ThyroSeq Performance on Cytology Smears: Evaluation of Modalities for Testing

Summary: Introduction: Fine needle aspiration (FNA) specimens can be utilized for molecular testing of indeterminate thyroid nodules for pre-operative triage. Collection of additional needle passes for molecular testing can lead to additional time commitment, tissue injury, or overuse of resources for transport, holding medium and storage. Performance of molecular testing as needed overcomes these challenges. Our study aimed to 1) evaluate the success rates of ThyroSeq molecular testing on thyroid cytology samples (smear and cell block) and 2) compare smear vs cell block in their performance with respect to molecular testing. Materials and Methods: We identified 61 thyroid FNA cases between 11/2023-3/2024 and their data on demographics, Bethesda diagnosis, tumor cellularity, and molecular test results were retrospectively reviewed. All 61 cases that were submitted for ThyroSeq v3 molecular testing included Papanicolaou (Pap) stained smears (72.1%; 44/61), Diff Quik (DQ) stained smears (19.6% ; 12/61) and formalin fixed paraffin embedded (FFPE) slides (8.2% ; 5/61). Results: Quantity was not sufficient for 4 (6.5%) samples; 93.4% (57 cases) showed successful molecular analysis. Of 57 cases both DNA-variant and RNA-fusion analysis was possible in 51 cases. 6/57 were successful but limited due to low cellularity and only DNA-variant analysis was possible. Our result indicated that molecular analysis from smears stained either with DQ (83.3%) or Pap (100%) offered significantly better success rates than unstained slides from cell block (60%) (Table1). There was no statistically significant difference in success rate between DQ and Pap (Fisher's Exact) (p>0.99) (Figure 1). Conclusions: In conclusion, our data show cytology smears with adequate cellularity can be successfully used for performing ThyroSeq v3 (success rate >93%). Fixation with air drying or alcohol does not impact molecular testing results. Utilizing cytology smears for molecular tests may circumvent the need for additional sampling, sparing patients from additional needle passes at the time of FNA.