tumor had no vascular invasion but had four complete capsular penetration foci. No metastatic neoplasms had incomplete capsular penetration only. There were no differences in the number of vascular invasion or complete capsular penetration foci between metastatic and nonmetastatic FCs and HCs and between metastatic FCs and HCs. Most metastatic neoplasms had vascular space invasion and complete capsular penetration. The number of complete capsular penetration or vascular invasion foci was not associated with the initial site of metastasis or the interval between the surgery and the metastasis.
