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

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.

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.

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).

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.