Performing Calcitonin Immunocytochemistry on an Additional ThinPrep Slide in Fine-Needle Aspiration Diagnosis of Medullary Thyroid Carcinoma

OBJECTIVES: This study reviewed the institutional experience of performing calcitonin immunostain on an additional ThinPrep slide in fine-needle aspiration (FNA) diagnosis of medullary thyroid carcinoma (MTC). METHODS: Thyroid FNA cases with MTC suspected or included in the differential diagnosis during cytologic evaluation and calcitonin immunostain performed were retrieved and reviewed. RESULTS: Calcitonin immunostain was performed in 132 cases with 41 positive, 81 negative, and 10 indeterminate results. All calcitonin-positive cases had a cytologic diagnosis of MTC while all calcitonin-negative cases were cytologically classified as non-MTCs except for two cases suspicious for MTC. In 10 cases with an indeterminate calcitonin result, diagnoses of non-MTC and suspicious for MTC were rendered in 6 and 4 cases, respectively. Histopathologic follow-up was available in 85 (64%) cases. All cytologically diagnosed MTC cases were confirmed on histopathology. In 3 MTC cases with an indeterminate calcitonin result, 1 case was misclassified cytologically as follicular neoplasm. The calculated sensitivity, specificity, positive predictive value, and negative predictive value of calcitonin immunostain were all 100% for diagnosing MTC. CONCLUSIONS: Our study demonstrates the feasibility of performing calcitonin immunostain on an additional ThinPrep slide. Calcitonin immunocytochemistry is a valuable adjunct test for FNA diagnosis and differential diagnosis of MTC.