Risk of malignancy in thyroid nodules with indeterminate (THY3f) cytology

Background Fine-needle aspiration cytology (FNAC) is an integral part of thyroid nodule assessment. Nodules with an indeterminate cytology (THY3a-f) require formal histological assessment to confirm benign or malignant pathology. This study aimed to provide data for an evidence-based approach for management of patients with THY3f nodules. Methods Retrospective review of patients who had a thyroid FNAC reported as suspicious of follicular neoplasm (THY3f) or showing atypia (THY3a) were identified, and clinical, operative and outcomes data were analysed. Results Between 2018 and 2020, 200 patients (167F:33M, median age 51 years (range:18-86 years)) had a THY3f cytology. Most presented with a palpable nodule (n=104; 68.4%). Overall, 152 (76.0%;130F:23M) underwent surgery and 31 (20.4%) were found to have a thyroid carcinoma (22 follicular carcinomas, 7 papillary carcinomas, 1 medullary thyroid carcinoma and 1 metastatic renal carcinoma). An additional incidental carcinoma (size: 0.7-13mm) was found in seven (4.6%). Among those with cancer, a completion thyroidectomy and radioactive iodine treatment was indicated in nine (<6% of the entire cohort). Previously suggested risk factors for malignancy, eg male gender, large tumour size (>4cm) or age, were not found to be associated with increased risk. During the same period, THY3a cytology was reported in 53 patients, of whom 29 underwent diagnostic surgery and 4 patients were found to have a thyroid cancer (follicular, n=3 and medullary, n=1). Conclusion One in five patients with features suspicious of a follicular neoplasm (THY3f) has a thyroid carcinoma. This risk is much lower for THY3a. This study reinforces the current recommendation for thyroid surgery in all patients with a reliable THY3f cytology, as no further stratifying risk factors could be identified.