Comprehensive evaluation of cytomorphologic, histologic, and molecular features of DICER1‐altered thyroid lesions on FNA: A multipractice experience

ABSTRACT Background DICER1 mutations, though infrequent, are encountered on preoperative molecular testing of indeterminate adult and pediatric thyroid fine‐needle aspiration (FNA) specimens. Yet, published cytomorphologic features of DICER1 ‐altered thyroid lesions are limited. Cytomorphological features of DICER1 ‐altered thyroid lesions were examined in a multipractice FNA cohort with clinical, radiological, and histologic data. Methods The cohort comprised 18 DICER1 ‐altered thyroid FNAs, with 14 having slides available and eight having corresponding surgical resections. Smears, ThinPrep, and formalin‐fixed cell block slides were reviewed and correlated with histology, when available. Clinical and radiologic data were obtained from the medical record. Results Most DICER1 ‐altered FNAs were classified as atypia of undetermined significance (94.4%). DICER1 mutations occurred in codons 1709 (50%), 1810 (27.8%), and 1813 (22.2%). One patient had an additional DICER1 p.D1822N variant in both of their FNAs. Lesions were often hypoechoic (35.3%) and solid (47.1%) on ultrasound. Notable cytomorphologic features include mixed but prominent microfollicular or crowded component, variable colloid, and insignificant nuclear atypia. On resection ( n = 10), histologic diagnoses ranged from benign follicular adenoma and low‐risk follicular thyroid carcinoma to high‐grade follicular–derived nonanaplastic thyroid carcinoma. Subcapsular infarct‐type change was the most common histologic change. There was no evidence of recurrence or metastasis in eight patients on limited follow‐up. Conclusion DICER1 ‐altered thyroid lesions occurred frequently in young females and FNAs show RAS‐like cytomorphology including crowded, mixed macro‐/microfollicular pattern, and bland nuclear features. On resection, DICER1 ‐altered thyroid lesions include benign (50%), low‐risk lesions (30%), or high‐risk malignancies (20%).