High Sensitivity of Fine‐Needle Aspiration Calcitonin in Detecting Medullary Thyroid Carcinoma Is Independent of Predefined Decisional Thresholds

The measurement of Calcitonin (Ctn) in fine-needle aspiration (FNA) washout fluids (FNA-Ctn) has demonstrated excellent sensitivity, significantly higher than FNA cytology, in detecting medullary thyroid carcinoma (MTC). However, the absence of a fixed cutoff value for FNA-Ctn poses a limitation. This study aimed to investigate whether the sensitivity of FNA-Ctn in detecting MTC varies with different cutoffs reported in the literature. A single-centre series of MTCs was retrospectively reviewed. The preoperative FNA-Ctn levels were re-evaluated using various thresholds previously reported in the literature, and the corresponding FNA-Ctn sensitivities were compared. Twenty-one MTCs were included (69% women; median age 59 years; median serum Ctn value 86 pg/mL; median MTC major diameter 10 mm). The median FNA-Ctn value was 2000 pg/mL (interquartile range 49-250). MTCs nodules were assessed at high risk (ACR TI-RADS 5) in 50% of cases, while 47.6% were cytologically malignant. Additionally, 42.9% of cases were assessed as stage III according to Union for International Cancer Control staging system (UICC). Serum Ctn was significantly lower in stage I (p = 0.04). FNA-Ctn was positively correlated with serum Ctn (Rho = 0.45; p = 0.04), while ACR TI-RADS assessment with MTC stage (Rho = 0.69; p = 0.003). FNA-Ctn sensitivity ranged from 95% to 100% based on the previously proposed FNA-Ctn cutoffs. The high sensitivity of FNA-Ctn in detecting MTC did not significantly differ when applying the previously proposed cutoffs. Given the absence of a universally applicable FNA-Ctn decisional threshold, institutions should establish their own diagnostic cutoffs. Future guidelines should incorporate these concepts to enhance clinical decision-making and patient outcomes.