Recurrence after ablation of Bethesda IV thyroid nodule: report of two cases.

Background: Bethesda IV follicular neoplasms [2-25% of fine-needle aspiration (FNA) diagnoses] pose significant diagnostic challenges due to their malignant potential. Thermal ablation (TA) has emerged as a minimally invasive alternative to surgery, though its safety for follicular neoplasms remains controversial given cytology's inability to assess capsular/vascular invasion-the diagnostic gold standard. Case Description: This case report analyzes two patients with Bethesda IV nodules treated with microwave ablation (MWA). Case 1 involved a 26-year-old female with a 5.1 cm nodule undergoing two MWA sessions, while Case 2 featured a 12-year-old female with a 2.6 cm nodule receiving single-session MWA. Both cases deviated from guideline-recommended dual preoperative FNAs. Both patients developed recurrence: Case 1: progression of thyroid carcinoma within the ablation zone and metastatic papillary carcinoma in cervical lymph nodes were surgically confirmed two years post-ablation. Case 2: a recurrent cystic-solid nodule (1.8 cm) was detected adjacent to the ablation zone during 9-month post-procedural surveillance. Notably, neither recurrence displayed typical follicular carcinoma histology, suggesting ablation-induced diagnostic obscuration. Conclusions: These first-reported recurrence cases after TA for Bethesda IV nodules underscore critical limitations: violations of pre-ablation diagnostic protocols [dual FNA/core needle biopsy (CNB)] exacerbated malignancy risks, while TA-induced obscuration of histopathological evaluation delayed cancer diagnosis. Current guidelines appropriately exclude follicular neoplasms from TA indications, with surgical resection remaining the gold standard. TA should be restricted to validated research protocols pending establishment of robust risk-stratification criteria.