Rare but Complex: Outcomes and Challenges in Managing Composite Follicular-Anaplastic Thyroid Cancer

Background: A subset of follicular thyroid cancers (FTC) can dedifferentiate into anaplastic thyroid cancer (ATC), forming composite FTC/ATC tumors. The effect of this dedifferentiation on survival outcomes remains unclear. This study aimed to characterize the clinicopathologic features of composite FTC/ATC tumors and assess their disease-specific survival (DSS). Patients and Methods: The Surveillance, Epidemiology, and End Results (SEER) database (2000–2020) was used to identify patients with FTC, composite FTC/ATC, and ATC. Propensity score matching (PSM) was performed on the basis of age, sex, race, and histologic subtype. Clinicopathologic characteristics, DSS, and treatment outcomes were compared. Results: A total of 60 patients with composite FTC/ATC were matched to 180 patients with FTC and 180 patients with ATC. Compared with FTC, composite tumors were associated with larger size (p < 0.001), extra-thyroidal extension (ETE) (p < 0.001), lymph node (p < 0.001) and distant metastases (p = 0.002), more external beam radiation (p < 0.001), and chemotherapy (p < 0.001), but less radioactive iodine (RAI) (p < 0.001). Compared with ATC, composite tumors showed less ETE (p = 0.01), fewer lymph node metastases (p = 0.01), less chemotherapy (p = 0.002), and more RAI (p = 0.002). The median survival of patients with co-FTC/ATC was 7 months (p < 0.001). Advanced age (hazard ratio, HR 1.05; 95% confidence intervals, CI 1.02–1.08) and incomplete thyroidectomy (HR 2.58, 95% CI 1.20–5.56) predicted worse DSS. Conclusions: Composite FTC/ATC tumors have survival outcomes that fall between those of FTC and ATC. Total thyroidectomy is a key component of treatment, as incomplete thyroidectomy is linked to poorer survival. Further research is needed to explore how the proportion of ATC within composite tumors influences survival outcomes.