Clinicopathologic Features of Fatal Non-Anaplastic Follicular Cell-Derived Thyroid Carcinomas

Background: The vast majority of thyroid cancers, in particular the non-anaplastic follicular cell-derived thyroid carcinomas (non-ANA FCDC), are considered indolent tumors with very low mortality. Hence, it is crucial to analyze the subgroup of these patients who die of disease (DOD) in order to identify clinicopathologic features predictive of disease-specific mortality. Methods: All non-ANA FCDC operated at a tertiary cancer center between 1985 and 2010 who were DOD were identified and submitted to a meticulous clinicopathologic analysis. Results: Out of 3750 non-ANA FCDC, 58 (1.5%) DOD cases were identified. The DOD group was composed of 33 (57%) poorly differentiated carcinomas (PDTC), 14 (24%) tall-cell variant papillary thyroid carcinomas (TCVPTC), four (7%) Hurthle cell carcinomas, three (5%) papillary microcarcinomas, two (3%) classical variant PTC, and two (3%) follicular variant PTC. Twenty-seven (47%) patients presented with distant metastases (DM), 28 (48%) developed DM during follow-up, while the remaining three (5%) had locally advanced non-resectable recurrence. Gross extension beyond the thyroid (GET) was present in 36 (62%) and extensive vascular invasion (VI) in 21 (36%) of cases. All microcarcinomas had PDTC in their clinically apparent cervical lymph nodes at presentation. Encapsulated thyroid carcinomas were responsible for 17% of DOD cases, and all had extensive VI and/or DM at presentation. All patients had at least one of these high-risk features at diagnosis: DM at presentation, PDTC, GET, and/or extensive VI. The majority of patients died from DM (n = 51; 88%), three (5%) from locoregional disease, three (5%) from both, and one (2%) from unknown cause. Conclusions: PDTC and TCVPTC are responsible for the vast majority of deaths in differentiated thyroid carcinomas, while the few fatal classical, follicular variant PTC and microcarcinomas all harbor a PDTC component, DM, or GET. Encapsulated differentiated thyroid carcinoma with focal capsular and/or VI without DM at presentation does not seem to cause death. Lack of DM at presentation, PDTC, GET, and extensive VI identify thyroid carcinomas that are at almost no risk of DOD. The vast majority of patients die of DM rather than locoregional invasion, prompting the need for effective systemic treatment.