Cytomorphologic features of thyroid nodules harboring thyroid adenoma-associated fusion: A single institutional review of 11 cases

Background: Thyroid adenoma-associated (THADA) fusions are being identified more frequently with increased preoperative molecular testing on indeterminate thyroid fine-needle aspirates (FNA). However, data on cytomorphologic features of THADA-fusion thyroid lesions are limited. We examined cytomorphologic features of a THADA-fusion thyroid FNA cohort with clinical, radiologic, and histologic data.

Methods: Our THADA fusion FNA cohort included 11 specimens from 11 patients with a mean age and F:M ratio of 56.5 years and 2.7:1, respectively. Ten cases had FNA slides available for review and five cases had histopathology. Air-dried Diff-Quik and alcohol-fixed Papanicolaou smears, and formalin-fixed cell blocks were reviewed and findings were correlated with the matched resection, when available. Clinical and radiologic data were obtained from the medical record.

Results: THADA fusion thyroid lesions were hypoechoic or isoechoic (80%), solid (80%) with well-demarcated margins (100%) on ultrasound. Notable cytomorphologic features include crowded or prominent microfollicular groups (100%), scant colloid (91%), and mild nuclear atypia with nuclear grooves (100%), insignificant nuclear pleomorphism (100%), and without intranuclear pseudoinclusions (100%). All cases were classified as either atypia of undetermined significance (91%) or suspicious for follicular neoplasm (9%). Five resected cases were all low-risk lesions (benign and malignant) with no evidence of recurrence in the limited available follow-up.

Conclusion: THADA-fusion thyroid FNAs show a crowded or prominent microfollicular pattern with mild nuclear atypia and without intranuclear pseudoinclusions, which overlaps with the cytomorphology of other RAS-like thyroid lesions. In our cohort, resected THADA lesions were low risk on histologic analysis.