Molecular diagnosis of indeterminate thyroid FNA cytology results

While thyroid nodules are frequent clinical findings, the incidence of thyroid cancer is low and the accurate classification and risk stratification of nodules remains a clinical challenge. After initial clinical and ultrasound cancer risk stratification, fine needle aspiration cytology (FNAC) differentiates malignant and benign thyroid nodules. Yet, FNAC cannot distinguish between benign and malignant follicular tumors in 20% of FNACs classified as indeterminate thus requiring a “diagnostic” thyroid surgery. Of these FNACs, malignancy will be found in approximately 20%. The other 80% of indeterminate FNAC patients will have undergone an unnecessary thyroid surgery with all the potential complications of the surgery. Therefore, we aim to better stratify cytologically indeterminate FNAC by analyzing an optimized and extended mutation panel and miRNA classifiers.

To date, others attempting to improve the diagnostic gap for the indeterminate FNAC deficit by molecular testing have used additional fresh FNA material as an input. That approach, however, requires an intent for molecular testing a priori; if the intent is unknown or if extra material is not collected at the point of care, then molecular testing is not possible. In contrast, we have previously established and validated a method to use the available material from the routine FNA smear or residual liquid-based FNA material. Major advantages of our approach are that exactly the same sample that was analyzed by the cytopathologist is used for the molecular analysis and integrated and focused molecular diagnostics is performed only for those FNAs that the cytopathologist classifies as indeterminate.

Since our proof of principle studies were completed, further mutations and miRNA classifiers have been identified that could reduce the fraction of papillary thyroid cancer with unknown molecular markers to <5%. Therefore, we aim to further improve the accuracy of the molecular testing by analyzing an improved set of cancer specific somatic mutations. These mutations will be analyzed by targeted next generation sequencing and MassARRAY technology. Furthermore, the quantification of recently defined miRNA classifiers in mutation-negative samples will further improve the accuracy of molecular testing. This molecular diagnostic approach will be evaluated in the setting of a guideline-oriented multidisciplinary thyroid cancer and thyroid nodule clinic in Calgary which also deals with the 20% indeterminate cytologies out of 1700 FNAs analyzed annually by Calgary Lab Services.