Institution-Specific Risk of Differentiated Thyroid Carcinoma in Follicular Neoplasms, Bethesda Category IV
Background: The Bethesda System for Reporting Thyroid Cytopathology (BSRTC) provides uniformity in reporting cytopathology results to help guide surgical decisions based on the risk of malignancy (ROM) within each of six categories. The three indeterminate categories have malignancy rates that vary widely between institutions. We recently reported our institution-specific rates of malignancy within atypia/follicular lesion of undetermined significance (AUS/FLUS) as 46% and suspicious for malignancy as 91%, out of 187 and 42 surgical cases, respectively. We have since accumulated enough patients to report our malignancy rates in Bethesda category IV, follicular neoplasms (FN)/suspicious for follicular neoplasm (SFN). The malignancy rates of FN/SFN vary in the literature from 14%-34%, and represents the most diverse pathology spectrum within the BSRTC categories.
Methods: We conducted a retrospective analysis of index thyroid nodule cytology and thyroid histopathology in patients who underwent thyroidectomy for lesions demonstrating Bethesda IV (FN/SFN) cytopathology between January 2010 and October 2018, in a high-volume tertiary endocrine head and neck surgery service.
Results: We identified 265 cytology cases with FN/SFN in 245 patients, of which 79 patients (32%) had a thyroidectomy. Within this group, 33 patients (42%) were found to have malignancy, with 11/33 patients (33%) harboring follicular thyroid carcinoma (FTC), and 2/33 patients (6%) demonstrating medullary thyroid carcinoma (MTC). Male patients were significantly more likely to have malignancy compared to female patients (67% vs 34%, p=0.03).
Conclusion: ROM within indeterminate nodules varies widely, with malignancy rates within our FN/SFN cohort at 42%, and 39% of patients demonstrating FTC or MTC, highlighting the importance of institution-specific malignancy rates combined with individual risk factors when making treatment decisions.