Relationship between histopathological findings and RAI in non-papillary follicular cell–derived thyroid carcinoma

Context: Detailed studies considering the distinct types of non-papillary follicular cell-derived thyroid carcinoma in terms of radioactive iodine (RAI) avidity are limited. Objective: This study aimed to investigate clinicopathological characteristics, relationship between histopathological findings, including solid/trabecular/insular (STI) components and RAI treatment (RAIT), in patients with non-papillary follicular cell-derived thyroid carcinoma. Methods: This study included 114 patients with non-papillary follicular cell-derived thyroid carcinoma who underwent first RAIT between January 2005 and August 2025. Results: Of the 114 patients, 5 (4%) were minimally invasive follicular thyroid carcinoma (MI-FTC), 41 (36%) were encapsulated angio-invasive FTC (EA-FTC), 23 (20%) were widely invasive FTC (WI-FTC), 22 (19%) were differentiated high-grade thyroid carcinoma (DHGTC), 21 (18%) were poorly differentiated thyroid carcinoma (PDTC), and 2 (2%) were others, respectively. The median percentages of STI component was 40% (range, 0%-10%) in all patients, 5% (range, 0%-50%) in MI-FTC, 30% (range, 0%-100%) in EA-FTC, 30% (range, 0%-100%) in WI-FTC, 28% (range, 5%-70%) in DHGTC, and 70% (range, 50%-100%) in PDTC, respectively. Among the 114 patients, the rate of RAI avidity was 60% (95% CI, 49%-71%) (47/78) in lung, 89% (95% CI, 79%-96%) (59/66) was in bone, 100% (13/13) in lymph node, and 100% (3/3) in other sites, respectively. Conclusion: RAI avidity was relatively high in non-papillary follicular cell-derived thyroid carcinoma, regardless of the histological type and percentage of STI components.