[Cytopathological features of thyroid tumors with DICER1 mutation].

<b>Objective:</b> To investigate the cytopathological features of thyroid tumor with DICER1 mutation. <b>Methods:</b> A retrospective study on the preoperative cell smear was conducted on thyroid tumors with DICER1 gene mutations detected by Sanger sequencing in the Department of Pathology Shanghai Sixth People's Hospital affiliated to Shanghai Jiaotong University School of Medicine from May 2022 to November 2024. <b>Results:</b> Totally 163 cases with histological features indicating DICER1 mutation related thyroid tumor underwent Sanger sequencing. Fifteen cases were confirmed to harbor DICER1 mutation (15/163,9.2%). Fourteen of 15 patients were female, and only 1 was male; average age 42(31,47) years. Eight cases presented with D1709 hotspot mutation (8/15) and 7 cases with the E1813 hotspot mutation (7/15) and there was no statistical significant difference between mutation rate of different hotspot (<i>F</i>=0.620，<i>P</i>=0.438). All specimens were stained with hematoxylin-eosin staining. A moderate number of cells were observed for all cases, predominantly with macrofollicular pattern and rare small papillae. The cell nuclei were mainly uniform, small, round and dark, slightly enlarged or medium-sized. Several cases could also present RAS-like nuclear features: 3 cases showed visible nuclear grooves. According to the expert consensus on the cytopathological diagnosis of thyroid fine needle aspiration (version 2023),the cytopathological diagnostic categories were: Ⅱ, 6 cases; Ⅲ, 2 cases; Ⅳ, 6 cases; and Ⅴ, 1 case. Postoperative histological diagnoses included follicular thyroid carcinoma in 2 cases, high-grade differentiated thyroid carcinoma in 1 case, follicular thyroid adenoma in 3 cases, follicular thyroid tumor of undetermined malignant potential in 4 case, and thyroid follicular nodular disease in 5 cases. <b>Conclusions:</b> The morphological features of DICER1-mutated thyroid tumors are predominant macrofollicles, with uniformly small round and dark nuclei. It is difficult to identify typically diagnostic atrophic follicles on cell smear, leading to a preoperative diagnosis of benign non-neoplastic or indeterminate category. Therefore, it is necessary to carefully observe the macro-follicles and small round dark nuclear features, which is necessary to suggest a genetic test of DICER1 gene and to confirm the diagnosis before surgery.