BACKGROUND This study aimed to analyze the temporal changes of the clinicopathologic characteristics, and the long-term outcomes, of various types of anaplastic thyroid cancer (ATC) and poorly differentiated thyroid cancer (PDTC).

METHODS A retrospective analysis was conducted on patients with ATC and PDTC who were treated from 1985 to 2013. The outcome measures included the clinical response to treatment and the survival rates of three separate thyroid cancer groups: ATC, PDTC, and differentiated thyroid cancer (DTC) with anaplastic foci.

RESULTS The five-year disease-specific survival rate was significantly higher, both in DTC with anaplastic foci and in PTDC (81.3% and 65.8%, respectively), than it was in ATC (14.3%; p < 0.001). The proportion of cases of DTC with anaplastic foci has been increasing over time, while that of ATC has decreased. The survival rate was found to be significantly higher in resectable tumors (71.4% and 26.5%, respectively; p < 0 .001). In ATC, external beam radiation therapy showed longer survival rates than did surgery-based treatment in unresectable tumors (19.2 vs. 7.7 months, p = 0.006). Adjuvant treatment with external beam radiation or radioactive iodine increased survival duration in PDTC and in DTC with anaplastic foci. Lymphatic invasion was the most significant postoperative prognosticator in ATC (p = 0.013).

CONCLUSIONS The choice of treatment of ATC and PDTC could be modified according to resectability and lymphatic invasion of the cancer.