An old friend, a new insight: Calcitonin measurement in serum and aspiration needle washout fluids significantly increases the early and accurate detection of medullary thyroid cancer

Abstract Background The sensitivity of cytological (CY) evaluation after fine‐needle aspiration (FNA) for detecting medullary thyroid carcinoma (MTC) is a subject of controversy. The routine use of serum calcitonin (CT) in patients with thyroid nodules is not universally adopted. The authors conducted CT screening of FNA washout fluid (FNA‐CT) to address the diagnostic challenges. The objective was to assess the contributions of serum CT, FNA cytology (FNA‐CY), and FNA‐CT to the diagnosis. Methods Between February 2019 and June 2022 (group 1), the authors prospectively screened the CT of patients with thyroid nodules. Both FNA‐CY and FNA‐CT were performed for patients with persistently elevated CT values. The sensitivity of FNA‐CY, serum CT, and FNA‐CT for accurate diagnosis was evaluated. Additionally, the authors retrospectively examined data from patients with thyroid nodules before CT screening (2008–2019) (group 2). They compared the characteristics of MTC patients in groups 1 and 2. Results MTC was identified in 30 patients (0.25%) in group 1 and 19 (0.07%) in group 2. A FNA‐CT cutoff value of 4085.5 pg/mL detected MTC with a sensitivity of 96.8%, and a serum CT cutoff value of 28.3 pg/mL detected MTC with a sensitivity of 86.7%. In contrast, FNA‐CY detected MTC with a sensitivity of 42.4%. In group 1, 18 patients (60%) with MTC were diagnosed with microcarcinoma, whereas only two patients (10.5%) in group 2 had microcarcinoma. Conclusions This study detected MTC earlier by routinely measuring serum CT in all patients with nodular thyroid disease and performing FNA‐CT in those with elevated values. FNA‐CT and serum CT sensitivities were significantly higher than those of FNA‐CY. This study revealed different FNA‐CT cutoff values compared to other studies, emphasizing the need for determining clinic‐specific cutoff values.